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                      GUIDE TO THE STUDY OF FISHES

[Illustration]

[Illustration]




                                A GUIDE

                                  TO

                          THE STUDY OF FISHES

                                  BY
                          DAVID STARR JORDAN
           _President of Leland Stanford Junior University_

          _With  Frontispieces and 427 Illustrations_

                            IN TWO VOLUMES
                                VOL I.


                                    "I am the wiser in respect to
                                    all knowledge and the better
                                    qualified for all fortunes for
                                    knowing that there is a minnow in
                                    that brook."--_Thoreau_

[Illustration]

                               NEW YORK
                        HENRY HOLT AND COMPANY
                                 1905

                            Copyright, 1905
                                  BY
                        HENRY HOLT AND COMPANY
                         Published March, 1905

                                  To
                            Theodore Gill,
        Ichthyologist, Philosopher, Critic, Master in Taxonomy,
                       this volume is dedicated.




PREFACE


This work treats of the fish from all the varied points of view of the
different branches of the study of Ichthyology. In general all traits
of the fish are discussed, those which the fish shares with other
animals most briefly, those which relate to the evolution of the group
and the divergence of its various classes and orders most fully. The
extinct forms are restored to their place in the series and discussed
along with those still extant.

In general, the writer has drawn on his own experience as an
ichthyologist, and with this on all the literature of the science.
Special obligations are recognized in the text. To Dr. Charles
H. Gilbert, he is indebted for a critical reading of most of his
proof-sheets; to Dr. Bashford Dean, for criticism of the proof-sheets
of the chapters on the lower fishes; to Dr. William Emerson Ritter, for
assistance in the chapters on _Protochordata_; to Dr. George Clinton
Price, for revision of the chapters on lancelets and lampreys, and to
Mr. George Clark, Secretary of Stanford University, for assistance of
various kinds, notably in the preparation of the index. To Dr. Theodore
Gill, he has been for many years constantly indebted for illuminating
suggestions, and to Dr. Barton Warren Evermann, for a variety of
favors. To Dr. Richard Rathbun, the writer owes the privilege of using
illustrations from the "Fishes of North and Middle America" by Jordan
and Evermann. The remaining plates were drawn for this work by Mary
H. Wellman, Kako Morita, and Sekko Shimada. Many of the plates are
original. Those copied from other authors are so indicated in the text.

No bibliography has been included in this work. A list of writers
so complete as to have value to the student would make a volume of
itself. The principal works and their authors are discussed in the
chapter on the History of Ichthyology, and with this for the present
the reader must be contented.

The writer has hoped to make a book valuable to technical students,
interesting to anglers and nature lovers, and instructive to all who
open its pages.

                                           DAVID STARR JORDAN.

  PALO ALTO, SANTA CLARA COUNTY, CAL.,
                  October, 1904.




ERRATA[1]

VOL. I


  Frontispiece, for _Paramia quinqueviltata_ read _Paramia quinquevittata_
  Page xiii, line 10, _for_ Filefish _read_ Tilefish
         39, " 15, _for_ Science _read_ Sciences
         52, lines 4 and 5, transpose hypocoracoid and hypercoracoid
        115, line 24, for _Hexagramidae_ read _Hexagrammidae_
        162, " 7, The female salmon does as much as the male in
             covering the eggs.
        169, last line, _for_ immmediately _read_ immediately
        189, legend, _for_ Miaki _read_ Misaki
        313, line 26, _for_ sand-pits _read_ sand-spits
        322, " 7 and elsewhere, for Wood's Hole read Woods Hole
        324, " 15, for _Roceus_ read _Roccus_
        327, " next to last, for _masquinonqy_ read _masquinongy_
        357, " 5, _for_ Filefish _read_ Tilefish
        361, " 26, _for_ 255 feet _read_ 25 feet
        368, " 26, _for_ infallibility _read_ fallibility
        414, " 22, _for_ West Indies _read_ East Indies
        419, " 23, _for_-99 _read_-96
        420, " 28, _for_ were _read_ are
        428, " 24, _for_ Geffroy, St. Hilaire _read_ Geoffroy St. Hilaire
        428, " 25, _for_ William Kitchener Parker _read_ William
             Kitchen Parker
        462, " 32, for _Enterpneusta_ read _Enteropneusta_




CONTENTS

VOL. I.


  CHAPTER I.

  THE LIFE OF THE FISH (_Lepomis megalotis_).

                                                                   PAGE

  What is a Fish?--The Long-eared Sunfish.--Form of the Fish.--Face
  of the Fish.--How the Fish Breathes.--Teeth of the Fish.--How the
  Fish Sees.--Color of the Fish.--The Lateral Line.--The Fins of
  the Fish.--The Skeleton of the Fish.--The Fish in Action.--The
  Air-bladder.--The Brain of the Fish.--The Fish's Nest.               3

  CHAPTER II.

  THE EXTERIOR OF THE FISH.

  Form of Body.--Measurement of the Fish.--The Scales or
  Exoskeleton.--Ctenoid and Cycloid Scales.--Placoid Scales.--Bony
  and Prickly Scales.--Lateral Line.--Function of the Lateral
  Line.--The Fins of Fishes.--Muscles.                                16

  CHAPTER III.

  THE DISSECTION OF THE FISH.

  The Blue-green Sunfish.--The Viscera.--Organs of Nutrition.--The
  Alimentary Canal.--The Spiral Valve.--Length of the Intestine.      26

  CHAPTER IV.

  THE SKELETON OF THE FISH.

  Specialization of the Skeleton.--Homologies of Bones of
  Fishes.--Parts of the Skeleton.--Names of Bones of Fishes.--Bones
  of the Cranium.--Bones of the Jaws.--The Suspensorium of the
  Mandible.--Membrane Bones of Head.--Branchial Bones.--The
  Gill-arches.--The Pharyngeals.--The Vertebral Column.--The
  Interneurals and Interhaemals.--The Pectoral Limb.--The
  Shoulder-girdle.--The Posterior Limb.--Degeneration.--The Skeleton
  in Primitive Fishes.--The Skeleton of Sharks.--The Archipterygium.  34

  CHAPTER V.

  MORPHOLOGY OF THE FINS OF FISHES.

  Origin of the Fins of Fishes.--Origin of the Paired
  Fins.--Development of the Paired Fins in the
  Embryo.--Evidences of Palaeontology.--Current Theories as
  to Origin of Paired Fin.--Balfour's Theory of the Lateral
  Fold.--Objections.--Objections to Gegenbaur's Theory.--Kerr's
  Theory of Modified External Gills.--Uncertain Conclusions.--Forms
  of the Tail in Fishes.--Homologies of the Pectoral Limb.--The
  Girdle in Fishes other than Dipnoans.                               62

  CHAPTER VI.

  THE ORGANS OF RESPIRATION.

  How Fishes Breathe.--The Gill Structures.--The Air-bladder.--Origin
  of the Air-bladder.--The Origin of Lungs.--The Heart of the
  Fish.--The Flow of Blood.                                           91

  CHAPTER VII.

  THE NERVOUS SYSTEM.

  The Nervous System.--The Brain of the Fish.--The Pineal Organ.--The
  Brain of Primitive Fishes.--The Spinal Cord.--The Nerves.          109

  CHAPTER VIII.

  THE ORGANS OF SENSE.

  The Organs of Smell.--The Organs of Sight.--The Organs of
  Hearing.--Voices of Fishes.--The Sense of Taste.--The Sense of
  Touch.                                                             115

  CHAPTER IX.

  THE ORGANS OF REPRODUCTION.

  The Germ-cells.--The Eggs of Fishes.--Protection of the
  Eggs.--Sexual Modification.                                        124

  CHAPTER X.

  THE EMBRYOLOGY AND GROWTH OF FISHES.

  Post-embryonic Development.--General Laws of Development.--The
  Significance of Facts of Development.--The Development of the
  Bony Fishes.--The Larval Development of Fishes.--Peculiar
  Larval Forms.--The Development of Flounders.--Hybridism.--The
  Age of Fishes.--Tenacity of Life.--Effect of Temperature
  on Fishes.--Transportation of Fishes.--Reproduction of Lost
  Parts.--Monstrosities among Fishes.                                131

  CHAPTER XI.

  INSTINCTS, HABITS, AND ADAPTATIONS.

  The Habits of Fishes.--Irritability of Animals.--Nerve-cells
  and Fibers.--The Brain or Sensorium.--Reflex
  Action.--Instinct.--Classification of Instincts.--Variability
  of Instincts.--Adaptations to Environment.--Flight of
  Fishes.--Quiescent Fishes.--Migratory Fishes.--Anadromous
  Fishes.--Pugnacity of Fishes.--Fear and Anger in Fishes.--Calling
  the Fishes.--Sounds of Fishes.--Lurking Fishes.--The Unsymmetrical
  Eyes of the Flounder.--Carrying Eggs in the Mouth.                 152

  CHAPTER XII.

  ADAPTATIONS OF FISHES.

  Spines of the Catfishes.--Venomous Spines.--The Lancet of the
  Surgeon-fish.--Spines of the Sting-ray.--Protection through
  Poisonous Flesh of Fishes.--Electric Fishes.--Photophores or
  Luminous Organs.--Photophores in the Iniomous Fishes.--Photophores
  of Porichthys.--Globefishes.--Remoras.--Sucking-disks of
  Clingfishes.--Lampreys and Hogfishes.--The Swordfishes.--The
  Paddle-fishes.--The Sawfishes.--Peculiarities of Jaws and
  Teeth.--The Angler-fishes.--Relation of Number of Vertebrae to
  Temperature, and the Struggle for Existence.--Number of Vertebrae:
  Soft-rayed Fishes; Spiny-rayed Fishes; Fresh-water Fishes; Pelagic
  Fishes.--Variations in Fin-rays.--Relation of Numbers to Conditions
  of Life.--Degeneration of Structures.--Conditions of Evolution
  among Fishes.                                                      179

  CHAPTER XIII.

  COLORS OF FISHES.

  Pigmentation.--Protective Coloration.--Protective Markings.--Sexual
  Coloration.--Nuptial Coloration.--Coral-reef Fishes.--Recognition
  Marks.--Intensity of Coloration.--Fading of Pigments in
  Spirits.--Variation in Pattern.                                    226

  CHAPTER XIV.

  GEOGRAPHICAL DISTRIBUTION OF FISHES.

  Zoogeography.--General Laws of Distribution.--Species Absent
  through Barriers.--Species Absent through Failure to Maintain
  Foothold.--Species Changed through Natural Selection.--Extinction
  of Species.--Barriers Checking Movements of Marine
  Species.--Temperature the Central Fact in Distribution.--Agency
  of Ocean Currents.--Centers of Distribution.--Distribution of
  Marine Fishes.--Pelagic Fishes.--Bassalian Fishes.--Littoral
  Fishes.--Distribution of Littoral Fishes by Coast Lines.--Minor
  Faunal Areas.--Equatorial Fishes most Specialized.--Realms of
  Distribution of Fresh-water Fishes.--Northern Zone.--Equatorial
  Zone.--Southern Zone.--Origin of the New Zealand Fauna.            237

  CHAPTER XV.

  ISTHMUS BARRIERS SEPARATING FISH FAUNAS.

  The Isthmus of Suez.--The Fish Fauna of Japan.--Fresh-water Faunas
  of Japan.--Faunal Areas of Marine Fishes of Japan.--Resemblance
  of Japanese and Mediterranean Fish Faunas.--Significance of
  Resemblances.--Differences between Japanese and Mediterranean Fish
  Faunas.--Source of Faunal Resemblances.--Effects of Direction of
  Shore Lines.--Numbers of Genera in Different Faunas.--Significance
  of Rare Forms.--Distribution of Shore-fishes.--Extension
  of Indian Fauna.--The Isthmus of Suez as a Barrier to
  Distribution.--Geological Evidences of Submergence of Isthmus of
  Suez.--The Cape of Good Hope as a Barrier to Fishes.--Relations of
  Japan to the Mediterranean Explained by Present Conditions.--The
  Isthmus of Panama as a Barrier to Distribution.--Unlikeness
  of Species on the Shores of the Isthmus of Panama.--Views of
  Dr. Guenther on the Isthmus of Panama.--Catalogue of Fishes of
  Panama.--Conclusions of Evermann & Jenkins.--Conclusions of Dr.
  Hill.--Final Hypothesis as to Panama.                              255

  CHAPTER XVI.

  DISPERSION OF FRESH-WATER FISHES.

  The Dispersion of Fishes.--The Problem of Oatka
  Creek.--Generalizations as to Dispersion.--Questions Raised by
  Agassiz.--Conclusions of Cope.--Questions Raised by Cope.--Views
  of Guenther.--Fresh-water Fishes of North America.--Characters of
  Species.--Meaning of Species.--Special Creation Impossible.--Origin
  of American Species of Fishes.                                     282

  CHAPTER XVII.

  DISPERSION OF FRESH-WATER FISHES. (_Continued._)

  Barriers to Dispersion of Fresh-water Fishes: Local
  Barriers.--Favorable Waters Have Most Species.--Watersheds.--How
  Fishes Cross Watersheds.--The Suletind.--The
  Cassiquiare.--Two-Ocean Pass.--Mountain Chains.--Upland
  Fishes.--Lowland Fishes.--Cuban Fishes.--Swampy Watersheds.--The
  Great Basin of Utah.--Arctic Species in Lakes.--Causes of
  Dispersion still in Operation.                                     297

  CHAPTER XVIII.

  FISHES AS FOOD FOR MAN.

  The Flesh of Fishes.--Relative Rank of Food-fishes.--Abundance
  of Food-fishes.--Variety of Tropical Fishes.--Economic
  Fisheries.--Angling.                                               320

  CHAPTER XIX.

  DISEASES OF FISHES.

  Contagious Diseases: Crustacean Parasites.--Myxosporidia or
  Parasitic Protozoa.--Parasitic Worms: Trematodes, Cestodes.--The
  Worm of the Yellowstone.--The Heart Lake Tape-worm.--Thorn-head
  Worms.--Nematodes.--Parasitic Fungi.--Earthquakes.--Mortality of
  Filefish.                                                          340

  CHAPTER XX.

  THE MYTHOLOGY OF FISHES.

  The Mermaid.--The Monkfish.--The Bishop-fish.--The Sea-serpent.    359

  CHAPTER XXI.

  THE CLASSIFICATION OF FISHES.

  Taxonomy.--Defects in Taxonomy.--Analogy and Homology.--Coues
  on Classification.--Species as Twigs of a Genealogical
  Tree.--Nomenclature.--The Conception of Genus and
  Species.--The Trunkfishes.--Trinomial Nomenclature.--Meaning
  of Species.--Generalization and Specialization.--High and Low
  Forms.--The Problem of the Highest Fishes.                         367

  CHAPTER XXII.

  THE HISTORY OF ICHTHYOLOGY.

  Aristotle.--Rondelet.--Marcgraf.--Osbeck.--Artedi.--Linnaeus.--
  Forskal.--Risso.--Bloch.--Lacepede.--Cuvier.--Valenciennes.--
  Agassiz.--Bonaparte.--Guenther.--Boulenger.--Le Sueur.--Mueller.--
  Gill.--Cope.--Luetken.--Steindachner.--Vaillant.--Bleeker.--
  Schlegel.--Poey.--Day.--Baird.--Garman.--Gilbert.--Evermann.--
  Eigenmann.--Zittel.--Traquair.--Woodward.--Dean.--Eastman.--Hay.--
  Gegenbaur.--Balfour.--Parker.--Dollo.                              387

  CHAPTER XXIII.

  THE COLLECTION OF FISHES.

  How to Secure Fishes.--How to Preserve Fishes.--Value of
  Formalin.--Records of Fishes.--Eternal Vigilance.                  429

  CHAPTER XXIV.

  THE EVOLUTION OF FISHES.

  The Geological Distribution of Fishes.--The Earliest
  Sharks.--Devonian Fishes.--Carboniferous Fishes.--Mesozoic
  Fishes.--Tertiary Fishes.--Factors of Extinction.--Fossilization
  of a Fish.--The Earliest Fishes.--The Cyclostomes.--The
  Ostracophores.--The Arthrodires.--The Sharks.--Origin of the
  Shark.--The Chimaeras.--The Dipnoans.--The Crossopterygians.--The
  Actinopteri.--The Bony Fishes.                                     435

  CHAPTER XXV.

  THE PROTOCHORDATA.

  The Chordate Animals.--The Protochordates.--Other Terms Used
  in Classification.--The Enteropneusta.--Classification of
  Enteropneusta.--Family Harrimaniidae.--Balanoglossidae.--Low
  Organization of Harrimaniidae.                                      460

  CHAPTER XXVI.

  THE TUNICATES, OR ASCIDIANS.

  Structure of Tunicates.--Development of
  Tunicates.--Reproduction of Tunicates.--Habits of
  Tunicates.--Larvacea.--Ascidiacea.--Thaliacea.--Origin of
  Tunicates.--Degeneration of Tunicates.                             467

  CHAPTER XXVII.

  THE LEPTOCARDII, OR LANCELETS.

  The Lancelet.--Habits of Lancelets.--Species of Lancelets.--Origin
  of Lancelets.                                                      482

  CHAPTER XXVIII.

  THE CYCLOSTOMES, OR LAMPREYS.

  The Lampreys.--Structure of the Lamprey.--Supposed Extinct
  Cyclostomes.--Conodontes.--Orders of Cyclostomes.--The
  Hyperotreta, or Hagfishes.--The Hyperoartia, or Lampreys.--Food
  of Lampreys.--Metamorphosis of Lampreys.--Mischief Done by
  Lampreys.--Migration or "Running" of Lampreys.--Requisite
  Conditions for Spawning with Lampreys.--The Spawning Process with
  Lampreys.--What Becomes of Lampreys after Spawning?                486

  CHAPTER XXIX.

  THE CLASS ELASMOBRANCHII, OR SHARK-LIKE FISHES.

  The Sharks.--Characters of Elasmobranchs.--Classification
  of Elasmobranchs.--Subclasses of Elasmobranchs.--The
  Selachii.--Hasse's Classification of Elasmobranchs.--Other
  Classifications of Elasmobranchs.--Primitive Sharks.--Order
  Pleuropterygii.--Order Acanthodii.--Dean on Acanthodii.--Order
  Ichthyotomi.                                                       506

  CHAPTER XXX.

  THE TRUE SHARKS.

  Order Notidani.--Family Hexanchidae.--Family
  Chlamydoselachidae.--Order Asterospondyli.--Suborder
  Cestraciontes.--Family Heterodontidae.--Edestus and its
  Allies.--Onchus.--Family Cochliodontidae.--Suborder Galei.--Family
  Scyliorhinidae.--The Lamnoid, or Mackerel-sharks.--Family
  Mitsukurinidae, the Goblin-sharks.--Family Alopiidae,
  or Thresher-sharks.--Family Pseudotriakidae.--Family
  Lamnidae.--Man-eating Sharks.--Family Cetorhinidae, or Basking
  Sharks.--Family Rhineodontidae.--The Carcharioid Sharks, or
  Requins.--Family Sphyrnidae, or Hammer-head Sharks.--The
  Order of Tectospondyli.--Suborder Cyclospondyli.--Family
  Squalidae.--Family Dalatiidae.--Family Echinorhinidae.--Suborder
  Rhinae.--Family Pristiophoridae, or Saw-sharks.--Suborder
  Batoidei, or Rays.--Pristididae, or Sawfishes.--Rhinobatidae,
  or Guitar-fishes.--Rajidae, or Skates.--Narcobatidae,
  or Torpedoes.--Petalodontidae.--Dasyatidae, or
  Sting-rays.--Myliobatidae.--Family Psammodontidae.--Family Mobulidae. 523

  CHAPTER XXXI.

  THE HOLOCEPHALI, OR CHIMAERAS.

  The Chimaeras.--Relationship of Chimaeras.--Family
  Chimaeridae.--Rhinochimaeridae.--Extinct
  Chimaeroids.--Ichthyodorulites.                                     561

  CHAPTER XXXII.

  THE CLASS OSTRACOPHORI.

  Ostracophores.--Nature of Ostracophores.--Orders of
  Ostracophores.--Order Heterostraci.--Order Osteostraci.--Order
  Antiarcha.--Order Anaspida.                                        568

  CHAPTER XXXIII.

  ARTHRODIRES.

  The Arthrodires.--Occurrence of
  Arthrodires.--Arthrognathi.--Anarthrodira.--Stegothalami.--
  Arthrodira.--Temnothoraci.--Arthrothoraci.--Relations of
  Arthrodires.--Suborder Cycliae.--Palaeospondylus.--Gill on
  Palaeospondylus.--Views as to the Relationships of Palaeospondylus:
  Huxley, Traquair, 1890. Traquair, 1893. Traquair, 1897. Smith
  Woodward, 1892. Dawson, 1893. Gill, 1896. Dean, 1896. Dean, 1898.
  Parker & Haswell, 1897. Gegenbaur, 1898.--Relationships of
  Palaeospondylus                                                     581

  CHAPTER XXXIV.

  THE CROSSOPTERYGII.

  Class Teleostomi.--Subclass Crossopterygii.--Order of
  Amphibians.--The Fins of Crossopterygians.--Orders of
  Crossopterygians.--Haplistia.--Rhipidistia.--Megalichthyidae.--Order
  Actinistia.--Order Cladistia.--The Polypteridae                     598

  CHAPTER XXXV.

  SUBCLASS DIPNEUSTI, OR LUNGFISHES.

  The Lungfishes.--Classification of Dipnoans.--Order
  Ctenodipterini.--Order Sirenoidei.--Family
  Ceratodontidae.--Development of Neoceratodus.--Lepidosirenidae.--Kerr
  on the Habits of Lepidosiren                                       609


FOOTNOTES:

[1] For most of this list of errata I am indebted to the kindly
interest of Dr. B. W. Evermann.




LIST OF ILLUSTRATIONS

VOL. I.


                                                                    PAGE

  _Lepomis megalotis_, Long-eared Sunfish                              2

  _Lepomis megalotis_, Long-eared Sunfish                              4

  _Eupomotis gibbosus_, Common Sunfish                                 7

  _Ozorthe dictyogramma_, a Japanese Blenny                            9

  _Eupomotis gibbosus_, Common Sunfish                                13

  _Monocentris japonicas_, Pine-cone Fish                             16

  _Diodon hystrix_, Porcupine-fish                                    17

  _Nemichthys avocetta_, Thread-eel                                   17

  _Hippocampus hudsonius_, Sea-horse                                  17

  _Peprilus paru_, Harvest-fish                                       18

  _Lophius litulon_, Anko or Fishing-frog                             18

  _Epinephelus adscensionis_, Rock-hind or Cabra Mora                 20

  Scales of _Acanthoessus bronni_                                     21

  Cycloid Scale                                                       22

  _Porichthys porosissimus_, Singing-fish                             23

  _Apomotis cyanellus_, Blue-green Sunfish                            27

  _Chiasmodon niger_, Black Swallower                                 29

  Jaws of a Parrot-fish, _Sparisoma aurofrenatum_                     30

  _Archosargus probatocephalus_, Sheepshead                           31

  _Campostoma anomalum_, Stone-roller                                 33

  _Roccus lineatus_, Striped Bass                                     35

  _Roccus lineatus._ Lateral View of Cranium                          36

  _Roccus lineatus._ Superior View of Cranium                         37

  _Roccus lineatus._ Inferior View of Cranium                         38

  _Roccus lineatus_. Posterior View of Cranium                        40

  _Roccus lineatus._ Face-bones, Shoulder and Pelvic Girdles, and
    Hyoid Arch                                                        42

  Lower Jaw of _Amia calva_, showing Gular Plate                      43

  _Roccus lineatus._ Branchial Arches                                 46

  Pharyngeal Bone and Teeth of European Chub, _Leuciscus cephalus_    47

  Upper Pharyngeals of Parrot-fish, _Scarus strongylocephalus_        47

  Lower Pharyngeal Teeth of Parrot-fish, _Scarus strongylocephalus_   47

  Pharyngeals of Italian Parrot-fish, _Sparisoma cretense_            48

  _Roccus lineatus_, Vertebral Column and Appendages                  48

  Basal Bone of Dorsal Fin, _Holoptychius leptopterus_                49

  Inner View of Shoulder-girdle of Buffalo-fish, _Ictiobus bubalus_
                                                                      51

  _Pterophryne tumida_, Sargassum-fish.                               52

  Shoulder-girdle of _Sebastolobus alascanus_.                        52

  Cranium of _Sebastolobus alascanus_.                                53

  Lower Jaw and Palate of _Sebastolobus alascanus_.                   54

  Maxillary and Premaxillary of _Sebastolobus alascanus_.             55

  Part of Skeleton of _Selene vomer_.                                 55

  Hyostylic Skull of _Chiloscyllium indicum_, a Scyliorhinoid Shark.  56

  Skull of _Heptranchias indicus_, a Notidanoid Shark.                56

  Basal Bones of Pectoral Fin of Monkfish, _Squatina_.                56

  Pectoral Fin of _Heterodontus philippi_.                            57

  Pectoral Fin of _Heptranchias indicus_.                             57

  Shoulder-girdle of a Flounder, _Paralichthys californicus_.         58

  Shoulder-girdle of a Toadfish, _Batrachoides pacifici_.             59

  Shoulder-girdle of a Garfish, _Tylosurus fodiator_.                 59

  Shoulder-girdle of a Hake, _Merluccius productus_.                  60

  _Cladoselache fyleri_, Restored.                                    65

  Fold-like Pectoral and Ventral Fins of _Cladoselache fyleri_.       65

  Pectoral Fin of a Shark, _Chiloscyllium_.                           66

  Skull and Shoulder-girdle of _Neoceratodus forsteri_, showing
    archipterygium.                                                   68

  _Acanthoessus wardi_.                                               69

  Shoulder-girdle of _Acanthoessus_.                                  69

  Pectoral Fin of _Pleuracanthus_.                                    69

  Shoulder-girdle of _Polypterus bichir_.                             70

  Arm of a Frog.                                                      71

  _Pleuracanthus decheni_.                                            74

  Embryos of _Heterodontus japonicas_, a Cestraciont Shark.           75

  _Polypterus congicus_, a Crossopterygian Fish with External Gills.  78

  Heterocercal Tail of Sturgeon, _Acipenser sturio_.                  80

  Heterocercal Tail of Bowfin, _Amia calva_.                          82

  Heterocercal Tail of Garpike, _Lepisosteus osseus_.                 82

  _Coryphaenoides carapinus_, showing Leptocercal Tail.                83

  Heterocercal Tail of Young Trout, _Salmo fario_.                    83

  Isocercal Tail of Hake, _Merluccius productus_.                     84

  Homocercal Tail of a Flounder, _Paralichthys californicus_.         84

  Gephyrocercal Tail of _Mola mola_.                                  85

  Shoulder-girdle of _Amia calva_.                                    86

  Shoulder-girdle of a Sea-catfish, _Selenaspis dowi_.                86

  Clavicles of a Sea-catfish, _Selenaspis dowi_.                      87

  Shoulder-girdle of a Batfish, _Ogcocephalus radiatus_.              88

  Shoulder-girdle of a Threadfin, _Polydactylus approximans_.         89

  Gill-basket of Lamprey.                                             92

  Weberian Apparatus and Air-bladder of Carp.                         93

  Brain of a Shark, _Squatina squatina_.                             110

  Brain of _Chimaera monstrosa_.                                      110

  Brain of _Polypterus annectens_.                                   110

  Brain of a Perch, _Perca flavescens_.                              111

  _Petromyzon marinus unicolor._ Head of Lake Lamprey, showing Pineal
    Body.                                                            111

  _Chologaster cornutus_, Dismal-swamp Fish.                         116

  _Typhlichthys subterraneus_, Blind Cavefish.                       116

  _Anableps dovii_, Four-eyed Fish.                                  117

  _Ipnops murrayi._                                                  118

  _Boleophthalmus chinensis_, Pond-skipper.                          118

  _Lampetra wilderi_, Brook Lamprey.                                 120

  _Branchiostoma lanceolatum_, European Lancelet.                    120

  _Pseudupeneus maculatus_, Goatfish.                                122

  _Xiphophorus helleri_, Sword-tail Minnow.                          124

  _Cymatogaster aggregatus_, White Surf-fish, Viviparous, with
    Young.                                                           125

  _Goodea luitpoldi_, a Viviparous Fish.                             126

  Egg of _Callorhynchus antarcticus_, the Bottle-nosed Chimaera.      127

  Egg of the Hagfish, _Myxine limosa_.                               127

  Egg of Port Jackson Shark, _Heterodontus philippi_.                128

  Development of Sea-bass, _Centropristes striatus_.                 135

  _Centropristes striatus_, Sea-bass.                                137

  _Xiphias gladius_, Young Sword-fish.                               139

  _Xiphias gladius_, Sword-fish.                                     139

  Larva of the Sail-fish, _Istiophorus_, Very Young.                 140

  Larva of Brook Lamprey, _Lampetra wilderi_, before Transformation.
                                                                     140

  _Anguilla chrisypa_, Common Eel.                                   140

  Larva of Common Eel, _Anguilla chrisypa_, called _Leptocephalus
    grassii_.                                                        141

  Larva of Sturgeon, _Acipenser sturio_.                             141

  Larva of _Chaetodon sedentarius_.                                   142

  _Chaetodon capistratus_, Butterfly-fish.                            142

  _Mola mola_, Very Early Larval Stage of Headfish, called _Centaurus
    booeps_.                                                          143

  _Mola mola_, Early Larval Stage called _Molacanthus nummularis_.   144

  _Mola mola_, Advanced Larval Stage.                                144

  _Mola mola_, Headfish, Adult.                                      146

  _Albula vulpes_, Transformation of Ladyfish from Larva to Young.   147

  Development of the Horsehead-fish, _Selene vomer_.                 148

  _Salanx hyalocranius_, Ice-fish.                                   149

  _Dallia pectoralis_, Alaska Blackfish.                             149

  _Ophiocephalus barca_, Snake-headed China-fish.                    150

  _Carassius auratus_, Monstrous Goldfish.                           151

  Jaws of _Nemichthys avocetta_.                                     156

  _Cypsilurus californicus_, Flying-fish.                            157

  _Ammocrypta clara_, Sand-darter.                                   158

  _Fierasfer acus_, Pearl-fish, issuing from a Holothurian.          159

  _Gobiomorus gronovii_, Portuguese Man-of-war Fish.                 160

  Tide Pools of Misaki.                                              161

  _Ptychocheilus oregonensis_, Squaw-fish.                           162

  _Ptychocheilus grandis_, Squaw-fish, Stranded as the Water Falls.  164

  Larval Stages of _Platophrys podas_, a Flounder of the
    Mediterranean, showing Migration of Eye.                         174

  _Platophrys lunatus_, the Wide-eyed Flounder.                      175

  Young Flounder Just Hatched, with Symmetrical Eyes.                175

  _Pseudopleuronectes americanus_, Larval Flounder.                  176

  _Pseudopleuronectes americanus_, Larval Flounder (more advanced
    stage).                                                          176

  Face View of Recently-hatched Flounder.                            177

  _Schilbeodes furiosus_, Mad-Tom.                                   179

  _Emmydrichthys vulcanus_, Black Nohu or Poison-fish.               180

  _Teuthis bahianus_, Brown Tang.                                    181

  _Stephanolepis hispidus_, Common Filefish.                         182

  _Tetraodon meleagris._                                             183

  _Balistes carolinensis_, the Trigger-fish.                         184

  _Narcine brasiliensis_, Numbfish.                                  185

  _Torpedo electricus_, Electric Catfish.                            186

  _Astroscopus guttatus_, Star-gazer.                                187

  _AEthoprora lucida_, Headlight-fish.                                188

  _Corynolophus reinhardti_, showing Luminous Bulb.                  188

  _Etmopterus lucifer._                                              189

  _Argyropelecus olfersi._                                           190

  Luminous Organs and Lateral Line of Midshipman, _Porichthys
    notatus_.                                                        192

  Cross-section of Ventral Phosphorescent Organ of Midshipman,
    _Porichthys notatus_.                                            193

  Section of Deeper Portion of Phosphorescent Organ, _Porichthys
    notatus_.                                                        194

  _Leptecheneis naucrates_, Sucking-fish or Pegador.                 197

  _Caularchus maeandricus_, Clingfish.                                198

  _Polistotrema stouti_, Hagfish.                                    199

  _Pristis zysron_, Indian Sawfish.                                  200

  _Pristiophorus japonicus_, Saw-shark.                              201

  Skeleton of Pike, _Esox lucius_.                                   203

  Skeleton of Red Rockfish, _Sebastodes miniatus_.                   214

  Skeleton of a Spiny-rayed Fish of the Tropics, _Holacanthus
    ciliaris_.                                                       214

  Skeleton of the Cowfish, _Lactophrys tricornis_.                   215

  _Crystallias matsushimae_, Liparid.                                 218

  _Sebastichthys maliger_, Yellow-backed Rockfish.                   218

  _Myoxocephalus scorpius_, European Sculpin.                        219

  _Hemitripterus americanus_, Sea-raven.                             220

  _Cyclopterus lumpus_, Lumpfish.                                    220

  _Psychrolutes paradoxus_, Sleek Sculpin.                           221

  _Pallasina barbata_, Agonoid-fish.                                 221

  _Amblyopsis spelaeus_, Blindfish of the Mammoth Cave.               221

  _Lucifuga subterranea_, Blind Brotula.                             222

  _Hypsypops rubicunda_, Garibaldi.                                  227

  _Synanceia verrucosa_, Gofu or Poison-fish.                        229

  _Alticus saliens_, Lizard-skipper.                                 230

  _Etheostoma camurum_, Blue-breasted Darter.                        231

  _Liuranus semicinctus_ and _Chlevastes colubrinus_, Snake-eels.    233

  Coral Reef at Apia.                                                234

  _Rudarius ercodes_, Japanese Filefish.                             241

  _Tetraodon setosus_, Globefish.                                    244

  _Dasyatis sabina_, Sting-ray.                                      246

  _Diplesion blennioides_, Green-sided Darter.                       247

  _Hippocampus mohnikei_, Japanese Sea-horse.                        250

  _Archoplites interruptus_, Sacramento Perch.                       258

  Map of the Continents, Eocene Time.                                270

  _Caulophryne jordani_, Deep-sea Fish of Gulf Stream.               276

  _Exerpes asper_, Fish of Rock-pools, Mexico.                       276

  _Xenocys jessiae._                                                  279

  _Ictalurus punctatus_, Channel Catfish.                            280

  Drawing the Net on the Beach of Hilo, Hawaii.                      281

  _Semotilus atromaculatus_, Horned Dace.                            285

  _Leuciscus lineatus_, Chub of the Great Basin.                     287

  _Melletes papilio_, Butterfly Sculpin.                             288

  _Scartichthys enosimae_, a Fish of the Rock-pools of the Sacred
    Island of Enoshima, Japan.                                       294

  _Halichoeres bivittatus_, the Slippery Dick.                       297

  _Peristedion miniatum._                                            299

  Outlet of Lake Bonneville.                                         303

  _Hypocritichthys analis_, Silver Surf-fish.                        309

  _Erimyzon sucetta_, Creekfish or Chub-sucker.                      315

  _Thaleichthys pretiosus_, Eulachon or Ulchen.                      320

  _Plecoglossus altivelis_, the Japanese Ayu.                        321

  _Coregonus clupeiformis_, the Whitefish.                           321

  _Mullus auratus_, the Golden Surmullet.                            322

  _Scomberomorus maculatus_, the Spanish Mackerel.                   322

  _Lampris luna_, the Opah or Moonfish.                              323

  _Pomatomus saltatrix_, the Bluefish.                               324

  _Centropomus undecimalis_, the Robalo.                             324

  _Chaetodipterus faber_, the Spadefish.                              325

  _Micropterus dolomieu_, the Small-mouthed Black Bass.              325

  _Salvelinus fontinalis_, the Speckled Trout.                       326

  _Salmo irideus_, the Rainbow Trout.                                326

  _Salvelinus oquassa_, the Rangeley Trout.                          326

  _Salmo gairdneri_, the Steelhead Trout.                            327

  _Salmo henshawi_, the Tahoe Trout.                                 327

  _Salvelinus malma_, the Dolly Varden Trout.                        327

  _Thymallus signifer_, the Alaska Grayling.                         328

  _Esox lucius_, the Pike.                                           328

  _Pleurogrammus monopterygius_, the Atka-fish.                      328

  _Chirostoma humboldtianum_, the Pescado blanco.                    329

  _Pseudupeneus maculatus_, the Red Goatfish.                        329

  _Pseudoscarus guacamaia_, Great Parrot-fish.                       330

  _Mugil cephalus_, Striped Mullet.                                  330

  _Lutianus analis_, Mutton-snapper.                                 331

  _Clupea harengus_, Herring.                                        331

  _Gadus callarias_, Codfish.                                        331

  _Scomber scombrus_, Mackerel.                                      332

  _Hippoglossus hippoglossus_, Halibut.                              332

  Fishing for Ayu with Cormorants.                                   333

  Fishing for Ayu. Emptying Pouch of Cormorant.                      335

  Fishing for Tai, Tokyo Bay.                                        338

  _Brevoortia tyrannus_, Menhaden.                                   340

  _Exonautes unicolor_, Australian Flying-fish.                      341

  _Rhinichthys atronasus_, Black-nosed Dace.                         342

  _Notropis hudsonius_, White Shiner.                                343

  _Ameiurus catus_, White Catfish.                                   344

  _Catostomus ardens_, Sucker.                                       348

  _Oncorhynchus tschawytscha_, Quinnat Salmon.                       354

  _Oncorhynchus tschawytscha_, Young Male.                           355

  _Ameiurus nebulosus_, Catfishes.                                   358

  "Le Monstre Marin en Habit de Moine".                              360

  "Le Monstre Marin en Habit d'Eveque".                              361

  _Regalecus russelli_, Oarfish.                                     362

  _Regalecus glesne_, Glesnaes Oarfish.                               363

  _Nemichthys avocetta_, Thread-eel.                                 365

  _Lactophrys tricornis_, Horned Trunkfish.                          373

  _Ostracion cornutum_, Horned Trunkfish.                            376

  _Lactophrys bicaudalis_, Spotted Trunkfish.                        377

  _Lactophrys bicaudalis_, Spotted Trunkfish (Face).                 377

  _Lactophrys triqueter_, Spineless Trunkfish.                       378

  _Lactophrys trigonus_, Hornless Trunkfish.                         378

  _Lactophrys trigonus_, Hornless Trunkfish (Face).                  379

  Bernard Germain de Lacepede.                                       399

  Georges Dagobert Cuvier.                                           399

  Louis Agassiz.                                                     399

  Johannes Mueller.                                                   399

  Albert Guenther.                                                    403

  Franz Steindachner.                                                403

  George Albert Boulenger.                                           403

  Robert Collett.                                                    403

  Spencer Fullerton Baird.                                           407

  Edward Drinker Cope.                                               407

  Theodore Nicholas Gill.                                            407

  George Brown Goode.                                                407

  Johann Reinhardt.                                                  409

  Edward Waller Claypole.                                            409

  Carlos Berg.                                                       409

  Edgar R. Waite.                                                    409

  Felipe Poey y Aloy.                                                413

  Leon Vaillant.                                                     413

  Louis Dollo.                                                       413

  Decio Vinciguerra.                                                 413

  Bashford Dean.                                                     417

  Kakichi Mitsukuri.                                                 417

  Carl H. Eigenmann.                                                 417

  Franz Hilgendorf.                                                  417

  David Starr Jordan.                                                421

  Herbert Edson Copeland.                                            421

  Charles Henry Gilbert.                                             421

  Barton Warren Evermann.                                            421

  Ramsay Heatley Traquair.                                           425

  Arthur Smith Woodward.                                             425

  Karl A. Zittel.                                                    425

  Charles R. Eastman.                                                425

  Fragment of Sandstone from Ordovician Deposits.                    435

  Fossil Fish Remains from Ordovician Rocks.                         436

  _Dipterus valenciennesi._                                          437

  _Hoplopteryx lewesiensis._                                         438

  _Paratrachichthys prosthemius_, Berycoid fish.                     439

  _Cypsilurus heterurus_, Flying-fish.                               440

  _Lutianidae_, Schoolmaster Snapper.                                 440

  _Pleuronichthys decurrens_, Decurrent Flounder.                    441

  _Cephalaspis lyelli_, Ostracophore.                                444

  _Dinichthys intermedius_, Arthrodire.                              445

  _Lamna cornubica_, Mackerel-shark or Salmon-shark.                 447

  _Raja stellulata_, Star-spined Ray.                                448

  _Harriotta raleighiana_, Deep-sea Chimaera.                         449

  _Dipterus valenciennesi_, Extinct Dipnoan.                         449

  _Holoptychius giganteus_, Extinct Crossopterygian.                 451

  _Platysomus gibbosus_, Ancient Ganoid fish.                        452

  _Lepisosteus platystomus_, Short-nosed Gar.                        452

  _Palaeoniscum macropomum_, Primitive Ganoid fish.                   453

  _Diplomystus humilis_, Fossil Herring.                             453

  _Holcolepis lewesiensis_.                                          454

  _Elops saurus_, Ten-pounder.                                       454

  _Apogon semilineatus_, Cardinal-fish.                              455

  _Pomolobus aestivalis_, Summer Herring.                             455

  _Bassozetus catena._                                               456

  _Trachicephalus uranoscopus._                                      456

  _Chlarias breviceps_, African Catfish.                             457

  _Notropis whipplii_, Silverfin.                                    457

  _Gymnothorax moringa._                                             458

  _Seriola lalandi_, Amber-fish.                                     458

  Geological Distribution of the Families of Elasmobranchs.          459

  "Tornaria" Larva of _Glossobalanus minutus_.                       463

  _Glossobalanus minutus._                                           464

  _Harrimania maculosa._                                             465

  Development of Larval Tunicate to Fixed Condition.                 471

  Anatomy of Tunicate.                                               472

  _Ascidia adhaerens._                                                474

  _Styela yacutatensis._                                             475

  _Styela greeleyi._                                                 476

  _Cynthia superba._                                                 476

  _Botryllus magnus_, Compound Ascidian.                             477

  _Botryllus magnus._                                                478

  _Botryllus magnus_, a Single Zooid.                                479

  _Aplidiopsis jordani_, a Compound Ascidian.                        479

  _Oikopleura_, Adult Tunicate of Group Larvacea.                    480

  _Branchiostoma californiense_, California Lancelet.                484

  Gill-basket of Lamprey.                                            485

  _Polygnathus dubium._                                              488

  _Polistotrema stouti_, Hagfish.                                    489

  _Petromyzon marinus_, Lamprey.                                     491

  _Petromyzon marinus unicolor_, Mouth Lake Lamprey.                 492

  _Lampetra wilderi_, Sea Larvae Brook Lamprey.                       492

  _Lampetra wilderi_, Mouth Brook Lamprey.                           492

  _Lampetra camtschatica_, Kamchatka Lamprey.                        495

  _Entosphenus tridentatus_, Oregon Lamprey.                         496

  _Lampetra wilderi_, Brook Lamprey.                                 505

  Fin-spine of _Onchus tenuistriatus_.                               509

  Section of Vertebrae of Sharks, showing Calcification.              510

  _Cladoselache fyleri._                                             514

  _Cladoselache fyleri_, Ventral View.                               515

  Teeth of _Cladoselache fyleri_.                                    515

  _Acanthoessus wardi._                                              515

  _Diplacanthus crassissimus._                                       517

  _Climatius scutiger._                                              518

  _Pleuracanthus decheni._                                           519

  _Pleuracanthus decheni_, Restored.                                 520

  Head-bones and Teeth of _Pleuracanthus decheni_.                   520

  Teeth of _Didymodus bohemicus_.                                    520

  Shoulder-girdle and Pectoral Fins of _Cladodus neilsoni_.          521

  Teeth of _Cladodus striatus_.                                      522

  _Hexanchus griseus_, Griset or Cow-shark.                          523

  Teeth of _Heptranchias indicus_.                                   524

  _Chlamydoselachus anguineus_, Frill-shark.                         525

  _Heterodontus francisci_, Bullhead-shark.                          526

  Lower Jaw of _Heterodontus philippi_.                              526

  Teeth of Cestraciont Sharks.                                       527

  Egg of Port Jackson Shark, _Heterodontus philippi_.                527

  Tooth of _Hybodus delabechei_.                                     528

  Fin-spine of _Hybodus basanus_.                                    528

  Fin-spine of _Hybodus reticulatus_.                                528

  Fin-spine of _Hybodus canaliculatus_.                              529

  Teeth of Cestraciont Sharks.                                       529

  _Edestus vorax_, Supposed to be a Whorl of Teeth.                  529

  _Helicoprion bessonowi_, Teeth of.                                 530

  Lower Jaw of _Cochliodus contortus_.                               531

  _Mitsukurina owstoni_, Goblin-shark.                               535

  _Scapanorhynchus lewisi_, Under Side of Snout.                     536

  Tooth of _Lamna cuspidata_.                                        537

  _Isuropsis dekayi_, Mackerel-shark.                                537

  Tooth of _Isurus hastalis_.                                        538

  _Carcharodon mega odon._                                           539

  _Cetorhinus maximus_, Basking-shark.                               540

  _Galeus zyopterus_, Soup-fin Shark.                                541

  _Carcharias lamia_, Cub-shark.                                     542

  Teeth of _Corax pristodontus_.                                     543

  _Sphyrna zygaena_, Hammer-head Shark.                               544

  _Squalus acanthias_, Dogfish.                                      545

  _Etmopterus lucifer._                                              546

  Brain of Monkfish, _Squatina squatina_.                            547

  _Pristiophorus japonicus_, Saw-shark.                              548

  _Pristis pectinatus_, Sawfish.                                     550

  _Rhinobatus lentiginosus_, Guitar-fish.                            551

  _Raja erinacea_, Common Skate.                                     552

  _Narcine brasiliensis_, Numbfish.                                  553

  Teeth of _Janassa linguaeformis_.                                   554

  _Polyrhizodus radicans._                                           555

  _Dasyatis sabina_, Sting-ray.                                      556

  _Aetobatis narinari_, Eagle-ray.                                   558

  _Manta birostris_, Devil-ray or Sea-devil.                         559

  Skeleton of _Chimaera monstrosa_.                                   564

  _Chimaera colliei_, Elephant-fish.                                  565

  _Odontotodus schrencki_, Ventral Side.                             570

  _Odontotodus schrencki_, Dorsal Side.                              570

  Head of _Odontotodus schrencki_, from the Side.                    571

  _Limulus polyphemus_, Horseshoe Crab.                              572

  _Lanarkia spinosa._                                                574

  _Drepanaspis gmundenensis._                                        575

  _Pteraspis rostrata._                                              575

  _Cephalaspis lyelli_, Restored.                                    576

  _Cephalaspis dawsoni._                                             577

  _Pterichthyodes testudinarius._                                    578

  _Pterichthyodes testudinarius_, Side View.                         579

  _Birkenia elegans._                                                579

  _Lasianius problematicus._                                         580

  _Coccosteus cuspidatus_, Restored.                                 582

  Jaws of _Dinichthys hertzeri_.                                     583

  _Dinichthys intermedius_, an Arthrodire.                           584

  _Palaeospondylus gunni._                                            591

  Shoulder-girdle of _Polypterus bichir_.                            600

  Arm of a Frog.                                                     601

  _Polypterus congicus_, a Crossopterygian Fish.                     602

  Basal Bone of Dorsal Fin, _Holoptychius leptopterus_.              603

  _Gyroptychius microlepidotus._                                     604

  _Coelacanthus elegans_, showing Air-bladder.                       604

  _Undina gulo._                                                     605

  Lower Jaw of _Polypterus bichir_, from Below.                      606

  _Polypterus congicus._                                             607

  _Polypterus delhezi._                                              607

  _Erpetoichthys calabaricus._                                       608

  Shoulder-girdle of _Neoceratodus forsteri_.                        609

  _Phaneropleuron andersoni._                                        613

  Teeth of _Ceratodus runcinatus_.                                   614

  _Neoceratodus forsteri._                                           614

  Archipterygium of _Neoceratodus forsteri_.                         614

  Upper jaw of _Neoceratodus forsteri_.                              615

  Lower Jaw of _Neoceratodus forsteri_.                              616

  Adult Male of _Lepidosiren paradoxa_.                              619

  _Lepidosiren paradoxa._ Embryo Three Days before Hatching; Larva
    Thirteen Days after Hatching.                                    620

  Larva of _Lepidosiren paradoxa_ Forty Days after Hatching.         621

  Larva of _Lepidosiren paradoxa_ Thirty Days after Hatching.        621

  Larva of _Lepidosiren paradoxa_ Three Months after Hatching.       621

  _Protopterus dolloi._                                              622

[Illustration: FIG. 1.--Long-eared Sunfish, _Lepomis megalotis_
(Rafinesque). (From life by R. W. Shufeldt.)--Page 2.]




CHAPTER I

THE LIFE OF THE FISH

            A POPULAR ACCOUNT OF THE LIFE OF THE LONG-EARED
                     SUNFISH, _LEPOMIS MEGALOTIS_


=What is a Fish?=--A fish is a back-boned animal which lives in the
water and cannot ever live very long anywhere else. Its ancestors have
always dwelt in water, and most likely its descendents will forever
follow their example. So, as the water is a region very different from
the fields or the woods, a fish in form and structure must be quite
unlike all the beasts and birds that walk or creep or fly above ground,
breathing air and being fitted to live in it. There are a great many
kinds of animals called fishes, but in this all of them agree: all have
some sort of a back-bone, all of them breathe their life long by means
of gills, and none have fingers or toes with which to creep about on
land.

=The Long-eared Sunfish.=--If we would understand a fish, we must first
go and catch one. This is not very hard to do, for there are plenty
of them in the little rushing brook or among the lilies of the pond.
Let us take a small hook, put on it an angleworm or a grasshopper,--no
need to seek an elaborate artificial fly,--and we will go out to the
old "swimming-hole" or the deep eddy at the root of the old stump where
the stream has gnawed away the bank in changing its course. Here we
will find fishes, and one of them will take the bait very soon. In one
part of the country the first fish that bites will be different from
the first one taken in some other. But as we are fishing in the United
States, we will locate our brook in the centre of population of our
country. This will be to the northwest of Cincinnati, among the low
wooded hills from which clear brooks flow over gravelly bottoms toward
the Ohio River. Here we will catch sunfishes of certain species, or
maybe rock bass or catfish: any of these will do for our purpose. But
one of our sunfishes is especially beautiful--mottled blue and golden
and scarlet, with a long, black, ear-like appendage backward from his
gill-covers--and this one we will keep and hold for our first lesson
in fishes. It is a small fish, not longer than your hand most likely,
but it can take the bait as savagely as the best, swimming away with it
with such force that you might think from the vigor of its pull that
you have a pickerel or a bass. But when it comes out of the water you
see a little, flapping, unhappy, living plate of brown and blue and
orange, with fins wide-spread and eyes red with rage.

[Illustration: FIG. 2.--Long-eared Sunfish, _Lepomis megalotis_
(Rafinesque). (From Clear Creek, Bloomington, Indiana.) Family
_Centrarchidae_.]

=Form of the Fish.=--And now we have put the fish into a bucket of
water, where it lies close to the bottom. Then we take it home and
place it in an aquarium, and for the first time we have a chance to see
what it is like. We see that its body is almost elliptical in outline,
but with flat sides and shaped on the lower parts very much like a
boat. This form we see is such as to enable it to part the water as it
swims. We notice that its progress comes through the sculling motion of
its broad, flat tail.

=Face of a Fish.=--When we look at the sunfish from the front we see
that it has a sort of face, not unlike that of higher animals. The big
eyes, one on each side, stand out without eyelids, but the fish can
move them at will, so that once in a while he seems to wink. There
isn't much of a nose between the eyes, but the mouth is very evident,
and the fish opens and shuts it as it breathes. We soon see that it
breathes water, taking it in through the mouth and letting it flow over
the gills, and then out through the opening behind the gill-covers.

=How the Fish Breathes.=--If we take another fish--for we shall
not kill this one--we shall see that in its throat, behind the
mouth-cavity, there are four rib-like bones on each side, above the
beginning of the gullet. These are the gill-arches, and on each one
of them there is a pair of rows of red fringes called the gills. Into
each of these fringes runs a blood-vessel. As the water passes over it
the oxygen it contains is absorbed through the skin of the gill-fringe
into the blood, which thus becomes purified. In the same manner the
impurities of the blood pass out into the water, and go out through
the gill-openings behind. The fish needs to breathe just as we do,
though the apparatus of breathing is not the same. Just as the air
becomes loaded with impurities when many people breathe it, so does
the water in our jar or aquarium become foul if it is breathed over
and over again by fishes. When a fish finds the water bad he comes
to the surface to gulp air, but his gills are not well fitted to use
undissolved air as a substitute for that contained in water. The rush
of a stream through the air purifies the water, and so again does the
growth of water plants, for these in the sunshine absorb and break up
carbonic acid gas, and throw out oxygen into the water.

=Teeth of the Fish.=--On the inner side of the gill-arch we find some
little projections which serve as strainers to the water. These are
called gill-rakers. In our sunfish they are short and thick, seeming
not to amount to much but in a herring they are very long and numerous.

Behind the gills, at the opening of the gullet, are some roundish bones
armed with short, thick teeth. These are called pharyngeals. They form
a sort of jaws in the throat, and they are useful in helping the little
fish to crack shells. If we look at the mouth of our live fish, we
shall find that when it breathes or bites it moves the lower jaw very
much as a dog does. But it can move the upper jaw, too, a little, and
that by pushing it out in a queer fashion, as though it were thrust out
of a sheath and then drawn in. If we look at our dead fish, we shall
see that the upper jaw divides in the middle and has two bones on each
side. On one bone are rows of little teeth, while the other bone that
lies behind it has no teeth at all. The lower jaw has little teeth like
those of the upper jaw, and there is a patch of teeth on the roof of
the mouth also. In some sunfishes there are three little patches, the
vomer in the middle and the palatines on either side.

The tongue of the fish is flat and gristly. It cannot move it, scarce
even taste its food with it, nor can it use it for making a noise. The
unruly member of a fish is not its tongue, but its tail.

=How the Fish Sees.=--To come back to the fish's eye again. We say
that it has no eyelids, and so, if it ever goes to sleep, it must keep
its eyes wide open. The iris is brown or red. The pupil is round, and
if we could cut open the eye we should see that the crystalline lens
is almost a perfect sphere, much more convex than the lens in land
animals. We shall learn that this is necessary for the fish to see
under water. It takes a very convex lens or even one perfectly round
to form images from rays of light passing through the water, because
the lens is but little more dense than the water itself. This makes
the fish near-sighted. He cannot see clearly anything out of water or
at a distance. Thus he has learned that when, in water or out, he sees
anything moving quickly it is probably something dangerous, and the
thing for him to do is to swim away and hide as swiftly as possible.

In front of the eye are the nostrils, on each side a pair of openings.
But they lead not into tubes, but into a little cup lined with delicate
pink tissues and the branching nerves of smell. The organ of smell
in nearly all fishes is a closed sac, and the fish does not use the
nostrils at all in breathing. But they can indicate the presence of
anything in the water which is good to eat, and eating is about the
only thing a fish cares for.

=Color of the Fish.=--Behind the eye there are several bones on the
side of the head which are more or less distinct from the skull itself.
These are called membrane bones because they are formed of membrane
which has become bony by the deposition in it of salts of lime. One
of these is called the opercle, or gill-cover, and before it, forming
a right angle, is the preopercle, or false gill-cover. On our sunfish
we see that the opercle ends behind in a long and narrow flap, which
looks like an ear. This is black in color, with an edging of scarlet
as though a drop of blood had spread along its margin. When the fish
is in the water its back is dark greenish-looking, like the weeds and
the sticks in the bottom, so that we cannot see it very plainly. This
is the way the fish looks to the fishhawks or herons in the air above
it who may come to the stream to look for fish. Those fishes which
from above look most like the bottom can most readily hide and save
themselves. The under side of the sunfish is paler, and most fishes
have the belly white. Fishes with white bellies swim high in the water,
and the fishes who would catch them lie below. To the fish in the water
all outside the water looks white, and so the white-bellied fishes are
hard for other fishes to see, just as it is hard for us to see a white
rabbit bounding over the snow.

[Illustration: FIG. 3.--Common sunfish, _Eupomotis gibbosus_ (Linnaeus).
Natural size. (From life by R. W. Shufeldt.)]

But to be known of his own kind is good for the sunfish, and we may
imagine that the black ear-flap with its scarlet edge helps his mate
and friends to find him out, where they swim on his own level near
the bottom. Such marks are called recognition-marks, and a great many
fishes have them, but we have no certain knowledge as to their actual
purpose.

We are sure that the ear-flap is not an ear, however. No fishes have
any external ear, all their hearing apparatus being buried in the
skull. They cannot hear very much: possibly a great jar or splash
in the water may reach them, but whenever they hear any noise they
swim off to a hiding-place, for any disturbance whatever in the water
must arouse a fish's anxiety. The color of the live sunfish is very
brilliant. Its body is covered with scales, hard and firm, making
a close coat of mail, overlapping one another like shingles on a
roof. Over these is a thin skin in which are set little globules of
bright- matter, green, brown, and black, with dashes of scarlet,
blue, and white as well. These give the fish its varied colors. Some
coloring matter is under the scales also, and this especially makes
the back darker than the lower parts. The bright colors of the sunfish
change with its surroundings or with its feelings. When it lies in wait
under a dark log its colors are very dark. When it rests above the
white sands it is very pale. When it is guarding its nest from some
meddling perch its red shades flash out as it stands with fins spread,
as though a water knight with lance at rest, looking its fiercest at
the intruder.

When the sunfish is taken out of the water its colors seem to fade.
In the aquarium it is generally paler, but it will sometimes brighten
up when another of its own species is placed beside it. A cause of
this may lie in the nervous control of the muscles at the base of the
scales. When the scales lie very flat the color has one appearance.
When they rise a little the shade of color seems to change. If you let
fall some ink-drops between two panes of glass, then spread them apart
or press them together, you will see changes in the color and size
of the spots. Of this nature is the apparent change in the colors of
fishes under different conditions. Where the fish feels at its best
the colors are the richest. There are some fishes, too, in which the
male grows very brilliant in the breeding season through the deposition
of red, white, black, or blue pigments, or coloring matter, on its
scales or on its head or fins, this pigment being absorbed when the
mating season is over. This is not true of the sunfish, who remains
just about the same at all seasons. The male and female are 
alike and are not to be distinguished without dissection. If we examine
the scales, we shall find that these are marked with fine lines and
concentric striae, and part of the apparent color is due to the effect
of the fine lines on the light. This gives the bluish lustre or sheen
which we can see in certain lights, although we shall find no real blue
pigment under it. The inner edge of each scale is usually scalloped
or crinkled, and the outer margin of most of them has little prickly
points which make the fish seem rough when we pass our hand along his
sides.

[Illustration: FIG. 4.--_Ozorthe dictyogramma_ (Herzenstein). A
Japanese blenny, from Hakodate: showing increased number of lateral
lines, a trait characteristic of many fishes of the north Pacific.]

=The Lateral Line.=--Along the side of the fish is a line of peculiar
scales which runs from the head to the tail. This is called the lateral
line. If we examine it carefully, we shall see that each scale has a
tube from which exudes a watery or mucous fluid. Behind these tubes are
nerves, and although not much is known of the function of the tubes,
we can be sure that in some degree the lateral line is a sense-organ,
perhaps aiding the fish to feel sound-waves or other disturbances in
the water.

=The Fins of the Fish.=--The fish moves itself and directs its
course in the water by means of its fins. These are made up of stiff
or flexible rods growing out from the body and joined together by
membrane. There are two kinds of these rays or rods in the fins. One
sort is without joints or branches, tapering to a sharp point. The rays
thus fashioned are called spines, and they are in the sunfish stiff
and sharp-pointed. The others, known as soft rays, are made up of
many little joints, and most of them branch and spread out brush-like
at their tips. In the fin on the back the first ten of the rays are
spines, the rest are soft rays. In the fin under the tail there are
three spines, and in each fin at the breast there is one spine with
five soft rays. In the other fins all the rays are soft.

The fin on the back is called the dorsal fin, the fin at the end of
the tail is the caudal fin, the fin just in front of this on the
lower side is the anal fin. The fins, one on each side, just behind
the gill-openings are called the pectoral fins. These correspond to
the arms of man, the wings of birds, or the fore legs of a turtle or
lizard. Below these, corresponding to the hind legs, is the pair of
fins known as the ventral fins. If we examine the bones behind the
gill-openings to which the pectoral fins are attached, we shall find
that they correspond after a fashion to the shoulder-girdle of higher
animals. But the shoulder-bone in the sunfish is joined to the back
part of the skull, so that the fish has not any neck at all. In animals
with necks the bones at the shoulder are placed at some distance behind
the skull.

If we examine the legs of a fish, the ventral fins, we shall find
that, as in man, these are fastened to a bone inside called the
pelvis. But the pelvis in the sunfish is small and it is placed far
forward, so that it is joined to the tip of the "collar-bone" of the
shoulder-girdle and pelvis attached together. The caudal fin gives most
of the motion of a fish. The other fins are mostly used in maintaining
equilibrium and direction. The pectoral fins are almost constantly in
motion, and they may sometimes help in breathing by starting currents
outside which draw water over the gills.

=The Skeleton of the Fish.=--The skeleton of the fish, like that of
man, is made up of the skull, the back-bone, the limbs, and their
appendages. But in the fish the bones are relatively smaller, more
numerous, and not so firm. The front end of the vertebral column is
modified as a skull to contain the little brain which serves for all
a fish's activities. To the skull are attached the jaws, the membrane
bones, and the shoulder-girdle. The back-bone itself in the sunfish
is made of about twenty-four pieces, or vertebrae. Each of these has a
rounded central part, concave in front and behind. Above this is a
channel through which the great spinal cord passes, and above and below
are a certain number of processes or projecting points. To some of
these, through the medium of another set of sharp bones, the fins of
the back are attached. Along the sides of the body are the slender ribs.

=The Fish in Action.=--The fish is, like any other animal, a machine
to convert food into power. It devours other animals or plants,
assimilates their substance, takes it over into itself, and through its
movements uses up this substance again. The food of the sunfish is made
up of worms, insects, and little fishes. To seize these it uses its
mouth and teeth. To digest them it needs its alimentary canal, made of
the stomach with its glands and intestines. If we cut the fish open, we
shall find the stomach with its pyloric caeca, near it the large liver
with its gall-bladder, and on the other side the smaller spleen. After
the food is dissolved in the stomach and intestines the nutritious part
is taken up by the walls of the alimentary canal, whence it passes into
the blood.

The blood is made pure in the gills, as we have already seen. To send
it to the gills the fish has need of a little pumping-engine, and
this we shall find at work in the fish as in all higher animals. This
engine of stout muscle surrounding a cavity is called the heart. In
most fishes it is close behind the gills. It contains one auricle and
one ventricle only, not two of each as in man. The auricle receives
the impure blood from all parts of the body. It passes it on to the
ventricle, which, being thick-walled, is dark red in color. This passes
the blood by convulsive action, or heart-beating, on to the gills. From
these the blood is collected in arteries, and without again returning
to the heart it flows all through the body. The blood in the fish flows
sluggishly. The combustion of waste material goes on slowly, and so the
blood is not made hot as it is in the higher beasts and birds. Fishes
have relatively little blood; what there is is rather pale and cold and
has no swift current.

If we look about in the inside of a fish, we shall find close along the
lower side of the back-bone, covering the great artery, the dark red
kidneys. These strain out from the blood a certain class of impurities,
poisons made from nerve or muscle waste which cannot be burned away by
the oxygen of respiration.

=The Air-bladder.=--In the front part of the sunfish, just above
the stomach, is a closed sac, filled with air. This is called the
air-bladder, or swim-bladder. It helps the fish to maintain its place
in the water. In bottom fishes it is almost always small, while fishes
that rise and fall in the current generally have a large swim-bladder.
The gas inside it is secreted from the blood, for the sunfish has no
way of getting any air into it from the outside.

But the primal purpose of the air-bladder was not to serve as a float.
In very old-fashioned fishes it has a tube connecting it with the
throat, and instead of being an empty sac it is a true lung made up of
many lobes and parts and lined with little blood-vessels. Such fishes
as the garpike and the bowfin have lung-like air-bladders and gulp air
from the surface of the water.

In the very little sunfish, when he is just hatched, the air-bladder
has an air-duct, which, however, is soon lost, leaving only a closed
sac. From all this we know that the air-bladder is the remains of
what was once a lung, or additional arrangement for breathing. As the
gills furnish oxygen enough, the lung of the common fish has fallen
into disuse and thrifty Nature has used the parts and the space for
another and a very different purpose. This will serve to help us to
understand the swim-bladder and the way the fish came to acquire it as
a substitute for a lung.

=The Brain of the Fish.=--The movements of the fish, like those of
every other complex animal, are directed by a central nervous system,
of which the principal part is in the head and is known as the brain.
From the eye of the fish a large nerve goes to the brain to report
what is in sight. Other nerves go from the nostrils, the ears, the
skin, and every part which has any sort of capacity for feeling. These
nerves carry their messages inward, and when they reach the brain they
may be transformed into movement. The brain sends back messages to the
muscles, directing them to contract. Their contraction moves the fins,
and the fish is shoved along through the water. To scare the fish or to
attract it to its food or to its mate is about the whole range of the
effect that sight or touch has on the animal. These sensations changed
into movement constitute what is called reflex action, performance
without thinking of what is being done. With a boy, many familiar
actions may be equally reflex. The boy can also do many other things
"of his own accord," that is, by conscious effort. He can choose among
a great many possible actions. But a fish cannot. If he is scared, he
must swim away, and he has no way to stop himself. If he is hungry,
and most fishes are so all the time, he will spring at the bait. If
he is thirsty, he will gasp, and there is nothing else for him to do.
In other words, the activities of a fish are nearly all reflex, most
of them being suggested and immediately directed by the influence of
external things. Because its actions are all reflex the brain is very
small, very primitive, and very simple, nothing more being needed for
automatic movement. Small as the fish's skull-cavity is, the brain does
not half fill it.

[Illustration: FIG. 5.--Common Sunfish, _Eupomotis gibbosus_ (Linnaeus).
Natural size. (From life by R. W. Shufeldt.)--Page 13.]

The vacant space about the little brain is filled with a fatty fluid
mass looking like white of egg, intended for its protection. Taking the
dead sunfish (for the live one we shall look after carefully, giving
him every day fresh water and a fresh worm or snail or bit of beef), if
we cut off the upper part of the skull we shall see the separate parts
of the brain, most of them lying in pairs, side by side, in the bottom
of the brain-cavity. The largest pair is near the middle of the length
of the brain, two nerve-masses (or ganglia), each one round and hollow.
If we turn these over, we shall see that the nerves of the eye run into
them. We know then that these nerve-masses receive the impressions of
sight, and so they are called optic lobes. In front of the optic lobes
are two smaller and more oblong nerve-masses. These constitute the
cerebrum. This is the thinking part of the brain, and in man and in
the higher animals it makes up the greater part of it, overlapping and
hiding the other ganglia. But the fish has not much need for thinking
and its fore-brain or cerebrum is very small. In front of these are
two small, slim projections, one going to each nostril. These are
the olfactory lobes which receive the sensation of smell. Behind the
optic lobes is a single small lobe, not divided into two. This is the
cerebellum and it has charge of certain powers of motion. Under the
cerebellum is the medulla, below which the spinal cord begins. The
rest of the spinal cord is threaded through the different vertebrae
back to the tail, and at each joint it sends out nerves of motion and
receives nerves of sense. Everything that is done by the fish, inside
or outside, receives the attention of the little branches of the great
nerve-cord.

=The Fish's Nest.=--The sunfish in the spawning time will build some
sort of a nest of stones on the bottom of the eddy, and then, when the
eggs are laid, the male with flashing eye and fins all spread will
defend the place with a good deal of spirit. All this we call instinct.
He fights as well the first time as the last. The pressure of the eggs
suggests nest-building to the female. The presence of the eggs tells
the male to defend them. But the facts of the nest-building and nest
protection are not very well understood, and any boy who can watch them
and describe them truly will be able to add something to science.




CHAPTER II

THE EXTERIOR OF THE FISH


=Form of Body.=--With a glance at the fish as a living organism and
some knowledge of those structures which are to be readily seen without
dissection, we are prepared to examine its anatomy in detail, and to
note some of the variations which may be seen in different parts of the
great group.

In general fishes are boat-shaped, adapted for swift progress through
the water. They are longer than broad or deep and the greatest width is
in front of the middle, leaving the compressed paddle-like tail as the
chief organ of locomotion.

[Illustration: FIG. 6.--Pine-cone Fish, _Monocentris japonicus_
(Houttuyn). Waka, Japan.]

But to all these statements there are numerous exceptions. Some fishes
depend for protection, not on swiftness, but on the thorny skin or
a bony coat of mail. Some of these are almost globular in form, and
their outline bears no resemblance to that of a boat. The trunkfish
(_Ostracion_) in a hard bony box has no need of rapid progress.

[Illustration: FIG. 7.--Porcupine-fish, _Diodon hystrix_ (Linnaeus).
Tortugas Islands.]

[Illustration: FIG. 8.--Thread-eel, _Nemichthys avocetta_ Jordan and
Gilbert. Vancouver Island.]

[Illustration: FIG. 9.--Sea-horse, _Hippocampus hudsonius_ Dekay.
Virginia.]

[Illustration: FIG. 10.--Harvest-fish, _Peprilus paru_ (Linnaeus).
Virginia.]

[Illustration: FIG. 11.--Anko or Fishing-frog, _Lophius litulon_
(Jordan). Matsushima Bay, Japan. (The short line in all cases shows the
degree of reduction; it represents an inch of the fish's length.)]

The pine-cone fish (_Monocentris japonicus_) adds strong fin-spines to
its bony box, and the porcupine fish (_Diodon hystrix_) is covered with
long prickles which keep away all enemies.

Among swift fishes, there are some in which the body is much deeper
than long, as in _Antigonia_. Certain sluggish fishes seem to be all
head and tail, looking as though the body by some accident had been
omitted. These, like the headfish (_Mola mola_) are protected by a
leathery skin. Other fishes, as the eels, are extremely long and
slender, and some carry this elongation to great extremes. Usually the
head is in a line with the axis of the body, but in some cases, as the
sea-horse (_Hippocampus_), the head is placed at right angles to the
axis, and the body itself is curved and cannot be straightened without
injury. The type of the swiftest fish is seen among the mackerels and
tunnies, where every outline is such that a racing yacht might copy it.

The body or head of the fish is said to be compressed when it is
flattened sidewise, depressed when it is flattened vertically. Thus the
_Peprilus_ (Fig. 10) is said to be compressed, while the fishing-frog
(_Lophius_) (Fig. 11) has a depressed body and head. Other terms as
truncate (cut off short), attenuate (long-drawn out), robust, cuboid,
filiform, and the like may be needed in descriptions.

=Measurement of the Fish.=--As most fishes grow as long as they live,
the actual length of a specimen has not much value for purposes of
description. The essential point is not actual length, but relative
length. The usual standard of measurement is the length from the tip of
the snout to the base of the caudal fin. With this length the greatest
depth of the body, the greatest length of the head, and the length of
individual parts may be compared. Thus in the Rock Hind (_Epinephelus
adscensionis_), fig. 12, the head is contained 2-3/5 times in the
length, while the greatest depth is contained three times.

Thus, again, the length of the muzzle, the diameter of the eye, and
other dimensions may be compared with the length of the head. In the
Rock Hind, fig. 12, the eye is 5 in head, the snout is 4-2/5 in head,
and the maxillary 2-3/5. Young fishes have the eye larger, the body
slenderer, and the head larger in proportion than old fishes of the
same kind. The mouth grows larger with age, and is sometimes larger
also in the male sex. The development of the fins often varies a good
deal in some fishes with age, old fishes and male fishes having higher
fins when such differences exist. These variations are soon understood
by the student of fishes and cause little doubt or confusion in the
study of fishes.

[Illustration: FIG. 12.--Rock Hind or Cabra Mora of the West Indies,
_Epinephelus adscensionis_ (Osbeck). Family _Serranidae_.]

=The Scales, or Exoskeleton.=--The surface of the fish may be naked as
in the catfish, or it may be covered with scales, prickles, shagreen,
or bony plates. The hard covering of the skin, when present, is known
as the exoskeleton, or outer skeleton. In the fish, the exoskeleton,
whatever form it may assume, may be held to consist of modified scales,
and this is usually obviously the case. The skin of the fish may be
thick or thin, bony, horny, leathery, or papery, or it may have almost
any intermediate character. When protected by scales the skin is
usually thin and tender; when unprotected it may be ossified, as in
the sea-horse; horny, as in the headfish; leathery, as in the catfish;
or it may, as in the sea-snails, form a loose scarf readily detachable
from the muscles below.

The scales themselves may be broadly classified as ctenoid, cycloid,
placoid, ganoid, or prickly.

_Ctenoid and Cycloid Scales._--Normally formed scales are rounded in
outline, marked by fine concentric rings, and crossed on the inner
side by a few strong radiating ridges and folds. They usually cover
the body more or less evenly and are imbricated like shingles on a
roof, the free edge being turned backward. Such normal scales are of
two types, ctenoid or cycloid. Ctenoid scales have a comb-edge of
fine prickles or cilia; cycloid scales have the edges smooth. These
two types are not very different, and the one readily passes into the
other, both being sometimes seen on different parts of the same fish.
In general, however, the more primitive representatives of the typical
fishes, those with abdominal ventrals and without spines in the fins,
have cycloid or smooth scales. Examples are the salmon, herring,
minnow, and carp. Some of the more specialized spiny-rayed fishes,
as the parrot-fishes, have, however, scales equally smooth, although
somewhat different in structure. Sometimes, as in the eel, the cycloid
scales may be reduced to mere rudiments buried in the skin.

_Ctenoid_ scales are beset on the free edge by little prickles or
points, sometimes rising to the rank of spines, at other times soft and
scarcely noticeable, when they are known as ciliate or eyelash-like.
Such scales are possessed in general by the more specialized types of
bony fishes, as the perch and bass, those with thoracic ventrals and
spines in the fins.

[Illustration: FIG. 13.--Scales of _Acanthoessus bronni_ (Agassiz).
(After Dean.)]

_Placoid Scales._--Placoid scales are ossified papillae, minute,
enamelled, and close-set, forming a fine shagreen. These are
characteristic of the sharks; and in the most primitive sharks the
teeth are evidently modifications of these primitive structures. Some
other fishes have scales which appear shagreen-like to sight and
feeling, but only the sharks have the peculiar structure to which
Agassiz gave the name of placoid. The rough prickles of the filefishes
and some sculpins are not placoid, but are reduced or modified ctenoid
scales, scales narrowed and reduced to prickles.

_Bony and Prickly Scales._--Bony and prickly scales are found in
great variety, and scarcely admit of description or classification.
In general, prickly points on the skin are modifications of ctenoid
scales. Ganoid scales are thickened and covered with bony enamel, much
like that seen in teeth, otherwise essentially like cycloid scales.
These are found in the garpike and in many genera of extinct Ganoid
and Crossopterygian fishes. In the line of descent the placoid scale
preceded the ganoid, which in turn was followed by the cycloid and
lastly by the ctenoid scale. Bony scales in other types of fishes may
have nothing structurally in common with ganoid scales or plates,
however great may be the superficial resemblance.

[Illustration: FIG. 14.--Cycloid Scale.]

The distribution of scales on the body may vary exceedingly. In some
fishes the scales are arranged in very regular series; in others they
are variously scattered over the body. Some are scaly everywhere on
head, body, and fins. Others may have only a few lines or patches. The
scales may be everywhere alike, or they may in one part or another
be greatly modified. Sometimes they are transformed into feelers or
tactile organs. The number of scales is always one of the most valuable
of the characters by which to distinguish species.

=Lateral Line.=--The lateral line in most fishes consists of a series
of modified scales, each one provided with a mucous tube extending
along the side of the body from the head to the caudal fin. The canal
which pierces each scale is simple at its base, but its free edge is
often branched or ramified. In most spiny-rayed fishes it runs parallel
with the outline of the back. In most soft-rayed fishes it follows
rather the outline of the belly. It is subject to many variations. In
some large groups (_Gobiidae_, _Paeciliidae_) its surface structures are
entirely wanting. In scaleless fishes the mucous tube lies in the skin
itself. In some groups the lateral line has a peculiar position, as in
the flying-fishes, where it forms a raised ridge bounding the belly.
In many cases the lateral line has branches of one sort or another. It
is often double or triple, and in some cases the whole back and sides
of the fish are covered with lateral lines and their ramifications.
Sometimes peculiar sense-organs and occasionally eye-like luminous
spots are developed in connection with the lateral line, enabling the
fish to see in the black depths of the sea. These will be noticed in
another chapter.

_The Lateral Line as a Mucous Channel._--The more primitive condition
of the lateral line is seen in the sharks and chimaeras, in which fishes
it appears as a series of channels in or under the skin. These channels
are filled with mucus, which exudes through occasional open pores. In
many fishes the bones of the skull are cavernous, that is, provided
with cavities filled with mucus. Analogous to these cavities are the
mucous channels which in primitive fishes constitute the lateral line.

[Illustration: FIG. 15.--Singing Fish (with many lateral lines),
_Porichthys porosissimus_ (Cuv. and Val.). Gulf of Mexico.]

_Function of the Lateral Line._--The general function of the lateral
line with its tubes and pores is still little understood. As the
structures of the lateral line are well provided with nerves, it has
been thought to be an organ of sense of some sort not yet understood.
Its close relation to the ear is beyond question, the ear-sac being an
outgrowth from it.

"The original significance of the lateral line," according to Dr.
Dean,[2] "as yet remains undetermined. It appears intimately if not
genetically related to the sense-organs of the head and gill region
of the ancestral fish. In response to special aquatic needs, it may
thence have extended farther and farther backward along the median line
of the trunk, and in its later differentiation acquired its metameral
characters." In view of its peculiar nerve-supply, "the precise
function of this entire system of organs becomes especially difficult
to determine. Feeling, in its broadest sense, has safely been admitted
as its possible use. Its close genetic relationship to the hearing
organ suggests the kindred function of determining waves of vibration.
These are transmitted in so favorable a way in the aquatic medium that
from the side of theory a system of hypersensitive end-organs may well
have been established. The sensory tracts along the sides of the body
are certainly well situated to determine the direction of the approach
of friend, enemy, or prey."

=The Fins of Fishes.=--The organs of locomotion in the fishes are
knows as fins. These are composed of bony or cartilaginous rods or
rays connected by membranes. The fins are divided into two groups,
paired fins and vertical fins. The pectoral fins, one on either side,
correspond to the anterior limbs of the higher vertebrates. The ventral
fins below or behind them represent the hinder limbs. Either or both
pairs may be absent, but the ventrals are much more frequently abortive
than the pectorals. The insertion of the ventral fins may be abdominal,
as in the sharks and the more generalized of the bony fishes, thoracic
under the breast (the pelvis attached to the shoulder-girdle) or
jugular, under the throat. When the ventral fins are abdominal, the
pectoral fins are usually placed very low. The paired fins are not in
general used for progression in the water, but serve rather to enable
the fish to keep its equilibrium. With the rays, however, the wing-like
pectoral fins form the chief organ of locomotion.

The fin on the median line of the back is called the dorsal, that on
the tail the caudal, and that on the lower median line the anal fin.
The dorsal is often divided into two fins or even three. The anal is
sometimes divided, and either dorsal or anal fin may have behind it
detached single rays called finlets.

The rays composing the fin may be either simple or branched. The
branched rays are always articulated, that is, crossed by numerous
fine joints which render them flexible. Simple rays are also sometimes
articulate. Rays thus jointed are known as soft rays, while those rays
which are neither jointed nor branched are called spines. A spine is
usually stiff and sharp-pointed, but it may be neither, and some spines
are very slender and flexible, the lack of branches or joints being the
feature which distinguishes spine from soft ray.

The anterior rays of the dorsal and anal fins are spinous in most
fishes with thoracic ventrals. The dorsal fin has usually about ten
spines, the anal three, but as to this there is much variation in
different groups. When the dorsal is divided all the rays of the first
dorsal and usually the first ray of the second are spines. The caudal
fin has never true spines, though at the base of its lobes are often
rudimentary rays which resemble spines. Most spineless fishes have such
rudiments in front of their vertical fins. The pectoral, as a rule,
is without spines, although in the catfishes and some others a single
large spine may be developed. The ventrals when abdominal are usually
without spines. When thoracic each usually, but not always, consists
of one spine and five soft rays. When jugular the number of soft rays
may be reduced, this being a phase of degeneration of the fin. In
writing descriptions of fishes the number of spines may be indicated
by Roman numerals, those of the soft rays by Arabic. Thus D. XII-I, 17
means that the dorsal is divided, that the anterior portion consists
of twelve spines, the posterior of one spine and seventeen soft rays.
In some fishes, as the catfish or the salmon, there is a small fin on
the back behind the dorsal fin. This is known as the adipose fin, being
formed of fatty substance covered by skin. In a few catfishes, this
adipose fin develops a spine or soft rays.

=Muscles.=--The movements of the fins are accomplished by the muscles.
These organs lie along the sides of the body, forming the flesh of the
fish. They are little specialized, and not clearly differentiated as in
the higher vertebrates.

With the higher fishes there are several distinct systems of muscles
controlling the jaws, the gills, the eye, the different fins, and the
body itself. The largest of all is the great lateral muscle, composed
of flake-like segments (myocommas) which correspond in general with the
number of the vertebrae. In general the muscles of the fish are white
in color. In some groups, especially of the mackerel family, they are
deep red, charged with animal oils. In the salmon they are orange-red,
a color also due to the presence of certain oils.

In a few fishes muscular structures are modified into electric organs.
These will be discussed in a later chapter.

FOOTNOTES:

[2] Fishes Recent and Fossil, p. 52.




CHAPTER III

THE DISSECTION OF THE FISH


=The Blue-green Sunfish.=--The organs found in the abdominal cavity of
the fish may be readily traced in a rapid dissection. Any of the bony
fishes may be chosen, but for our purposes the sunfish will serve as
well as any. The names and location of the principal organs are shown
in the accompanying figure, from Kellogg's Zoology. It represents the
blue-green sunfish, _Apomotis cyanellus_, from the Kansas River, but
in these regards all the species of sunfishes are alike. We may first
glance at the different organs as shown in the sequence of dissection,
leaving a detailed account of each to the subsequent pages.

=The Viscera.=--Opening the body cavity of the fish, as shown in the
plate, we see below the back-bone a membranous sac closed and filled
with air. This is the air-bladder, a rudiment of that structure which
in higher vertebrates is developed as a lung. The alimentary canal
passes through the abdominal cavity extending from the mouth through
the pharynx and ending at the anus or vent. The stomach has the form
of a blind sac, and at its termination are a number of tubular sacs,
the pyloric caeca, which secrete a digestive fluid. Beyond the pylorus
extends the intestine with one or two loops to the anus. Connected with
the intestine anteriorly is the large red mass of the liver, with its
gall-bladder, which serves as a reservoir for bile, the fluid the liver
secretes. Farther back is another red glandular mass, the spleen.

In front of the liver and separated from it by a membrane is the heart.
This is of four parts. The posterior part is a thin-walled reservoir,
the sinus venosus, into which blood enters through the jugular vein
from the head and through the cardinal vein from the kidney. From the
sinus venosus it passes forward into a large thin-walled chamber, the
auricle.

[Illustration: FIG. 16.--Dissection of the Blue-green Sunfish,
_Apomotis cyanellus_ Rafinesque. (After Kellogg.)--27.] Next it flows
into the thick-walled ventricle, whence by the rhythmical contraction
of its walls it is forced into an arterial bulb which lies at the base
of the ventral aorta, which carries it on to the gills. After passing
through the fine gill-filaments, it is returned to the dorsal aorta,
a large blood-vessel which extends along the lower surface of the
back-bone, giving out branches from time to time.

The kidneys in fishes constitute an irregular mass under the back-bone
posteriorly. They discharge their secretions through the ureter to
a small urinary bladder, and thence into the urogenital sinus, a
small opening behind the anus. Into the same sinus are discharged the
reproductive cells in both sexes.

In the female sunfish the ovaries consist of two granular masses of
yellowish tissue lying just below and behind the swim-bladder. In the
spring they fill much of the body cavity and the many little eggs can
be plainly seen. When mature they are discharged through the oviduct to
the urogenital sinus. In some fishes there is no special oviduct and
the eggs pass into the abdominal cavity before exclusion.

In the male the reproductive organs have the same position as the
ovaries in the female. They are, however, much smaller in size and
paler in color, while the minute spermatozoa appear milky rather than
granular on casual examination. A _vas deferens_ leads from each of
these organs into the urogenital sinus.

The lancelets, lampreys, and hagfishes possess no genital ducts. In
the former the germ cells are shed into the atrial cavity, and from
there find their way to the exterior either through the mouth or the
atrial pore; in the latter they are shed directly into the body cavity,
from which they escape through the abdominal pores. In the sharks and
skates the Wolffian duct in the male, in addition to its function as
an excretory duct, serves also as a passage for the sperm, the testes
having a direct connection with the kidneys. In these forms there is a
pair of Muellerian ducts which serve as oviducts in the females; they
extend the length of the body cavity, and at their anterior end have an
opening which receives the eggs which have escaped from the ovary into
the body cavity. In some bony fishes as the eels and female salmon the
germ cells are shed into the body cavity and escape through genital
pores, which, however, may not be homologous with abdominal pores. In
most other bony fishes the testes and ovaries are continued directly
into ducts which open to the outside.

=Organs of Nutrition.=--The organs thus shown in dissection we may now
examine in detail.

[Illustration: FIG. 17.--Black Swallower, _Chiasmodon niger_ Johnson,
containing a fish larger than itself. Le Have Bank.]

The mouth of the fish is the organ or series of structures first
concerned in nutrition. The teeth are outgrowths from the skin,
primarily as modified papillae, aiding the mouth in its various
functions of seizing, holding, cutting, or crushing the various kinds
of food material. Some fishes feed exclusively on plants, some on
plants and animals alike, some exclusively on animals, some on the mud
in which minute plants and animals occur. The majority of fishes feed
on other fishes, and without much regard to species or condition. With
the carnivorous fishes, to feed represents the chief activity of the
organism. In proportion to the voracity of the fish is usually the size
of the mouth, the sharpness of the teeth, and the length of the lower
jaw.

The most usual type of teeth among fishes is that of villiform
bands. Villiform teeth are short, slender, even, close-set, making
a rough velvety surface. When the teeth are larger and more widely
separated, they are called cardiform, like the teeth of a wool-card.
Granular teeth are small, blunt, and sand-like. Canine teeth are those
projecting above the level of the others, usually sharp, curved, and
in some species barbed. Sometimes the canines are in front. In some
families the last tooth in either jaw may be a "posterior canine,"
serving to hold small animals in place while the anterior teeth crush
them. Canine teeth are often depressible, having a hinge at base.

[Illustration: FIG. 18.--Jaws of a Parrot-fish, _Sparisoma
aurofrenatum_ (Val.). Cuba.]

Teeth very slender and brush-like are called setiform. Teeth with
blunt tips are molar. These are usually enlarged and fitted for
crushing shells. Flat teeth set in mosaic, as in many rays and in the
pharyngeals of parrot-fishes, are said to be _paved_ or tessellated.
Knife-like teeth, occasionally with serrated edges, are found in
many sharks. Many fishes have incisor-like teeth, some flattened
and truncate like human teeth, as in the sheepshead, sometimes with
serrated edges. Often these teeth are movable, implanted only in
the skin of the lips. In other cases they are set fast in the jaw.
Most species with movable teeth or teeth with serrated edges are
herbivorous, while strong incisors may indicate the choice of snails
and crabs as food. Two or more of these different types may be found
in the same fish. The knife-like teeth of the sharks are progressively
shed, new ones being constantly formed on the inner margins of the jaw,
so that the teeth are marching to be lost over the edge of the jaw as
soon as each has fulfilled its function. In general the more distinctly
a species is a fish-eater, the sharper are the teeth. Usually fishes
show little discrimination in their choice of food; often they devour
the young of their own species as readily as any other. The digestive
process is rapid, and most fishes rapidly increase in size in the
process of development. When food ceases to be abundant the fishes grow
more slowly. For this reason the same species will grow to a larger
size in large streams than in small ones, in lakes than in brooks. In
most cases there is no absolute limit to growth, the species growing as
long as it lives. But while some species endure many years, others are
certainly very shortlived, and some may be even annual, dying after
spawning, perhaps at the end of the first season.

Teeth are wholly absent in several groups of fishes. They are, however,
usually present on the premaxillary, dentary, and pharyngeal bones.
In the higher forms, the vomer, palatines, and gill-rakers are rarely
without teeth, and in many cases the pterygoids, sphenoids, and the
bones of the tongue are similarly armed.

No salivary glands or palatine velum are developed in fishes. The
tongue is always bony or gristly and immovable. Sometimes taste-buds
are developed on it, and sometimes these are found on the barbels
outside the mouth.

[Illustration: FIG. 19.--Sheepshead (with incisor teeth), _Archosargus
probatocephalus_ (Walbaum). Beaufort, N. C.]

=The Alimentary Canal.=--The mouth-cavity opens through the pharynx
between the upper and lower pharyngeal bones into the oesophagus,
whence the food passes into the stomach. The intestinal tract is in
general divided into four portions--oesophagus, stomach, small and
large intestines. But these divisions of the intestines are not always
recognizable, and in the very lowest forms, as in the lancelet, the
stomach is a simple straight tube without subdivision.

In the lampreys there is a distinction only of the oesophagus with many
longitudinal folds and the intestine with but one. In the bony fishes
the stomach is an enlarged area, either siphon-shaped, with an opening
at either end, or else forming a blind sac with the openings for
entrance (cardiac) and exit (pyloric) close together at the anterior
end. In the various kinds of mullets (_Mugil_) and in the hickory shad
(_Dorosoma_), fishes which feed on minute vegetation mixed with mud,
the stomach becomes enlarged to a muscular gizzard, like that of a
fowl. Attached near the pylorus and pouring their secretions into the
duodenum or small intestine are the _pyloric caeca_. These are tubular
sacs secreting a pale fluid and often almost as long as the stomach or
as wide as the intestine. These may be very numerous as in the salmon,
in which case they are likely to become coalescent at base, or they be
few or altogether wanting.

Besides these appendages which are wanting in the higher vertebrates, a
pancreas is also found in the sharks and many other fishes. This is a
glandular mass behind the stomach, its duct leading into the duodenum
and often coalescent with the bile duct from the liver. The liver in
the lancelet is a long diverticulum of the intestine. In the true
fishes it becomes a large gland of irregular form, and usually but not
always provided with a gall-bladder as in the higher vertebrates. Its
secretions usually pass through a _ductus cholodechus_ to the duodenum.

The _spleen_, a dark-red lymphatic gland, is found attached to the
stomach in all fish-like vertebrates except the lancelet.

The lining membrane of the abdominal cavity is known as the
_peritoneum_, and the membrane sustaining the intestines from the
dorsal side, as in the higher vertebrates, is called the _mesentery_.
In many species the peritoneum is jet black, while in related forms it
may be pale in color. It is more likely to be black in fishes from deep
water and in fishes which feed on plants.

=The Spiral Valve.=--In the sharks or skates the rectum or large
intestine is peculiarly modified, being provided with a spiral valve,
with sometimes as many as forty gyrations. A spiral valve is also
present in the more ancient types of the true fishes as dipnoans,
crossopterygians, and ganoids. This valve greatly increases the
surface of the intestine, doing away with the necessity for length.
In the bowfin (_Amia_) and the garpike (_Lepisosteus_) the valve is
reduced to a rudiment of three or four convolutions near the end of the
intestine. In the sharks and skates the intestine opens into a cloaca,
which contains also the urogenital openings. In all fishes the latter
lie behind the orifice of the intestine. In the bony fishes and the
ganoids there is no cloaca.

[Illustration: FIG. 20.--Stone-roller, _Campostoma anomalum_
(Rafinesque). Family _Cyprinidae_. Showing nuptial tubercles and
intestines coiled about the air-bladder.]

=Length of the Intestine.=--In all fishes, as in the higher
vertebrates, the length of the alimentary canal is coordinated with the
food of the fish. In those which feed upon plants the intestine is very
long and much convoluted, while in those which feed on other fishes it
is always relatively short. In the stone-roller, a fresh-water minnow
(_Campostoma_) found in the Mississippi Valley, the excessively long
intestines filled with vegetable matter are wound spool-fashion about
the large air-bladder. In all other fishes the air-bladder lies on the
dorsal side of the intestinal canal.




CHAPTER IV

THE SKELETON OF THE FISH


=Specialization of the Skeleton.=--In the lowest form of fish-like
vertebrates (_Branchiostoma_), the skeleton consists merely of a
cartilaginous rod or notochord extending through the body just below
the spinal cord. In the lampreys, sharks, dipnoans, crossopterygians,
and sturgeons the skeleton is still cartilaginous, but grows
progressively more complex in their forms and relations. Among the
typical fishes the skeleton becomes ossified and reaches a very high
degree of complexity. Very great variations in the forms and relations
of the different parts of the skeleton are found among the bony fishes,
or teleostei. The high degree of specialization of these parts gives to
the study of the bones great importance in the systematic arrangement
of these fishes. In fact the true affinities of forms is better shown
by the bones than by any other system of organs. In a general way
the skeleton of the fish is homologous with that of man. The head in
the one corresponds to the head in the other, the back-bone to the
back-bone, and the paired fins, pectoral and ventral, to the arms and
legs.

=Homologies of Bones of Fishes.=--But this homology does not extend to
the details of structure. The bones of the arm of the specialized fish
are not by any means identical with the humerus, coracoid, clavicle,
radius, ulna, and carpus of the higher vertebrates. The vertebrate arm
is not derived from the pectoral fin, but both from a cartilaginous
shoulder-girdle with undifferentiated pectoral elements bearing
fin-rays, in its details unlike an arm and unlike the pectoral fin of
the specialized fish.

The assumption that each element in the shoulder-girdle and the
pectoral fin of the fish must correspond in detail to the arm of man
has led to great confusion in naming the different bones. Among the
many bones of the fish's shoulder-girdle and pectoral fin, three or
four different ones have successively borne the names of scapula,
clavicle, coracoid, humerus, radius, and ulna. None of these terms,
unless it be clavicle, ought by rights apply to the fish, for no bone
of the fish is a true homologue of any of these as seen in man. The
land vertebrates and the fishes have doubtless sprung from a common
stock, but this stock, related to the crossopterygians of the present
day, was unspecialized in the details of its skeleton, and from it the
fishes and the higher vertebrates have developed the widely diverging
lines.

[Illustration: FIG. 21.--Striped Bass, _Roccus lineatus_ (Bloch).
Potomac River.]

=Parts of the Skeleton.=--The skeleton may be divided into the head,
the vertebral column, and the limbs. The very lowest of the fish-like
forms (_Branchiostoma_) has no differentiated head or skull, but in all
the other forms the anterior part of the vertebral column is modified
to form a cranium for the protection of the brain. In the lampreys
there are no jaws or other appendages to the cranium.

In the sharks, dipnoans, crossopterygians, ganoids, and teleosts or
bony fishes, jaws are developed as well as a variety of other bones
around the mouth and throat. The jaw-bearing forms are sometimes known
by the general name of gnathostomes. In the sharks and their relatives
(rays, chimaeras, etc.) all the skeleton is composed of cartilage. In
the more specialized bony fishes, besides these bones we find also
series of membrane bones, more or less external to the skull and
composed of ossified dermal tissues. Membrane bones are not found in
the sharks and lampreys, but are developed in an elaborate coat of mail
in some extinct forms.

[Illustration: FIG. 22.--_Roccus lineatus._ Lateral view of cranium.

1. Vomer. 3. Prefrontal. 5. Sphenotic. 7. Epiotic. 9. Pterotic. 11.
Exoccipital. 13. Parasphenoid. 15. Prootic. 2. Ethmoid. 4. Frontal. 6.
Parietal. 8. Supraoccipital. 10. Opisthotic. 12. Basioccipital. 14.
Basisphenoid.]

[Illustration: FIG. 23.--_Roccus lineatus._ Superior view of cranium.

1. Vomer. 3. Prefrontal. 5. Sphenotic. 7. Epiotic. 9. Pterotic. 11.
Exoccipital. 2. Ethmoid. 4. Frontal. 6. Parietal. 8. Supraoccipital.
10. Opisthotic.]

[Illustration: FIG. 24.--_Roccus lineatus._ Inferior view of cranium.

1. Vomer. 4. Frontal. 7. Epiotic. 9. Pterotic. 11. Exoccipital.
13. Parasphenoid. 16. Alisphenoid. 3. Prefrontal. 5. Sphenotic. 8.
Supraoccipital. 10. Opisthotic. 12. Basioccipital. 15. Prootic.]

=Names of Bones of Fishes.=--In the study of the names of the bones of
fishes it will be more convenient to begin with a highly specialized
form in which each of the various structures is present and in its
normal position.

To this end we present a series of figures of a typical form, choosing,
after Starks, the striped bass (_Roccus lineatus_) of the Atlantic
coast of the United States. For this set of plates, drawn from nature
by Mrs. Chloe Lesley Starks, we are indebted to the courtesy of Mr.
Edwin Chapin Starks. The figures of the striped bass illustrate a
noteworthy paper on "The Synonymy of the Fish Skeleton," published by
the Washington Academy of Sciences in 1901.

=Bones of the Cranium.=--The _vomer_ (1) is the anterior part of the
roof of the mouth, armed with small teeth in the striped bass and in
many other fishes, but often toothless. The _ethmoid_ (2) lies behind
the vomer on the upper surface of the skull, and the _prefrontal_ (3)
projects on either side and behind the ethmoid, the nostrils usually
lying over or near it and near the nasal bone (51). Between the eyes
above are the two _frontal_ (4) bones joined by a suture. On the
side behind the posterior angle of the frontal is the _sphenotic_
(5) above the posterior part of the eye. Behind each frontal is the
_parietal_ (6). Behind the parietal and more or less turned inward
over the ear-cavity is the _epiotic_ (7). Between the parietals, and
in most fishes rising into a thin crest, is the _supraoccipital_ (8),
which bounds the cranium above and behind, its posterior margin being
usually a vertical knife-like edge. The _pterotic_ (9) forms a sort
of wing or free margin behind the epiotic and over the ear-cavity.
The _opisthotic_ (10) is a small, hard, irregular bone behind the
pterotic. The _exoccipital_ (11) forms a concave joint or condyle on
each side of the _basioccipital_ (12), by which the vertebral column
is joined to the skull. The _parasphenoid_ (13) forms a narrow ridge
of the roof of the mouth, connecting the vomer with the basioccipital.
In some fishes of primitive structure (_Salmo_, _Beryx_) there is
another bone, called orbitosphenoid, on the middle line above and
between the eyes. The _basisphenoid_ (14) is a little bone above
the myotome or tube in which runs the rectus muscle of the eye. It
descends toward the parasphenoid and is attached to the prootic. The
_prootic_ (15) is an irregular bone below the ear region and lying in
advance of the opisthotic. The _alisphenoid_ (16) is a small bone in
the roof of the mouth before the prootic. These sixteen bones (with a
loose bone of specialized form, the _otolith_, within the ear-cavity)
constitute the cranium. All are well developed in the striped bass and
in most fishes. In some specialized forms they are much distorted,
coossified, or otherwise altered, and their relations to each other may
be more or less changed. In the lower forms they are not always fully
differentiated, but in nearly all cases their homologies can be readily
traced. In the sharks and lampreys the skull constitutes a continuous
cartilaginous box without sutures. In the dipnoans and other forms
having a bony casque the superficial bones outside the cranium may not
correspond to the cartilaginous elements of the soft skull itself.

[Illustration: FIG. 25--_Roccus lineatus._ Posterior view of cranium.

  6. Parietal.
  7. Epiotic.
  8. Supraoccipital.
  9. Pterotic.
  10. Opisthotic.
  11. Exoccipital.
  12. Basioccipital.]

=Bones of the Jaws.=--The bones of the jaws are attached to the
cranium by membranes only, not by sutures, except in a few peculiarly
specialized forms.

_The Upper Jaw._--The _premaxillary_ (32) lies on either side and forms
the front of the upper jaw. Its upper posterior tip or premaxillary
spine projects backward almost at right angles with the rest of the
bone into a groove on the ethmoid. There is often a fold in the skin by
which this bone may be thrust out or protracted, as though drawn out of
a sheath. When the spines of the premaxillary are very long the upper
jaw may be thrust out for a considerable distance. The premaxillary is
also often known as intermaxillary.

Lying behind the premaxillary, its anterior end attached within
the angle of the premaxillary, is the _maxillary_ (31), or
_supramaxillary_, a flattened bone with expanded posterior tip. In the
striped bass this bone is without teeth, but in many less specialized
forms, as the salmon, it is provided with teeth and joined to the
premaxillary in a different fashion. In any case its position readily
distinguishes it. In some cases the maxillary is divided by one or more
sutures, setting off from it one or more extra maxillary (supplemental
maxillary) bones. This suture is absent in the striped bass, but
distinct in the black bass, and more than one suture is found in the
shad and herring. The roof of the mouth above is formed by a number of
bones, which, as they often possess teeth, may be considered with the
jaws. These are the _palatine_ bones (21), one on either side flanking
the vomer, the _pterygoid_ (20), behind it and articulating with it,
the _mesopterygoid_ (22), on the roof of the mouth toward the median
line, and the _metapterygoid_ (23), lying behind this. Although often
armed with teeth, these bones are to be considered of the general
nature of the membrane bones. In some degraded types of fishes (eels,
morays, congers) the premaxillary is indistinguishable, being united
with the vomer and palatines.

[Illustration: FIG. 26.--_Roccus lineatus._ Face-bones, shoulder and
pelvic girdles, and hyoid arch.

  17. Hyomandibular.
  18. Symplectic.
  19. Quadrate.
  20. Pterygoid.
  21. Palatine.
  22. Mesopterygoid.
  23. Metapterygoid.
  24. Preopercle.
  25. Opercle.
  26. Subopercle.
  27. Interopercle.
  28. Articular.
  29. Angular.
  30. Dentary.
  31. Maxillary.
  32. Premaxillary.
  33. Interhyal.
  34. Epihyal.
  35. Ceratohyal
  36. Basihyal.
  37. Glossohyal.
  38. Urohyal.
  39. Branchiostegal.
  49. Preorbital.
  50. Suborbital.
  51. Nasal.
  52. Supratemporal.
  53. Post-temporal.
  54. Supraclavicle.
  55. Clavicle.
  56. Postclavicle.
  57. Hypercoracoid.
  58. Hypocoracoid.
  60. Actinosts.
  61. Pectoral fin.
  62. Pelvic girdle.
  63. Ventral fin.]

The upper jaw of the shark is formed from the anterior portion of the
palatine bones, which are not separate from the quadrate, the whole
forming the palato-quadrate apparatus. In the himaera and the dipnoans
this apparatus is solidly united with the cranium. In these fishes the
true upper jaw, formed of maxillary and premaxillary, is wanting.

[Illustration: FIG. 27.--Lower jaw of _Amia calva_ (Linnaeus), showing
the gular plate.]

_The Lower Jaw._--The lower jaw or mandible is also complex, consisting
of two divisions or rami, right and left, joined in front by a suture.
The anterior part of each ramus is formed by the _dentary bone_ (30),
which carries the teeth. Behind this is the _articular bone_ (28),
which is connected by a joint to the _quadrate bone_ (19). At the lower
angle of the articular bone is the small _angular bone_ (29). In many
cases another small bone, which is called _splenial_, may be found
attached to the inner surface of the articular bone. This little bone
has been called coronoid, but it is doubtless not homologous with the
coronoid bone of reptiles. In a few fishes, _Amia_, _Elopidae_, and
certain fossil dipnoans, there is a bony gular plate, a membrane bone
across the throat behind the chin on the lower jaw.

=The Suspensorium of the Mandible.=--The lower jaw is attached to the
cranium by a chain of suspensory bones, which vary a good deal with
different groups of fishes. The articular is jointed with the flat
quadrate bone (19), which lies behind the pterygoid. A slender bone
passes upward (18) under the preopercle and the metapterygoid, forming
a connection above with a large flattish bone, the _hyomandibular_
(17), which in turn joins the cranium. The slender bone which thus keys
together the upper and lower elements, hyomandibular and quadrate,
forming the suspensorium of the lower jaw, is known as _symplectic_
(18). The hyomandibular is thought to be homologous with the stapes, or
stirrup-bone, of the ear in higher animals. In this case the symplectic
may be homologous with its small orbicular bone, and the malleus is
a transformation of the articular. The incus, or anvil-bone, may be
formed from part of Meckel's cartilage. All these homologies are
however extremely hypothetical. The core of the lower jaw is formed
of a cartilage called Meckel's cartilage, outside which the membrane
bones, dentary, etc., are developed. This cartilage forms the lower
jaw in sharks, true jaw-bones not being developed in these fishes. In
lampreys and lancelets there is no lower jaw.

=Membrane Bones of Face.=--The membrane bones lie on the surface of
the head, when they are usually covered by thin skin and have only a
superficial connection with the cranium. Such bones, formed of ossified
membrane, are not found in the earlier or less specialized fishes, the
lancelets and lampreys, nor in the sharks, rays, and chimaeras. They are
chiefly characteristic of the bony fishes, although in some of these
they have undergone degradation.

The _preorbital_ (49) lies before and below the eye, its edge more or
less parallel with that of the maxillary. It may be broad or narrow.
When broad it usually forms a sheath into which the maxillary slips.
The _nasal_ (51) lies before the preorbital, a small bone usually lying
along the spine of the premaxillary. Behind and below the eye is a
series of about three flat bones, the _suborbitals_ (50), small in the
striped bass, but sometimes considerably modified. In the great group
of loricate fishes (sculpins, etc.), the third suborbital sends a bony
process called the suborbital stay backward across the cheek toward the
preopercle. The suborbital stay is present in the rosefish. In some
cases, as in the gurnard, this stay covers the whole cheek with a bony
coat of mail. In some fishes, but not in the striped bass, a small
supraorbital bone exists over the eye, forming a sort of cap on an
angle of the frontal bone.

The largest uppermost flat bone of the gill-covers is known as the
_opercle_ (25). Below it, joined by a suture, is the _subopercle_
(26). Before it is the prominent ridge of the _preopercle_ (24), which
curves forward below and forms a more or less distinct angle, often
armed with serrations or spines. In some cases this armature is very
highly developed. The _interopercle_ (27) lies below the preopercle and
parallel with the lower limb.

=Branchial Bones.=--The bones of the branchial apparatus or gills are
very numerous and complex, as well as subject to important variations.
In many fishes some of these bones are coossified, and in other cases
some are wanting. The tongue may be considered as belonging to this
series, as the bones of the gills are attached to its axis below.

In the striped bass, as in most fishes, the tongue, gristly and
immovable, is formed anteriorly by a bone called the _glossohyal_
(37). Behind this are the _basihyals_ (36), and still farther back,
on the side, is the _ceratohyal_ (35). To the basihyals is attached
a bone extending downward and free behind the _urohyal_ (38). Behind
the ceratohyal and continuous with it is the _epihyal_ (34), to which
behind is attached the narrow _interhyal_ (33). On the under surface of
the _ceratohyal_ and the _epihyal_ are attached the _branchiostegals_
(39). These are slender rays supporting a membrane beneath the gills,
seven in number on each side in the striped bass, but much more
numerous in some groups of fishes. The gill membranes connecting the
branchiostegals are in the striped bass entirely separate from each
other. In other fishes they may be broadly joined across the fleshy
interspace between the gill-openings, known as the _isthmus_, or again
they may be grown fast to the isthmus itself, so that the gill-openings
of the two sides are widely separated.

=The Gill-arches.=--The gills are attached to four bony arches with
a fifth of the same nature, but totally modified by the presence of
teeth, and very rarely having on it any of the gill-fringes. The fifth
arch thus modified to serve in mastication instead of respiration is
known collectively as the _lower pharyngeals_ (46). Opposite these are
the _upper pharyngeals_ (45).

The gill-arches are suspended to the cranium from above by the
_suspensory pharyngeal_ (44). Each arch contains three parts--the
_epibranchial_ (43), above, the _ceratobranchial_ (42), forming the
middle part, and the _hypobranchial_ (41), the lower part articulating
with the series of _basibranchials_ (40) which lie behind the epihyal
of the tongue. On the three bones forming the first gill-arch are
attached numerous appendages called _gill-rakers_ (47). These
gill-rakers vary very greatly in number and form. In the striped bass
they are few and spear-shaped. In the shad they are very many and
almost as fine as hairs. In some fishes they form an effective strainer
in separating the food, or perhaps in keeping extraneous matter from
the gills. In some fishes they are short and lumpy, in others wanting
altogether.

[Illustration: FIG. 28.--_Roccus lineatus._ Branchial arches. (After
Starks.)

  40. Basibranchial.
  41. Hypobranchial.
  42. Ceratobranchial.
  43. Epibranchial.
  44. Suspensory pharyngeal.
  45. Upper pharyngeals.
  46. Lower pharyngeals.
  47. Gill-rakers.]

=The Pharyngeals.=--The hindmost gill-arch, as above stated, is
modified to form a sort of jaw. The tooth-bearing bones above, 2 to
4 pairs, are known as _upper pharyngeals_ (45), those below, single
pair, as _lower pharyngeals_ (46). Of these the lower pharyngeals are
most highly specialized and the most useful in classification. These
are usually formed much as in the striped bass. Occasionally they are
much enlarged, with large teeth for grinding. In many families the
lower pharyngeals are grown together in one large bone. In the suckers
(_Catostomidae_) the lower pharyngeal preserves its resemblance to a
gill-arch. In the carp family (_Cyprinidae_) retaining this resemblance,
it possesses highly specialized teeth.

=Vertebral Column.=--The vertebral column is composed of a series of
vertebrae, 24 in number in the striped bass and in many of the higher
fishes, but varying in different groups from 16 to 18 to upwards of
400, the higher numbers being evidence of unspecialized or more usually
degenerate structure.

Each vertebra consists of a double concave body or _centrum_
(66). Above it are two small projections often turned backward,
_zygapophyses_ (71), and two larger ones, _neurapophyses_ (67), which
join above to form the _neural spine_ (68) and thus form the _neural
canal_, through which passes the spinal cord from end to end of the
body.

[Illustration: FIG. 29.--Pharyngeal bone and teeth of European Chub,
_Leuciscus cephalus_ (Linnaeus). (After Seelye.)]

[Illustration: FIG. 30.--Upper pharyngeals of a Parrot-fish, _Scarus
strongylocephalus_.]

[Illustration: FIG. 31.--Lower pharyngeals of a Parrot-fish, _Scarus
strongylocephalus_ (Bleeker).]

Below in the vertebrae of the posterior half of the body the
_haemapophyses_ (69) unite to form the _haemal spine_ (70), and through
the _haemal canal_ thus formed passes a great artery. The vertebrae
having haemal as well as neural spines are known as _caudal vertebrae_,
and occupy the posterior part of the body, usually that behind the
attachment of the _anal fin_ (78).

The anterior vertebrae known as _abdominal vertebrae_, bounding the
body-cavity, possess neural spines similar to those of the caudal
vertebrae. In place, however, of the haemapophyses are projections known
as _parapophyses_ (72), which do not meet below, but extend outward,
forming the upper part of the wall of the abdominal cavity.

[Illustration: FIG. 32.--Pharyngeals of Italian Parrot-fish, _Sparisoma
cretense_ (L.). _a_, upper; _b_, lower.]

To the parapophyses, or near them, the ribs (73) are rather loosely
attached and each rib may have one or more accessory branches (74)
called _epipleurals_.

[Illustration: FIG. 33.--_Roccus lineatus._ Vertebral column and
appendages, with a typical vertebra. (After Starks.)

  64. Abdominal vertebrae.
  65. Caudal vertebrae.
  66. Centrum.
  67. Neurapophysis.
  68. Neural spine.
  69. Haemapophysis.
  70. Haemal spine.
  71. Zygapophysis.
  72. Parapophysis.
  73. Ribs.
  74. Epipleurals.
  75. Interneural.
  76. Dorsal fin.
  77. Interhaemal.
  78. Anal fin.
  79. Hypural.
  80. Caudal fin.]

In the striped bass the dorsal vertebrae are essentially similar in
form, but in some fishes, as the carp and the catfish, 4 or 5 anterior
vertebrae are greatly modified, coossified, and so arranged as to
connect the air-bladder with the organ of hearing. Fishes with vertebrae
thus altered are called _plectospondylous_.

In the garpike the vertebrae are convex anteriorly, concave behind,
being joined by ball-and-socket joints (opisthocoelian). In most
other fishes they are double concave (amplicoelian). In sharks the
vertebrae are imperfectly ossified, a number of terms, asterospondylous,
cyclospondylous, tectospondylous, being applied to the different stages
of ossification, these terms referring to the different modes of
arrangement of the calcareous material within the vertebra.

=The Interneurals and Interhaemals.=--The vertical fins are connected
with the skeletons by bones placed loosely in the flesh and not joined
by ligament or suture. Below the dorsal fin (76) lies a series of
these bones, dagger-shaped, with the point downward. These are called
_interneurals_ (75) and to these the spines and soft rays of the fin
are articulated.

In like fashion the spines and rays of the anal fin (18) are jointed
at base to bones called _interhaemals_ (77). In certain cases the
second interhaemal is much enlarged, made hollow and quill-shaped, and
in its concave upper end the tip of the air-bladder is received. This
structure is seen in the plume-fishes (_Calamus_). These two groups of
bones, interneural and interhaemal, are sometimes collectively called
_inter-spinals_. The flattened basal bone of the _caudal fin_ (80) is
known as _hypural_ (79).

[Illustration: FIG. 34.--Basal bone of dorsal fin, _Holoptychius
leptopterus_ (Agassiz). (After Woodward.)]

The tail of the striped bass, ending in a broad plate which supports
the caudal, is said to be homocercal. In more primitive forms the tail
is turned upward more or less, the fin being largely thrown to its
lower side. Such a tail as in the sturgeon is said to be heterocercal.
In the isocercal tail of the codfish and its relatives the vertebrae
are progressively smaller behind and the hypural plate is obsolete or
nearly so, the vertebrae remaining in the line of the axis of the body
and dividing the caudal fin equally. The simplest form of tail, called
diphycercal, is extended horizontally, tapering backward, the fin
equally divided above and below, without hypural plate. In any form of
the tail, it may through degeneration be attenuate or whip-like, a form
called leptocercal.

=The Pectoral Limb.=--The four limbs of the fish are represented by
the paired fins. The anterior limb is represented by the pectoral
fin and its basal elements with the shoulder-girdle, which in the
bony fishes reaches a higher degree of complexity than in any other
vertebrates. It is in connection with the shoulder-girdle that the
greatest confusion in names has occurred. This is due to an attempt
to homologize its parts with the shoulder-girdle (scapula, coracoid,
and clavicle) of higher vertebrates. But it is not evident that a
bony fish possesses a real scapula, coracoid, or even clavicle. The
parts of its shoulder-girdle are derived by one line of descent from
the undifferentiated elements of the cartilaginous shoulder-girdle of
ancestral crossopterygian or dipnoan forms. From a similar ancestry by
another line of differentiation has come the amphibian and reptilian
shoulder-girdle and its derivative, the girdle of birds and mammals.

=The Shoulder-girdle.=--In the higher fishes the uppermost bone of the
shoulder-girdle is called the _post-temporal_ (_suprascapula_) (53).
In the striped bass and in most fishes this bone is jointed to the
temporal region of the cranium. Sometimes, as in the trigger-fishes, it
is grown fast to the skull, but it usually rests lightly with the three
points of its upper end. In sharks and skates the shoulder-girdle,
which is formed of a continuous cartilage, does not touch the skull. In
the eels and their allies, it has, by degradation, lost its connection
and the post-temporal rests in the flesh behind the cranium.

The post-temporal sometimes projects behind through the skin and
may bear spines or serrations. In front of the post-temporal and a
little to the outside of it is the small _supratemporal_ (52) also
usually connecting the shoulder-girdle with the skull. Below the
post-temporal, extending downward and backward, is the flattish
_supraclavicle_ (_posterotemporal_) (54). To this is joined the long
_clavicle_ (_proscapula_) (55), which runs forward and downward in
the bony fishes, meeting its fellow on the opposite side in a manner
suggesting the wishbone of a fowl. Behind the base of the clavicle,
the sword-shaped post-clavicle (56) extends downward through the
muscles behind the base of the pectoral fin. In some fishes, as the
stickleback and the trumpet-fish, a pair of flattish or elongate bones
called _interclavicles_ (_infraclavicles_) lie between and behind the
lower part of the clavicle. These are not found in most fishes and are
wanting in the striped bass. They are probably in all cases merely
extensions of the hypocoracoid.

[Illustration: FIG. 35.--Inner view of shoulder-girdle of the
Buffalo-fish, _Ictiobus bubalus_ Rafinesque, showing the mesocoracoid
(59). (After Starks.)]

Two flat bones side by side lie at the base of the pectoral fin, their
anterior edges against the upper part of the clavicle. These are the
_hypercoracoid_ (57), above, and _hypocoracoid_ (58), below. These
have been variously called scapula, coracoid, humerus, radius, and
ulna, but being found in the higher fishes only and not in the higher
vertebrates, they should receive names not used for other structures.
The hypercoracoid is usually pierced by a round foramen or fenestra,
but in some fishes (cods, weavers) the fenestra is between the two
bones. Attached to the hypercoracoid in the striped bass are four
little bones shaped like an hour-glass. These are the _actinosts_ (60)
(_carpals_ or _pterygials_), which support the rays of the pectoral fin
(61). In most bony fishes these are placed much as in the striped bass,
but in certain specialized or aberrant forms their form and position
are greatly altered.

In the anglers (_Pediculati_) the "carpals" are much elongated, forming
a kind of arm, by which the fish can execute a motion not unlike
walking.

In the Alaska blackfish (_Dallia pectoralis_) the two coracoids are
represented by a thin, cartilaginous plate, imperfectly divided, and
there are no actinosts. In almost all bony fishes, however, these bones
are well differentiated and distinct. In most of the soft-rayed fishes
an additional V-shaped bone or arch exists on the inner surface of the
shoulder-girdle near the insertion of the hypercoracoid. This is known
as the _mesocoracoid_ (59). It is not found in the striped bass, but
is found in the carp, catfish, salmon, and all their allies.

[Illustration: FIG. 36.--Sargassum-fish, _Pterophryne tumida_ (Osbeck).
One of the Anglers. Family _Antennariidae_.]

[Illustration: FIG. 37.--Shoulder-girdle of _Sebastolobus alascanus_
Gilbert. (After Starks.)

  POT.   Post-temporal.
  CL.    Clavicle.
  PCL.   Postclavicle.
  HYC.   Hypercoracoid.
  HYPC.  Hypocoracoid.]

=The Posterior Limbs.=--The posterior limb or ventral fin (63) is
articulated to a single bone on either side, the _pelvic girdle_ (62).

[Illustration: FIG. 38.--Cranium of _Sebastolobus alascanus_ Gilbert.
(After Starks.)

  V.    Vomer.
  N.    Nasal.
  E.    Ethmoid.
  PF.   Prefrontal.
  FR.   Frontal.
  PAS.  Parasphenoid.
  ALS.  Alisphenoid.
  P.    Parietal.
  BA.   Basisphenoid.
  PRO.  Prootic.
  BO.   Basioccipital.
  SO.   Supraoccipital.
  EO.   Exoccipital.
  EPO.  Epiotic.
  SPO.  Sphenotic.
  PTO.  Pterotic.]

In the shark the pelvic girdle is rather largely developed, but in
the more specialized fishes it loses its importance. In the less
specialized of the bony fishes the pelvis is attached at a distance
from the head among the muscles of the side, and free from the
shoulder-girdle and other parts of the skeleton. The ventral fins
are then said to be abdominal. When very close to the clavicle, but
not connected with it, as in the mullet, the fin is still said to be
abdominal or subabdominal. In the striped bass the pelvis is joined by
ligament between the clavicles, near their tip. The ventral fins thus
connected, as seen in most spiny-rayed fishes, are said to be thoracic.
In certain forms the pelvis is thrown still farther forward and
attached at the throat or even to the chin. When the ventral fins are
thus inserted before the shoulder-girdle, they are said to be jugular.
Most of the fishes with spines in the fins have thoracic ventrals.
In the fishes with jugular ventrals these fins have begun a process
of degeneration by which the spines or soft rays or both are lost or
atrophied.

[Illustration: FIG. 39.--Lower jaw and palate of _Sebastolobus
alascanus_. (After Starks.)

  PA.    Palatine.
  MSPT.  Mesopterygoid.
  PT.    Pterygoid.
  MPT.   Metapterygoid.
  D.     Dentary.
  AR.    Articular.
  AN.    Angular.
  Q.     Quadrate.
  SY.    Symplectic.
  HM.    Hyomandibular.
  POP.   Preopercle.
  IOP.   Interopercle.
  SOP.   Subopercle.
  OP.    Opercle.]

=Degeneration.=--By degeneration or degradation in biology is meant
merely a reduction to a lower degree of complexity or specialization
in structure. If in the process of development of the individual some
particular organ loses its complexity it is said to be degenerate. If
in the geological history of a type the same change takes place the
same term is used. Degeneration in this sense is, like specialization,
a phase of adaptation. It does not imply disease, feebleness, or
mutilation, or any tendency toward extinction. It is also necessary to
distinguish clearly phases of primitive simplicity from the apparent
simplicity resulting from degeneration.

=The Skeleton in Primitive Fishes.=--To learn the names of bones we can
deal most satisfactorily with the higher fishes, those in which the
bony framework has attained completion. But to understand the origin
and relation of parts we must begin with the lowest types, tracing the
different stages in the development of each part of the system.

[Illustration: FIG. 40.--Maxillary and premaxillary of _Sebastolobus
alascanus_. M, maxillary; PM, premaxillary.]

In the lancelets (_Leptocardii_), the vertebral column consists
simply of a gelatinous notochord extending from one end of the fish
to the other, and pointed at both ends, no skull being developed. The
notochord never shows traces of segmentation, although cartilaginous
rods above it are thought to forecast apophyses. In these forms there
is no trace of jaws, limbs, or ribs.

[Illustration: FIG. 41.--Part of skeleton of _Selene vomer_ (Linnaeus).]

In the embryo of the bony fish a similar notochord precedes the
segmentation and ossification of the vertebral column. In most of the
extinct types of fishes a notochord more or less modified persisted
through life, the vertebrae being strung upon it spool fashion in
various stages of development. In the Cyclostomi (lampreys and
hagfishes) the limbs and lower jaw are still wanting, but a distinct
skull is developed. The notochord is still present, but its anterior
pointed end is wedged into the base of a cranial capsule, partly
membranous, partly cartilaginous. There is no trace of segmentation
in the notochord itself in these or any other fishes, but neutral
arches are foreshadowed in a series of cartilages on each side of the
spinal chord. The top of the head is protected by broad plates. There
are ring-like cartilages supporting the mouth and other cartilages in
connection with the tongue and gill structures.

[Illustration: FIG. 42.--Hyostylic skull of _Chiloscyllium indicum_, a
Scyliorhinoid Shark. (After Parker and Haswell.)]

[Illustration: FIG. 43.--Skull of _Heptranchias indicus_ (Gmelin), a
notidanoid shark. (After Parker and Haswell.)]

[Illustration: FIG. 44.--Basal bones of pectoral fin of Monkfish,
_Squatina_. (After Zittel.)]

=The Skeleton of Sharks.=--In the Elasmobranchs (sharks, rays,
chimaeras) the tissues surrounding the notochord are segmented and
in most forms distinct vertebrae are developed. Each of these has a
conical cavity before and behind, with a central canal through which
the notochord is continued. The form and degree of ossification of
these vertebrae differ materially in the different groups. The skull
in all these fishes is cartilaginous, forming a continuous undivided
box containing the brain and lodging the organs of sense. To the
skull in the shark is attached a suspensorium of one or two pieces
supporting the mandible and the hyoid structures. In the chimaera the
mandible is articulated directly with the skull, the hyomandibular
and quadrate elements being fused with the cranium. The skull in
such case is said to be _autostylic_, that is, with self-attached
mandible. In the shark it is said to be _hyostylic_, the hyomandibular
intervening. The upper jaw in the shark consists not of maxillary
and premaxillary but of palatine elements, and the two halves of the
lower jaw are representatives of Meckel's cartilage, which is the
cartilaginous centre of the dentary bone in the bony fishes. These
jaw-bones in the higher fishes are in the nature of membrane bones, and
in the sharks and their relatives all such bones are undeveloped. The
hyoid structures are in the shark relatively simple, as are also the
gill-arches, which vary in number. The vertical fins are supported by
interneural and interhaemal cartilages, to which the soft fin-rays are
attached without articulation.

[Illustration: FIG. 45.--Pectoral fin of _Heterodontus philippi_. (From
nature.)]

[Illustration: FIG. 46.--Pectoral fin of _Heptranchias indicus_
(Gmelin). (After Dean.)]

The shoulder-girdle is made of a single cartilage, touching the
back-bone at a distance behind the head. To this cartilage three
smaller ones are attached, forming the base of the pectoral fin. These
are called _mesopterygium_, _propterygium_, and _metapterygium_, the
first named being in the middle and more distinctly basal. These three
segments are subject to much variation. Sometimes one of them is
wanting; sometimes two are grown together. Behind these the fin-rays
are attached. In most of the skates the shoulder-girdle is more closely
connected with the anterior vertebrae, which are more or less fused
together.

[Illustration: FIG. 47.--Shoulder-girdle of a Flounder, _Paralichthys
californicus_ (Ayres).]

The pelvis, remote from the head, is formed, in the shark, of a single
or paired cartilage with smaller elements at the base of the fin-rays.
In the males a cartilaginous generative organ, known as the clasper, is
attached to the pelvis and the ventral fins. In the Elasmobranchs the
tail vertebrae are progressively smaller backward. If a caudal fin is
present, the last vertebrae are directed upward (_heterocercal_) and the
greater part of the fin is below the axis. In other forms (sting-rays)
the tail degenerates into a whip-like organ (_leptocercal_), often
without fins. In certain primitive sharks (Ichthyotomi), as well as in
the Dipnoi and Crossopterygii, the tail is _diphycercal_, the vertebrae
growing progressively smaller backward and not bent upward toward the
tip.

In the chimaeras (_Holocephali_) the notochord persists and is
surrounded by a series of calcified rings. The palate with the
suspensorium is coalesced with the skull, and the teeth are grown
together into bony plates.

[Illustration: FIG. 48.--Shoulder-girdle of a Toadfish, _Batrachoides
pacifici_ (Guenther).]

[Illustration: FIG. 49.--Shoulder-girdle of a Garfish, _Tylosurus
fodiator_ (Jordan and Gilbert).]

=The Archipterygium.=--The Dipnoans, Crossopterygians, and Ganoids
represent various phases of transition from the ancient cartilaginous
types to the modern bony fishes.

In the Ichthyotomous sharks, Dipnoans, and Crossopterygians the
segments of the pectoral limb are arranged axially, or one beyond
another. This type of fin has been called _archipterygium_ by
Gegenbaur, on the theory that it represents the condition shown on the
first appearance of the pectoral fin. This theory is now seriously
questioned, but it will be convenient to retain the name for the
pectoral fin with segmented axis fringed on one or both sides by soft
rays.

[Illustration: FIG. 50.--Shoulder-girdle of a Hake, _Merluccius
productus_ (Ayres).]

The archipterygium of the Dipnoan genus _Neoceratodus_ is thus
described by Dr. Guenther ("Guide to the Study of Fishes," p. 73): "The
pectoral limb is covered with small scales along the middle from the
root to the extremity, and is surrounded by a rayed fringe similar to
the rays of the vertical fins. A muscle split into numerous fascicles
extends all the length of the fin, which is flexible in every part and
in every direction. The cartilaginous framework supporting it is joined
to the scapular arch by a broad basal cartilage, generally single,
sometimes showing traces of a triple division. Along the middle of
the fin runs a jointed axis gradually becoming smaller and thinner
towards the extremity. Each joint bears on each side a three-, two-, or
one-jointed branch."

In the genus _Lepidosiren_, also a Dipnoan, the pectoral limb has the
same axial structure, but is without fin-rays, although in the breeding
season the posterior limb or ventral fin in the male is covered with
a brush of fine filaments. This structure, according to Prof. J. G.
Kerr,[3] is probably without definite function, but belongs to the
"category of modifications so often associated with the breeding season
(cf. the newts' crest) commonly called ornamental, but which are
perhaps more plausibly looked upon as expressions of the intense vital
activity of the organisms correlated with its period of reproductive
activity." Professor Kerr, however, thinks it not unlikely that this
brush of filaments with its rich blood-supply may serve in the function
of respiration, a suggestion first made by Professor Lankester.

FOOTNOTES:

[3] Philos. Trans., Lond., 1900.




CHAPTER V

MORPHOLOGY OF THE FINS


=Origin of the Fins of Fishes.=--One of the most interesting problems
in vertebrate morphology, and one of the most important from its
wide-reaching relations, is that of the derivation of the fins of
fishes. This resolves itself at once into two problems, the origin of
the median fins, which appear in the lancelets, at the very bottom of
the fish-like series, and the origin of the paired fins or limbs, which
are much more complex, and which first appear with the primitive sharks.

In this study the problem is to ascertain not what theoretically should
happen, but what, as a matter of fact, has happened in the early
history of the fish-like groups. That these structures, with the others
in the fish body, have sprung from simple origins, growing more complex
with the demands of varied conditions, and then at times again simple,
through degeneration, there can be no doubt. It is also certain that
each structure must have had some element of usefulness in all its
stages. In such studies we have, as Haeckel has expressed it, "three
ancestral documents, paleontology, morphology, and ontogeny"--the
actual history as shown by fossil remains, the sidelight derived from
comparison of structures, and the evidence of the hereditary influences
shown in the development of the individual. As to the first of these
ancestral documents, the evidence of paleontology is conclusive where
it is complete. But in very few cases are we sure of any series of
details. The records of geology are like a book with half its leaves
torn out, the other half confused, displaced, and blotted. Still
each record actually existing represents genuine history, and in
paleontology we must in time find our final court of appeal in all
matters of biological origins.

The evidence of comparative anatomy is most completely secured, but
it is often indecisive as to relative age and primitiveness of
origin among structures. As to ontogeny, it is, of course, true that
through heredity "the life-history of the individual is an epitome
of the life-history of the race." "Ontogeny repeats phylogeny," and
phylogeny, or line of descent of organisms and structures, is what we
are seeking. But here the repetition is never perfect, never nearly so
perfect in fact as Haeckel and his followers expected to find it. The
demands of natural selection may lead to the lengthening, shortening,
or distortion of phases of growth, just as they may modify adult
conditions. The interpolation of non-ancestral stages is recognized
in several groups. The conditions of the individual development may,
therefore, furnish evidence in favor of certain theories of origins,
but they cannot alone furnish the absolute proof.

In the process of development the median or vertical fins are doubtless
older than the paired fins or limbs, whatever be the origin of the
latter. They arise in a dermal keel which is developed in a web fitting
and accentuating the undulatory motion of the body. In the embryo of
the fish the continuous vertical fin from the head along the back and
around the tail precedes any trace of the paired fins.

In this elementary fin-fold slender supports, the rudiments of
fin-rays, tend to appear at intervals. These are called by Ryder
ray-hairs or actinotrichia. They are the prototype of fin-rays in the
embryo fish, and doubtless similarly preceded the latter in geological
time. In the development of fishes the caudal fin becomes more and more
the seat of propulsion. The fin-rays are strengthened, their basal
supports are more and more specialized, and the fin-fold ultimately
divides into distinct fins, the longest rays developed where most
needed.

That the vertical fins, dorsal, anal, and caudal, have their origin in
a median fold of the skin admits of no question. In the lowest forms
which bear fins these structures are dermal folds, being supported
by very feeble rays. Doubtless at first the vertical fins formed a
continuous fold, extending around the tail, this fold ultimately
broken, by atrophy of parts not needed, into distinct dorsal, anal,
and caudal fins. In the lower fishes, as in the earlier sharks, there
is an approach to this condition of primitive continuity, and in the
embryos of almost all fishes the same condition occurs. Dr. John A.
Ryder points out the fact that there are certain unexplained exceptions
to this rule. The sea-horse, pipefish, and other highly modified forms
do not show this unbroken fold, and it is wanting in the embryo of
the top-minnow, _Gambusia affinis_. Nevertheless the existence of a
continuous vertical fold in the embryo is the rule, almost universal.
The codfish with three dorsals, the Spanish mackerel with dorsal and
anal finlets, the herring with one dorsal, the stickleback with a
highly modified one, all show this character, and we may well regard it
as a certain trait of the primitive fish. This fold springs from the
ectoblast or external series of cells in the embryo. The fin-rays and
bony supports of the fins spring from the mesoblast or middle series
of cells, being thrust upward from the skeleton as supports for the
fin-fold.

=Origin of the Paired Fins.=--The question of the origin of the paired
fins is much more difficult and is still far from settled, although
many, perhaps the majority of recent writers favor the theory that
these fins are parts of a once continuous lateral fold of skin,
corresponding to the vertical fold which forms the dorsal, anal, and
caudal. In this view the lateral fold, at first continuous, became soon
atrophied in the middle, while at either end it is highly specialized,
at first into an organ of direction, then into fan-shaped and later
paddle-shaped organs of locomotion. According to another view, the
paired fins originated from gill structures, originally both close
behind the head, the ventral fin migrating backward with the progress
of evolution of the species.

=Evidence of Paleontology.=--If we had representations of all the early
forms of fishes arranged in proper sequence, we could decide once for
all, by evidence of paleontology, which form of fin appears first and
what is the order of appearance. As to this, it is plain that we do
not know the most primitive form of fin. Sharks of unknown character
must have existed long before the earliest remains accessible to us.
Hence the evidence of paleontology seems conflicting and uncertain. On
the whole it lends most support to the fin-fold theory. In the later
Devonian, a shark, _Cladoselache fyleri_, is found in which the paired
fins are lappet-shaped, so formed and placed as to suggest their
origin from a continuous fold of skin. In this species the dorsal fins
show much the same form. Other early sharks, constituting the order of
_Acanthodei_, have fins somewhat similar, but each preceded by a stiff
spine, which may be formed from coalescent rays.

[Illustration: FIG. 51.--_Cladoselache fyleri_ (Newberry), restored.
Upper Devonian of Ohio. (After Dean.)]

[Illustration: FIG. 52.--Fold-like pectoral and ventral fins of
_Cladoselache fyleri_. (After Dean.)]

Long after these appears another type of sharks represented by
_Pleuracanthus_ and _Cladodus_, in which the pectoral fin is a jointed
organ fringed with rays arranged serially in one or two rows. This
form of fin has no resemblance to a fold of skin, but accords better
with Gegenbaur's theory that the pectoral limb was at first a modified
gill-arch. In the Coal Measures are found also teeth of sharks
(_Orodontidae_) which bear a strong resemblance to still existing forms
of the family of _Heterodontidae_, which originates in the Permian. The
existing _Heterodontidae_ have the usual specialized form of shark-fin,
with three of the basal segments especially enlarged and placed side
by side, the type seen in modern sharks. Whatever the primitive
form of shark-fin, it may well be doubted whether any one of these
three (_Cladoselache_, _Pleuracanthus_, or _Heterodontus_) actually
represents it. The beginning is therefore unknown, though there is
some evidence that _Cladoselache_ is actually more nearly primitive
than any of the others. As we shall see, the evidence of comparative
anatomy may be consistent with either of the two chief theories, while
that of ontogeny or embryology is apparently inconclusive, and that of
paleontology is apparently most easily reconciled with the theory of
the fin-fold.

[Illustration: FIG. 53.--Pectoral fin of shark, _Chiloscyllium_. (After
Parker and Haswell.)]

=Development of the Paired Fins in the Embryo.=--According to Dr.
John A. Ryder ("Embryography of Osseous Fishes," 1882) "the paired
fins in Teleostei arise locally, as short longitudinal folds, with
perhaps a few exceptions. The pectorals of _Lepisosteus_ originate in
the same way. Of the paired fins, the pectoral or anterior pair seems
to be the first to be developed, the ventral or pelvic pair often not
making its appearance until after the absorption of the yolk-sac has
been completed, in other cases before that event, as in _Salmo_ and
in _Gambusia_. The pectoral fin undergoes less alteration of position
during its evolution than the posterior pair."

In the codfish (_Gadus callarias_) the pectoral fin-fold "appears as a
slight longitudinal elevation of the skin on either side of the body of
the embryo a little way behind the auditory vesicles, and shortly after
the tail of the embryo begins to bud out. At the very first it appears
to be merely a dermal fold, and in some forms a layer of cells extends
out underneath it from the sides of the body, but does not ascend
into it. It begins to develop as a very low fold, hardly noticeable,
and, as growth proceeds, its base does not expand antero-posteriorly,
but tends rather to become narrowed, so that it has a pedunculated
form. With the progress of this process the margin of the fin-fold
also becomes thinner at its distal border, and at the basal part
mesodermal cells make their appearance more noticeably within the inner
contour-line. The free border of the fin-fold grows out laterally and
longitudinally, expanding the portion outside of the inner contour-line
of the fin into a fan-shape. This distal thinner portion is at first
without any evidence of rays; further than that there is a manifest
tendency to a radial disposition of the histological elements of the
fin."

The next point of interest is found in the change of position of the
pectoral fin by a rotation on its base. This is associated with changes
in the development of the fish itself. The ventral fin is also, in most
fishes, a short horizontal fold and just above the preanal part of the
median vertical fold which becomes anal, caudal, and dorsal. But in
the top-minnow (_Gambusia_), of the order Haplomi, the ventral first
appears as "a little papilla and not as a fold, where the body-walls
join the hinder upper portion of the yolk-sac, a very little way in
front of the vent." "These two modes of origin," observes Dr. Ryder,
"are therefore in striking contrast and well calculated to impress us
with the protean character of the means at the disposal of Nature to
achieve one and the same end."

=Current Theories as to Origin of Paired Fins.=--There are three chief
theories as to the morphology and origin of the paired fins. The
earliest is that of Dr. Karl Gegenbaur, supported by various workers
among his students and colleagues. In his view the pectoral and ventral
fins are derived from modifications of primitive gill-arches. According
to this theory, the skeletal arrangements of the vertebrate limb are
derived from modifications of one primitive form, a structure made up
of successive joints, with a series of fin-rays on one or both sides
of it. To this structure Gegenbaur gives the name of archipterygium.
It is found in the shark, _Pleuracanthus_, in _Cladodus_, and in all
the Dipnoan and Crossopterygian fishes, its primitive form being still
retained in the Australian genus of Dipnoans, _Neoceratodus_. This
biserial archipterygium with its limb-girdle is derived from a series
of gill-rays attached to a branchial arch. The backward position of the
ventral fin is due to a succession of migrations in the individual and
in the species.

As to this theory, Mr. J. Graham Kerr observes:

[Illustration: FIG. 54.--Skull and shoulder-girdle of _Neoceratodus
forsteri_ (Guenther), showing the archipterygium.]

"The Gegenbaur theory of the morphology of vertebrate limbs thus
consists of two very distinct portions. The first, that the
archipterygium is the ground-form from which all other forms of
presently existing fin skeletons are derived, concerns us only
indirectly, as we are dealing here only with the _origin_ of the limbs,
i.e., their origin from other structures that were not limbs.

"It is the second part of the view that we have to do with, that
deriving the archipterygium, the skeleton of the primitive paired fin,
from a series of gill-rays and involving the idea that the limb itself
is derived from the septum between two gill-clefts.

"This view is based on the skeletal structures within the fin. It
rests upon (1) the assumption that the archipterygium is the primitive
type of fin, and (2) the fact that amongst the Selachians is found a
tendency for one branchial ray to become larger than the others, and,
when this has happened, for the base of attachment of neighboring rays
to show a tendency to migrate from the branchial arch on to the base
of the larger or, as we may call it, primary ray; a condition coming
about which, were the process to continue rather farther than it is
known to do in actual fact, would obviously result in a structure
practically identical with the archipterygium. Gegenbaur suggests that
the archipterygium actually has arisen in this way in phylogeny."

[Illustration: FIG. 55.--_Acanthoessus wardi_ (Egerton). Carboniferous.
Family _Acanthoessidae_. (After Woodward.)]

[Illustration: FIG. 56.--Shoulder-girdle of _Acanthoessus_. (After
Dean.)]

[Illustration: FIG. 57.--Pectoral fin of _Pleuracanthus_. (After Dean.)]

The fin-fold theory of Balfour, adopted by Dohrn, Weidersheim, Thacher,
Mivart, Ryder, Dean, Boulenger, and others, and now generally accepted
by most morphologists as plausible, is this: that "The paired limbs
are persisting and exaggerated portions of a fin-fold once continuous,
which stretched along each side of the body and to which they bear an
exactly similar phylogenetic relation as do the separate dorsal and
anal fins to the once continuous median fin-fold."

"This view, in its modern form, was based by Balfour on his observation
that in the embryos of certain Elasmobranchs the rudiments of the
pectoral and pelvic fins are at a very early period connected together
by a longitudinal ridge of thickened epiblast--of which indeed they
are but exaggerations. In Balfour's own words referring to these
observations: 'If the account just given of the development of the limb
is an accurate record of what really takes place, it is not possible
to deny that some light is thrown by it upon the first origin of the
vertebrate limbs. The facts can only bear one interpretation, viz.,
that the limbs are the remnants of continuous lateral fins.'

[Illustration: FIG. 58.--Shoulder-girdle of _Polypterus bichir_.
Specimen from the White Nile.]

"A similar view to that of Balfour was enunciated almost synchronously
by Thacher and a little later by Mivart--in each case based on
anatomical investigation of Selachians--mainly relating to the
remarkable similarity of the skeletal arrangements in the paired and
unpaired fins."

A third theory is suggested by Mr. J. Graham Kerr (_Cambridge Philos.
Trans._, 1899), who has recently given a summary of the theories on
this subject. Mr. Kerr agrees with Gegenbaur as to the primitive nature
of the archipterygium, but believes that it is derived, not from the
gill-septum, but from an external gill. Such a gill is well developed
in the young of all the living sharks, Dipnoans and Crossopterygians,
and in the latter types of fishes it has a form analogous to that of
the archipterygium, although without bony or cartilaginous axis.

We may now take up the evidence in regard to each of the different
theories, using in part the language of Kerr, the paragraphs in
quotation-marks being taken from his paper. We may first consider
Balfour's theory of the lateral fold.

=Balfour's Theory of the Lateral Fold.=--"The evidence in regard to
this view may be classed under three heads, as ontogenetic, comparative
anatomical, and paleontological. The ultimate fact on which it was
founded was Balfour's discovery that in certain Elasmobranch embryos,
but especially in _Torpedo_ (_Narcobatus_), the fin rudiments were,
at an early stage, connected by a ridge of epiblast. I am not able
to make out what were the other forms in which Balfour found this
ridge, but subsequent research, in particular by Mollier, a supporter
of the lateral-fold view, is to the effect that it does not occur in
such ordinary sharks as _Pristiurus_ and _Mustelus_, while it is to
be gathered from Balfour himself that it does not occur in _Scyllium_
(_Scyliorhinus_).

"It appears to me that the knowledge we have now that the longitudinal
ridge is confined to the rays and absent in the less highly specialized
sharks greatly diminishes its security as a basis on which to rest a
theory. In the rays, in correlation with their peculiar mode of life,
the paired fins have undergone (in secondary development) enormous
extension along the sides of the body, and their continuity in the
embryo may well be a mere foreshadowing of this.

[Illustration: FIG. 59.--Arm of a frog.]

"An apparently powerful support from the side of embryology came
in Dohrn and Rabl's discoveries that in _Pristiurus_ all the
interpterygial myotomes produce muscle-buds. This, however, was
explained away by the Gegenbaur school as being merely evidence of the
backward migration of the hind limb--successive myotomes being taken
up and left behind again as the limb moved farther back. As either
explanation seems an adequate one, I do not think we can lay stress
upon this body of facts as supporting either one view or the other.
The facts of the development of the skeleton cannot be said to support
the fold view; according to it we should expect to find a series of
metameric supporting rays produced which later on become fused at
their bases. Instead of this we find a _longitudinal_ bar of cartilage
developing quite continuously, the rays forming as projections from its
outer side.

"The most important evidence for the fold view from the side of
comparative anatomy is afforded by (1) the fact that the limb derives
its nerve supply from a large number of spinal nerves, and (2) the
extraordinary resemblance met with between the skeletal arrangements
of paired and unpaired fins. The believers in the branchial arch
hypothesis have disposed of the first of these in the same way as they
did the occurrence of interpterygial myotomes, by looking on the nerves
received from regions of the spinal cord anterior to the attachment of
the limb as forming a kind of trail marking the backward migration of
the limb.

"The similarity in the skeleton is indeed most striking, though
its weight as evidence has been recently greatly diminished by the
knowledge that the apparently metameric segmentation of the skeletal
and muscular tissues of the paired fins is quite secondary and does not
at all agree with the metamery of the trunk. What resemblance there is
may well be of a homoplastic character when we take into account the
similarity in function of the median and unpaired fins, especially in
such forms as _Raja_, where the anatomical resemblances are especially
striking. There is a surprising dearth of paleontological evidence in
favor of this view."

The objection to the first view is its precarious foundation. Such
lateral folds are found only in certain rays, in which they may be
developed as a secondary modification in connection with the peculiar
form of these fishes. Professor Kerr observes that this theory must be
looked upon and judged: "Just as any other view at the present time
regarding the nature of the vertebrate limb, rather as a speculation,
brilliant and suggestive though it be, than as a logically constructed
theory of the now known facts. It is, I think, on this account
allowable to apply to it a test of a character which is admittedly very
apt to mislead, that of 'common sense.'

"If there is any soundness in zoological speculation at all, I think
it must be admitted that the more primitive vertebrates were creatures
possessing a notochordal axial skeleton near the dorsal side, with the
main nervous axis above it, the main viscera below it, and the great
mass of muscle lying in myotomes along its sides. Now such a creature
is well adapted to movements of the character of lateral flexure, and
not at all for movements in the sagittal plane--which would be not only
difficult to achieve, but would tend to alternately compress and extend
its spinal cord and its viscera. Such a creature would swim through the
water as does a Cyclostome, or a _Lepidosiren_, or any other elongated
vertebrate without special swimming organs. Swimming like this,
specialization for more and more rapid movement would mean flattening
of the tail region and is extension into an at first not separately
mobile median tail-fold. It is extremely difficult to my mind to
suppose that a new purely _swimming_ arrangement should have arisen
involving up-and-down movement, and which, at its first beginnings,
while useless as a swimming organ itself, must greatly detract from the
efficiency of that which already existed."

=Objections to Gegenbaur's Theory.=--We now return to the Gegenbaur
view--that the limb is a modified gill-septum.

"Resting on Gegenbaur's discovery already mentioned, that the gill-rays
in certain cases assume an arrangement showing great similarity to that
of the skeletal elements of the archipterygium, it has, so far as I
am aware, up to the present time received no direct support whatever
of a nature comparable with that found for the rival view in the fact
that, in certain forms at all events, the limbs actually do arise in
the individual in the way that the theory holds they did in phylogeny.
No one has produced either a form in which a gill-septum becomes the
limb during ontogeny, or the fossil remains of any form which shows an
intermediate condition.

"The portion of Gegenbaur's view which asserts that the biserial
archipterygial fin is of an extremely primitive character is supported
by a large body of anatomical facts, and is rendered further probable
by the great frequency with which fins apparently of this character
occur amongst the oldest known fishes. On the lateral-fold view we
should have to regard these as independently evolved, which would
imply that fins of this type are of a very perfect character, and in
that case we may be indeed surprised at their so complete disappearance
in the more highly developed forms, which followed later on."

[Illustration: FIG. 60.--_Pleuracanthus decheni_ (Goldfuss). (After
Dean.)]

As to Gegenbaur's theory it is urged that no form is known in which a
gill-septum develops into a limb during the growth of the individual.
The main thesis, according to Professor Kerr, "that the archipterygium
was derived from gill-rays, is supported only by evidence of an
indirect character. Gegenbaur in his very first suggestion of his
theory pointed out, as a great difficulty in the way of its acceptance,
the position of the limbs, especially of the pelvic limbs, in a
position far removed from that of the branchial arches. This difficulty
has been entirely removed by the brilliant work of Gegenbaur's
followers, who have shown from the facts of comparative anatomy and
embryology that the limbs, and the hind limbs especially, actually
have undergone, and in ontogeny do undergo, an extensive backward
migration. In some cases Braus has been able to find traces of this
migration as far forward as a point just behind the branchial arches.
Now, when we consider the numbers, the enthusiasm, and the ability of
Gegenbaur's disciples, we cannot help being struck by the fact that the
_only_ evidence in favor of this derivation of the limbs has been that
which tends to show that a migration of the limbs backwards has taken
place from a region somewhere near the last branchial arch, and that
they have failed utterly to discover any intermediate steps between
gill-rays and archipterygial fin. And if for a moment we apply the test
of common sense we cannot but be impressed by the improbability of the
evolution of a gill-septum, which in all the lower forms of fishes is
fixed firmly in the body-wall, and beneath its surface, into an organ
of locomotion.

[Illustration: FIG. 61.--Embryos of _Heterodontus japonicus_ Maclay and
Macleay, a Cestraciont shark, showing the backward migration of the
gill-arches and the forward movement of the pectoral fin. _a_, _b_,
_c_, representing different stages of growth. (After Dean.)]

"May I express the hope that what I have said is sufficient to show in
what a state of uncertainty our views are regarding the morphological
nature of the paired fins, and upon what an exceedingly slender basis
rest both of the two views which at present hold the field?"

As to the backward migration of the ventral fins, Dr. Bashford Dean has
recently brought forward evidence from the embryo of a very ancient
type of shark (_Heterodontus japonicus_) that this does not actually
occur in that species. On the other hand, we have a forward migration
of the pectoral fin, which gradually takes its place in advance
of the hindmost gill-arches. The accompanying cut is from Dean's
paper, "Biometric Evidence in the Problem of the Paired Limbs of the
Vertebrates" (American Naturalist for November, 1902). Dean concludes
that in _Heterodontus_ "there is no evidence that there has ever been a
migration of the fins in the Gegenbaurian sense." "The gill region, at
least in its outer part, shows no affinity during proportional growth
with the neighboring region of the pectoral fin. In fact from an early
stage onward, they are evidently growing in opposite directions."

=Kerr's Theory of Modified External Gills.=--"It is because I feel
that in the present state of our knowledge neither of the two views I
have mentioned has a claim to any higher rank than that of extremely
suggestive speculations that I venture to say a few words for the third
view, which is avowedly a mere speculation.

"Before proceeding with it I should say that I assume the serial
homology of fore and hind limbs to be beyond dispute. The great and
deep-seated resemblances between them are such as to my mind seem not
to be adequately explicable except on this assumption.

"In the Urodela (salamanders) the external gills are well-known
structures--serially arranged projections from the body-wall near the
upper ends of certain of the branchial arches. When one considers the
ontogenetic development of these organs, from knob-like outgrowth
from the outer face of the branchial arch, covered with ectoderm and
possessing a mesoblastic core, and which frequently if not always
appear before the branchial clefts are open, one cannot but conclude
that they are morphologically projections of the outer skin and
that they have nothing whatever to do with the gill-pouches of the
gut-wall. Amongst the Urodela one such gill projects from each of the
first three branchial arches. In _Lepidosiren_ there is one on each
of the branchial arches I-IV. In _Polypterus_ and _Calamoichthys_
(_Erpetoichthys_) there is one on the hyoid arch. Finally, in many
Urodelan larvae we have present at the same time as the external gills
a pair of curious structures called balancers. At an early stage of my
work on _Lepidosiren_, while looking over other vertebrate embryos
and larvae for purposes of comparison, my attention was arrested by
these structures, and further examinations, by section or otherwise,
convinced me that there were serial homologues of the external gills,
situated on the mandibular arch. On then looking up the literature,
I found that I was by no means first in this view. Rusconi had long
ago noticed the resemblance, and in more recent times both Orr and
Maurer had been led to the same conclusion as I had been. Three
different observers having been independently led to exactly the same
conclusions, we may, I think, fairly enough regard the view I have
mentioned of the morphological nature of the balancers as probably a
correct one.

"Here, then, we have a series of homologous structures projecting
from each of the series of visceral arches. They crop up on the
Crossopterygii, the Dipnoi, and the Urodela, i.e., in three of the most
archaic of the groups of Gnathostomata. But we may put it in another
way. The groups in which they do not occur are those whose young
possess a very large yolk-sac (or which are admittedly derived from
such forms). Now wherever we have a large yolk-sac we have developed on
its surface a rich network of blood-vessels for purposes of nutrition.
But such a network _must necessarily_ act as an extraordinarily
efficient organ of respiration, and did we not know the facts we
might venture to prophesy that in forms possessing it any other small
skin-organ of respiration would tend to disappear.

"No doubt these external gills are absent also in a few of the
admittedly primitive forms such as, e.g., (_Neo-_) _Ceratodus_.
But I would ask that in this connection one should bear in mind
one of the marked characteristics of external gills--their great
regenerative power. This involves their being extremely liable to
injury and consequently a source of danger to their possessor. Their
absence, therefore, in certain cases may well have been due to natural
selection. On the other hand, the _presence_ in so many lowly forms
of these organs, the general close similarity in structure that runs
through them in different forms, and the exact correspondence in their
position and relations to the body can, it seems to me, _only_ be
adequately explained by looking on them as being homologous structures
inherited from a common ancestor and consequently of great antiquity in
the vertebrate stem."

As to the third theory, Professor Kerr suggests tentatively that the
external gill may be the structure modified to form the paired limbs.
Of the homology of fore and hind limbs and consequently of their like
origin there can be no doubt.

The general gill-structures have, according to Kerr, "the primary
function of respiration. They are also, however, provided with an
elaborate muscular apparatus comprising elevators, depressors, and
adductors, and larvae possessing them may be seen every now and then
to give them a sharp backward twitch. They are thus _potentially_
motor organs. In such a Urodele as _Amblystoma_ their homologues on
the mandibular arch are used as supporting structures against a solid
substratum exactly as are the limbs of the young _Lepidosiren_.

[Illustration: FIG. 62.--_Polypterus congicus_, a _Crossopterygian_
fish from the Congo River. Young, with external gills. (After
Boulenger.)]

"I have, therefore, to suggest that the more ancient Gnathostomata
possessed a series of potentially motor, potentially supporting
structures projecting from their visceral arches; it was inherently
extremely probable that these should be made use of when actual
supporting, and motor appendages had to be developed in connection with
clambering about a solid substratum. If this had been so, we should
look upon the limb as a modified external gill; the limb-girdle, with
Gegenbaur, as a modified branchial arch.

"This theory of the vertebrate paired limb seems to me, I confess, to
be a more plausible one on the face of it than either of the two which
at present hold the field. If untrue, it is so dangerously plausible
as to surely deserve more consideration than it appears to have had.
One of the main differences between it and the other two hypotheses
is that, instead of deriving the swimming-fin from the walking and
supporting limb, it goes the other way about. That this is the safer
line to take seems to me to be shown by the consideration that a very
small and rudimentary limb could _only_ be of use if provided with a
fixed _point d'appui_. Also on this view, the pentadactyle limb and the
swimming-fin would probably be evolved independently from a simple form
of limb. This would evade the great difficulties which have beset those
who have endeavored to establish the homologies of the elements of the
pentadactyle limb with those of any type of fully formed fin."

=Uncertain Conclusions.=--In conclusion we may say that the evidence
of embryology in this matter is inadequate, though possibly favoring
on the whole the fin-fold theory; that of morphology is inconclusive,
and probably the final answer may be given by paleontology. If the
records of the rocks were complete, they would be decisive. At
present we have to decide which is the more primitive of two forms
of pectoral fin actually known among fossils. That of _Cladoselache_
is a low, horizontal fold of skin, with feeble rays, called by Cope
_ptychopterygium_. That of _Pleuracanthus_ is a jointed paddle-shaped
appendage with a fringe of rays on either side. In the theory of
Gegenbaur and Kerr _Pleuracanthus_ must be, so far as the limbs are
concerned, the form nearest the primitive limb-bearing vertebrate. In
Balfour's theory _Cladoselache_ is nearest the primitive type from
which the other and with it the archipterygium of later forms may be
derived.

Boulenger and others question even this, believing that the
archipterygium in _Pleuracanthus_ and other primitive sharks and
that in _Neoceratodus_ and its Dipnoan and Crossopterygian allies
and ancestors have been derived independently, not the latter
from the former. In this view there is no real homology between
the archipterygium in the sharks possessing it and that in the
_Dipnoans_ and _Crossopterygians_. In the one theory the type of
_Pleuracanthus_ would be ancestral to the other sharks on the one
hand, and to Crossopterygians and all higher vertebrates on the other.
With the theory of the origin of the pectoral from a lateral fold,
_Pleuracanthus_ would be merely a curious specialized offshoot from the
primitive sharks, without descendants and without special significance
in phylogeny.

As elements bearing on this decision we may note that the tapering
unspecialized diphycercal tail of _Pleuracanthus_ seems very primitive
in comparison with the short heterocercal tail of _Cladoselache_. This
evidence, perhaps deceptive, is balanced by the presence on the head of
_Pleuracanthus_ of a highly specialized serrated spine, evidence of a
far from primitive structure. Certainly neither the one genus nor the
other actually represents the primitive shark. But as _Cladoselache_
appears in geological time, long before _Pleuracanthus_, _Cladodus_,
or any other shark with a jointed, archipterygial fin, the burden
of proof, according to Dean, rests with the followers of Gegenbaur.
If the remains found in the Ordovician at Canon City referred to
Crossopterygians are correctly interpreted, we must regard the shark
ancestry as lost in pre-Silurian darkness, for in sharks of some sort
the Crossopterygians apparently must find their remote ancestry.

[Illustration: FIG. 63.--Heterocercal tail of Sturgeon, _Acipenser
sturio_ (Linnaeus). (After Zittel.)]

=Forms of the Tail in Fishes.=--In the process of development the
median or vertical fins are, as above stated, older than the paired
fins or limbs, whatever be the origin of the latter. They arise in a
dermal keel, its membranes fitting and accentuating the undulatory
motion of the body.

In this elementary fin-fold slender supports (actinotrichia), the
rudiments of fin-rays, appear at intervals. In those fins of most
service in the movement of the fish, the fin-rays are strengthened, and
their basal supports specialized.

Dean calls attention to the fact that in fishes which swim, when
adult, by an undulatory motion, the paired fins tend to disappear, as
in the eel and in all eel-like fishes, as blennies and eel-pouts.

The form of the tail at the base of the caudal fin varies in the
different groups. In most primitive types, as in most embryonic
fishes, the vertebrae grow smaller to the last (diphycercal). In
others, also primitive, the end of the tail is directed upward, and
the most of the caudal fin is below it. Such a tail is seen in most
sharks, in the sturgeon, garpike, bowfin, and in the Ganoid fishes.
It is known as heterocercal, and finally in ordinary fishes the tail
becomes homocercal or fan-shaped, although usually some trace of the
heterocercal condition is traceable, gradually growing less with the
process of development.

Since Professor Agassiz first recognized, in 1833, the distinction
between the heterocercal and homocercal tail, this matter has been the
subject of elaborate investigation and a number of additional terms
have been proposed, some of which are in common use.

A detailed discussion of these is found in a paper by Dr. John A.
Ryder "On the Origin of Heterocercy" in the Report of the U. S. Fish
Commissioner for 1884. In this paper a dynamic or mechanical theory
of the causes of change of form is set forth, parts of this having a
hypothetical and somewhat uncertain basis.

Dr. Ryder proposes the name _archicercal_ to denote the cylindroidal
worm-like caudal end of the larva of fishes and amphibians before they
acquire median fin-folds. The term _lophocercal_ is proposed by Ryder
for the form of caudal fin which consists of a rayless fold of skin
continuous with the skin of the tail, the inner surfaces of this fold
being more or less nearly in contact. To the same type of tail Dr.
Jeffries Wyman in 1864 gave the name _protocercal_. This name was used
for the tail of the larval ray when it acquires median fin-folds. The
term implies, what cannot be far from true, that this form of tail is
the first in the stages of evolution of the caudal fin.

To the same type of tail Mr. Alexander Agassiz gave, in 1877, the name
of _leptocardial_, on the supposition that it represented the adult
condition of the lancelet. In this creature, however, rudimentary
basal rays are present, a condition differing from that of the early
embryos.

The diphycercal tail, as usually understood, is one in which the
end of the vertebral column bears "not only hypural but also epural
intermediary pieces which support rays." The term is used for the
primitive type of tail in which the vertebrae, lying horizontally, grow
progressively smaller, as in _Neoceratodus_, _Protopterus_, and other
Dipnoans and Crossopterygians. The term was first applied by McCoy to
the tails of the Dipnoan genera _Diplopterus_ and _Gyroptychius_, and
for tails of this type it should be reserved.

[Illustration: FIG. 64.--Heterocercal tail of Bowfin, _Amia calva_
(Linnaeus). (After Zittel.)]

[Illustration: FIG. 65.--Heterocercal tail of Garpike, _Lepisosteus
osseus_ (Linnaeus).]

The heterocercal tail is one in which the hindmost vertebrae are bent
upwards. The term is generally applied to those fishes only in which
this bending is considerable and is externally evident, as in the
sharks and Ganoids. The character disappears by degrees, changing
sometimes to diphycercal or leptocercal by a process of degeneration,
or in ordinary fishes becoming _homocercal_. Dr. Ryder uses the term
heterocercal for all cases in which any up-bending of the axis takes
place, even though it involves the modification of but a single
vertebra. With this definition, the tail of salmon, herring, and
even of most bony fishes would be considered heterocercal, and most
or all of these pass through a heterocercal stage in the course of
development. The term is, however, usually restricted to those forms in
which the curving of the axis is evident without dissection.

[Illustration: FIG. 66.--_Coryphaenoides carapinus_ (Goode and Bean),
showing leptocercal tail. Gulf Stream.]

The homocercal tail is the fan-shaped or symmetrical tail common
among the Teleosts, or bony fishes. In its process of development
the individual tail is first archicercal, then lophocercal, then
diphycercal, then heterocercal, and lastly homocercal. A similar order
is indicated by the sequence of fossil fishes in the rocks, although
some forms of diphycercal tail may be produced by degeneration of the
heterocercal tail, as suggested by Dr. Dollo and Dr. Boulenger, who
divide diphycercal tails into primitive and secondary.

The peculiar tapering tail of the cod, the vertebrae growing
progressively smaller behind, is termed _isocercal_ by Professor Cope.
This form differs little from diphycercal, except in its supposed
derivation from the homocercal type. A similar form is seen in eels.

[Illustration: FIG. 67.--Heterocercal tail of Young Trout, _Salmo
fario_ (Linnaeus). (After Parker and Haswell.)]

The term _leptocercal_ has been suggested by Gaudry, 1883, for those
tails in which the vertebral column ends in a point. We may, perhaps,
use it for all such as are attenuate, ending in a long point or whip,
as in the _Macrouridae_, or grenadiers, the sting-rays, and in various
degenerate members of almost every large group.

The term _gephyrocercal_ is devised by Ryder for fishes in which the
end of the vertebral axis is aborted in the adult, leaving the caudal
elements to be inserted on the end of this axis, thus bridging over
the interval between the vertical fins, as the name (~gephyros~,
bridge; ~kerkos~, tail) is intended to indicate. Such a tail has been
recognized in four genera only, _Mola_, _Ranzania_, _Fierasfer_, and
_Echiodon_, the head-fishes and the pearl-fishes.

[Illustration: FIG. 68.--Isocercal tail of Hake, _Merluccius productus_
(Ayres).]

[Illustration: FIG. 69.--Homocercal tail of a Flounder, _Paralichthys
californicus_.]

The part of the body of the fish which lies behind the vent is known
as the urosome. The urostyle is the name given to a modified bony
structure, originally the end of the notochord, turned upward in most
fishes. The term _opisthure_ is suggested by Ryder for the exserted tip
of the vertebral column, which in some larvae (_Lepisosteus_) and in
some adult fishes (_Fistularia_, _Chimaera_) projects beyond the caudal
fin. The urosome, or posterior part of the body, must be regarded as
a product of evolution and specialization, its function being largely
that of locomotion. In the theoretically primitive fish there is no
urosome, the alimentary canal, as in the worm, beginning at one end of
the body and terminating at the other.

[Illustration: FIG. 70.--Gephyrocercal tail of _Mola mola_ (Linnaeus).
(After Ryder.)]

=Homologies of the Pectoral Limb.=--Dr. Gill has made an elaborate
attempt to work out the homologies of the bones of the pectoral
limb.[4] From his thesis we take the following:

"The following are assumed as premises that will be granted by all
zootomists:

"1. Homologies of parts are best determinable, _ceteris paribus_, in
the most nearly related forms.

"2. Identification should proceed from a central or determinate point
outwards.

"The applications of these principles are embodied in the following
conclusions:

"1. The forms that are best comparable and that are most nearly related
to each other are the Dipnoi, an order of fishes at present represented
by _Lepidosiren_, _Protopterus_, and _Ceratodus_, and the Batrachians
as represented by the _Ganocephala_, Salamanders, and Salamander-like
animals.

"2. The articulation of the anterior member with the shoulder-girdle
forms the most obvious and determinable point for comparison in the
representatives of the respective classes.

[Illustration: FIG. 71.--Shoulder-girdle of _Amia calva_ (Linnaeus).]

[Illustration: FIG. 72.--Shoulder-girdle of a Sea Catfish, _Selenaspis
dowi_.]

=The Girdle in Dipnoans.=--"The proximal element of the anterior limb
in the Dipnoi has almost by common consent been regarded as homologous
with the _humerus_ of the higher vertebrates.

"The humerus of Urodele Batrachians, as well as the extinct Ganocephala
and Labyrinthodontia, is articulated chiefly with the coracoid.
Therefore the element of the shoulder-girdle with which the humerus
of the Dipnoi is articulated must also be regarded as the _coracoid_
(subject to the proviso hereinafter stated), unless some specific
evidence can be shown to the contrary. No such evidence has been
produced.

"The scapula in the Urodele and other Batrachians is entirely or almost
wholly excluded from the glenoid foramen, and above the coracoid.
Therefore the corresponding element in Dipnoi must be the _scapula_.

"The other elements must be determined by their relation to the
preceding, or to those parts from or in connection with which they
originate. All those elements in _immediate_ connection with the
pectoral fin and the scapula must be homologous as a whole with the
coraco-scapular plate of the Batrachians; that is, it is infinitely
more probable that they represent, as a whole or as dismemberments
therefrom, the coraco-scapular element than that they independently
originated. But the homogeneity of that coraco-scapular element
forbids the identification of the several elements of the fish's
shoulder-girdle with regions of the Batrachian's coraco-scapular plate.

[Illustration: FIG. 73.--Clavicles of a Sea Catfish, _Selenaspis dowi_
(Gill).]

"And it is equally impossible to identify the fish's elements with
those of the higher reptiles or other vertebrates which have developed
from the Batrachians. The elements in the shoulder-girdles of the
distantly separated classes _may_ be (to use the terms introduced
by Dr. Lankester) homoplastic, but they _are not_ homogenetic.
Therefore they must be named accordingly. The element of the Dipnoan's
shoulder-girdle, continuous downward from the scapula, and to which the
coracoid is closely applied, may be named _ectocoracoid_.

"Neither the scapula in Batrachians nor the cartilaginous extension
thereof, designated suprascapula, is dissevered from the coracoid.
Therefore there is an _a priori_ improbability against the homology
with the scapula of any part having a distant and merely ligamentous
connection with the humerus-bearing element. Consequently, as an
element better representing the scapula exists, the element named
scapula (by Owen, Guenther, etc.) cannot be the homologue of the scapula
of Batrachians. On the other hand, its more intimate relations with the
skull and the mode of development indicate that it is rather an element
originating and developed in more intimate connection with the skull.
It may therefore be considered, with Parker, as a _post-temporal_.

"The shoulder-girdle in the Dipnoi is connected by an azygous
differentiated cartilage, swollen backwards. It is more probable that
this is the homologue of the _sternum_ of Batrachians, and that in the
latter that element has been still more differentiated and specialized
than that it should have originated _de novo_ from an independently
developed nucleus."

[Illustration: FIG. 74.--Shoulder-girdle of a Batfish, _Ogcocephalus
radiatus_ (Mitchill).]

=The Girdle in Fishes Other than Dipnoans.=--"Proceeding from the
basis now obtained, a comparative examination of other types of fishes
successively removed by their affinities from the Lepidosirenids may be
instituted.

"With the humerus of the Dipnoans, the element of the Polypterids
(single at the base, but immediately divaricating and with its limbs
bordering an intervening cartilage which supports the pectoral and
its basilar ossicles) must be homologous. But it is evident that the
external elements of the so-called carpus of the teleosteoid Ganoids
are homologous with that element in Polypterids. Therefore those
elements cannot be carpal, but must represent the humerus.

[Illustration: FIG. 75.--Shoulder-girdle of a Threadfin, _Polydactylus
approximans_ (Lay and Bennett).]

"The element with which the homologue of the humerus, in Polypterids,
is articulated must be homologous with the analogous element in
Dipnoans, and therefore with the _coracoid_. The coracoid of
Polypterids is also evidently homologous with the corresponding
element in the other Ganoids, and the latter consequently must be
also _coracoid_. It is equally evident, after a detailed comparison,
that the single coracoid element of the Ganoids represents the three
elements developed in the generalized Teleosts (Cyprinids, etc.) in
connection with the basis of the pectoral fin, and, such being the
case, the nomenclature should correspond. Therefore the upper element
may be named _hypercoracoid_; the lower, _hypocoracoid_; and the
transverse or median, _mesocoracoid_.

"The two elements of the arch named by Parker, in _Lepidosiren_,
'supraclavicle' (scapula) and 'clavicle' (ectocoracoid) seem to be
comparable together, and as a whole, with the single element carrying
the humerus and pectoral fin in the Crossopterygians (_Polypterus_
and _Calamoichthys_) and other fishes, and therefore not identical
respectively with the 'supraclavicle' and 'clavicle' (except in part)
recognized by him in other fishes. As this compound bone, composed of
the scapula and ectocoracoid fused together, has received no name which
is not ambiguous or deceptive in its homologous allusions, it may be
designated as _proscapula_.

"The post-temporal of the Dipnoans is evidently represented by the
analogous element in the Ganoids generally, as well as in the typical
fishes. The succeeding elements (outside those already alluded to)
appear from their relations to be developed from or in connection
with the post-temporal, and not from the true scapular apparatus;
they may therefore be named _post-temporal_, _posterotemporal_, and
_teleo-temporal_. It will be thus seen that the determinations here
adopted depend mainly (1) on the interpretation of the homologies of
the elements with which the pectoral limbs are articulated, and (2) on
the application of the term 'coracoid.' The name 'coracoid,' originally
applied to the process so called in the human scapula and subsequently
extended to the independent element homologous with it in birds
and other vertebrates, has been more especially retained (e.g., by
Parker in mammals, etc.) for the region including the glenoid cavity.
On the assumption that this may be preferred by some zootomists,
the preceding terms have been applied. But if the name should be
restricted to the proximal element, nearest the glenoid cavity, in
which ossification commences, the name _paraglenal_ given by Duges
to the cartilaginous glenoid region can be adopted, and the coracoid
would then be represented (in part) rather by the element so named by
Owen. That eminent anatomist, however, reached his conclusion (only in
part the same as that here adopted) by an entirely different course of
reasoning, and by a process, as it may be called, of elimination; that
is, recognizing first the so-called 'radius' and 'ulna,' the 'humerus,'
the 'scapula,' and the 'coracoid' were successively identified from
their relations to the elements thus determined and because they were
numerically similar to the homonymous parts among higher vertebrates."

FOOTNOTES:

[4] Catalogue of the Families of Fishes, 1872.




CHAPTER VI

THE ORGANS OF RESPIRATION


=How Fishes Breathe.=--The fish breathes the air which is dissolved in
water. It cannot use the oxygen which is a component part of water, nor
can it, as a rule, make use of atmospheric air. The amount of oxygen
required for the low vegetative processes of the fish is comparatively
small. According to Dr. Guenther, a man consumes 50,000 times as much
oxygen as a tench. But some fishes demand more oxygen than others.
Some, like the catfish or the loach, will survive long out of water,
while others die almost instantly if removed from their element or if
the water is allowed to become foul. In most cases the temperature
of the blood of the fish is but little above that of the water in
which they live, but in the mackerel and other muscular fishes the
temperature of the body may be somewhat higher.

Some fishes which live in mud, especially in places which become dry
in summer, have special contrivances by which they can make use of
atmospheric air. In a few primitive fishes (Dipnoans, Crossopterygians,
Ganoids) the air-bladder retains its original function of a lung. In
other cases some peculiar structure exists in connection with the
gills. Such a contrivance for holding water above the gills is seen in
the climbing perch of India (_Anabas scandens_) and other members of
the group called Labyrinthici.

In respiration, in fishes generally, the water is swallowed through the
mouth and allowed to pass out through the gill-openings, thus bathing
the gills. In a few of the lower types a breathing-pore takes the place
of the gill-openings.

The gills, or branchiae, are primarily folds of the skin lining the
branchial cavity. In most fishes they form fleshy fringes or laminae
throughout which the capillaries are distributed. In the embryos of
sharks, skates, chimaeras, lung-fishes, and Crossopterygians external
gills are developed, but in the more specialized forms these do not
appear outside the gill-cavity. In some of the sharks, and especially
the rays, a spiracle or open foramen remains behind the eye. Through
this spiracle, leading from the outside into the cavity of the mouth,
water is drawn downwards to pass outward over the gills. The presence
of this breathing-hole permits these animals to lie on the bottom
without danger of inhaling sand.

[Illustration: FIG. 76.--Gill-basket of Lamprey. (After Dean.)]

=The Gill-structures.=--The three main types of gills among fishes are
the following: (_a_) the purse-shaped gills found in the hagfishes and
lampreys, known as a class as Marsipobranchs, or purse-gills. These
have a number (5 to 12) of sac-like depressions on the side of the
body, lined with gill-fringes and capillaries, the whole supported
by an elaborate branchial basket formed of cartilage. (_b_) The
plate-gills, found among the sharks, rays, and chimaeras, thence called
Elasmobranchs, or plate-gills. In these the gill-structures are flat
laminae, attached by one side to the gill-arches. (_c_) The fringe-gills
found in ordinary fishes, in which the gill-filaments containing
the capillaries are attached in two rows to the outer edge of each
gill-arch. The so-called tuft-gills (Lophobranchs) of the sea-horse
and pipefish are like these in structure, but the filaments are long,
while the arches are very short. In most of the higher fishes a small
accessory gill (pseudobranchia) is developed in the skin of the inner
side of the opercle.

=The Air-bladder.=--The air-bladder, or swim-bladder, must be classed
among the organs of respiration, although in the higher fishes its
functions in this regard are rudimentary, and in some cases it has
taken collateral functions (as a hydrostatic organ of equilibrium, or
perhaps as an organ of hearing) which have no relation to its original
purpose.

[Illustration: FIG. 77.--Weberian apparatus and air-bladder of Carp.
(From Guenther, after Weber.)]

The air-bladder is an internal sac possessed by many fishes, but not
by all. It lies in the dorsal part of the abdominal cavity above the
intestines and below the kidneys. In some cases it is closely adherent
to the surrounding tissues. In others it is almost entirely free, lying
almost loose in the cavity of the body. In some cases it is enclosed in
a bony capsule. In the allies of the carp and catfish, which form the
majority of fresh-water fishes, its anterior end is connected through a
chain of modified vertebrae to the ear. Sometimes its posterior end fits
into an enlarged and hollow interhaemal bone. Sometimes, again, a mass
of muscle lies in front of it or is otherwise attached to it. Sometimes
it is divided into two or three parts by crosswise constrictions.
Sometimes it is constricted longitudinally, and at other times it
has attached to it a complication of supplemental tubes of the same
character as the air-bladder itself. In still other cases it is divided
by many internal partitions into a cellular body, similar to the lung
of the higher vertebrates, though the cells are coarser and less
intricate. This condition is evidently more primitive than that of the
empty sac.

The homology of the air-bladder with the lung is evident. This is often
expressed in the phrase that the lung is a developed air-bladder.
This is by no means true. To say that the air-bladder is a modified
and degenerate lung is much nearer the truth, although we should
express the fact more exactly to say that both air-bladder and lung
are developed from a primitive cellular breathing-sac, originally a
diverticulum from the ventral walls of the oesophagus.

The air-bladder varies in size as much as in form. In some fishes it
extends from the head to the tail, while in others it is so minute
as to be scarcely traceable. It often varies greatly in closely
related species. The common mackerel (_Scomber scombrus_) has no
air-bladder, while in the closely related colias or chub mackerel
(_Scomber japonicus_) the organ is very evident. In other families,
as the rockfishes (_Scorpaenidae_), genera with and those without the
air-bladder are scarcely distinguishable externally. In general, fishes
which lie on the bottom, those which inhabit great depths, and those
which swim freely in the open sea, as sharks and mackerel, lack the
air-bladder. In the sharks, rays, and chimaeras there is no trace of an
air-bladder. In the mackerel and other bony fishes without it, it is
lost in the process of development.

The air-bladder is composed of two layers of membrane, the outer
one shining, silvery in color, with muscular fibres, the inner well
supplied by blood-vessels. The gas within the air-bladder must be in
most cases secreted from the blood-vessels. In river fishes it is said
to be nearly pure nitrogen. In marine fishes it is mostly oxygen, with
from 6 to 10 per cent of carbonic-acid gas, while in the deep-sea
fishes oxygen is greatly in excess. In _Lopholatilus_, a deep-sea fish,
Professor R. W. Tower finds 66 to 69 per cent of oxygen. In _Trigla
lyra_ Biot records 87 per cent. In _Dentex dentex_, a shore fish of
Europe, 40 per cent of oxygen was found in the air-bladder. Fifty
per cent is recorded from the European porgy, _Pagrus pagrus_. In a
fish dying from suffocation the amount of carbonic-acid gas (CO2)
is greatly increased, amounting, according to recent researches of
Professor Tower on the weak-fish, _Cynoscion regalis_, to 24 to 29 per
cent. This shows conclusively that the air-bladder is to some degree a
reservoir of oxygen secreted from the blood, to which channel it may
return through a kind of respiration.

The other functions of the air-bladder have been subject to much
question and are still far from understood. The following summary of
the various views in this regard we copy from Professor Tower's paper
on "The Gas in the Swim-bladder of Fishes":

"The function of the swim-bladder of fishes has attracted the attention
of scientists for many centuries. The role that this structure plays in
the life of the animal has been interpreted in almost as many ways as
there have been investigators, and even now there is apparently much
doubt as to the true functions of the swim-bladder. Consequently any
additional data concerning this organ are of immediate scientific value.

"Aristotle, writing about the noises made by fishes, states that
'some produce it by rubbing the gill-arches ...; others by means of
the air-bladder. Each of these fishes contains air, by rubbing and
moving of which the noise is produced.' The bladder is thus considered
a sound-producing organ, and it is probable that he arrived at this
result by his own investigations.

"Borelli (De Motu Animalium, 1680) attributed to the air-bladder
a hydrostatic function which enabled the fish to rise and fall in
the water by simply distending or compressing the air-bladder. This
hypothesis, which gives to the fish a volitional control over the
air-bladder--it being able to compress or distend the bladder at
pleasure--has prevailed, to a greater or less degree, from the time
of Borelli to the present. To my knowledge, however, there are no
investigations which warrant such a theory, while, on the other hand,
there are many facts, as shown by Moreau's experiment, which distinctly
contradict this belief. Delaroche (Annales du Mus. d'Hist. Nat.,
tome XIV, 1807-1809) decidedly opposed the ideas of Borelli, and yet
advanced an hypothesis similar to it in many respects. Like Borelli,
he said that the fish could compress or dilate the bladder by means
of certain muscles, but this was to enable the fish to keep the same
specific gravity as the surrounding medium, and thus be able to remain
at any desired depth (and not to rise or sink). This was also disproved
later by Moreau. Delaroche proved that there existed a constant
exchange between the air in the air-bladder and the air in the blood,
although he did not consider the swim-bladder an organ of respiration.

"Biot (1807), Provencal and Humboldt (1809), and others made chemical
analyses of the gas in the swim-bladder, and found 1 to 5 per cent of
CO_{2}, 1 to 87 per cent of O_{2}, and the remainder nitrogen. The most
remarkable fact discovered about this mixture was that it frequently
consisted almost entirely of oxygen, the per cent of oxygen increasing
with the depth of the water inhabited by the fish. The reasons for this
phenomenon have never been satisfactorily explained.

"In 1820 Weber described a series of paired ossicles which he
erroneously called stapes, malleus, and incus, and which connected the
air-bladder in certain fishes with a part of the ear--the atrium sinus
imparis. Weber considered the swim-bladder to be an organ by which
sounds striking the body from the outside are intensified, and these
sounds are then transmitted to the ear by means of the ossicles. The
entire apparatus would thus function as an organ of hearing. Weber's
views remained practically uncontested for half a century, but recently
much has been written both for and against this theory. Whatever the
virtues of the case may be, there is certainly an inviting field for
further physiological investigations regarding this subject, and more
especially on the phenomena of hearing in fishes.

"Twenty years later Johannes Mueller described, in certain Siluroid
fishes, a mechanism, the so-called 'elastic-spring' apparatus, attached
to the anterior portion of the air-bladder, which served to aid the
fish in rising and sinking in the water according as the muscles of
this apparatus were relaxed or contracted to a greater or lesser
degree. This interpretation of the function of the 'elastic-spring'
mechanism was shown by Soerensen to be untenable. Mueller also stated
that in some fish, at least, there was an exchange of gas between
blood and air-bladder--the latter having a respiratory function--and
regarded the gas in the air-bladder as the result of active secretion.
In _Malapterurus_ (_Torpedo electricus_) he stated that it is a
sound-producing organ.

"Hasse, in 1873, published the results of his investigations on the
functions of the ossicles of Weber, stating that their action was that
of a manometer, acquainting the animal with the degree of pressure that
is exerted by the gases in the air-bladder against its walls. This
pressure necessarily varies with the different depths of water which
the fish occupies. Hasse did not agree with Weber that the ear is
affected by the movements of these ossicles.

"One year later Dufosse described in some fishes an air-bladder
provided with extrinsic muscles by whose vibration sound was produced,
the sound being intensified by the air-bladder, which acted as a
resonator. He also believed that certain species produced a noise by
forcing the gas from the air-bladder through a pneumatic duct.

"At about the same time Moreau published his classical work on the
functions of the air-bladder. He proved by ingenious experiments that
many of the prevailing ideas about the action of the air-bladder were
erroneous, and that this organ serves to equilibrate the body of the
fish with the water at any level. This is not accomplished quickly,
but only after sufficient time for the air in the bladder to become
adjusted to the increase or decrease in external pressure that has
taken place. The fish, therefore, makes no use of any muscles in
regulating the volume of its air-bladder. The animal can accommodate
itself only gradually to considerable changes in depth of water, but
can live equally comfortably at different depths, provided that the
change has been gradual enough. Moreau's experiments also convinced
him that the gas is actually secreted into the air-bladder, and
that there is a constant exchange of gas between it and the blood.
In these investigations he has also noticed that section of the
sympathetic-nerve fibres supplying the walls of the air-bladder hastens
the secreting of the gas into the empty bladder. Since then Bohr has
shown that section of the vagus nerve causes the secretion to cease.
Moreau noticed in one fish (_Trigla_) having an air-bladder supplied
with muscles that the latter served to make the air-bladder produce
sound.

"Again, in 1885, the Weberian mechanism was brought to our attention
with a new function attributed to it by Sagemehl who stated that this
mechanism exists not for any auditory purposes, nor to tell the fish
at what level of the water it is swimming, but to indicate to the fish
the variations in the atmospheric pressure. Soerensen tersely contrasts
the views of Hasse and Sagemehl by saying that 'Hasse considers the
air-bladder with the Weberian mechanism as a manometer; Sagemehl
regards it as a barometer.' The theory of Sagemehl has, naturally
enough, met with little favor. Soerensen (1895) held that there is but
little evidence for attributing to the air-bladder the function of a
lung. It is to be remembered, however, that, according to Soerensen's
criterion no matter what exchange of gases takes place between blood
and air-bladder, it cannot be considered an organ of respiration,
'unless its air is renewed by mechanical respiration.'

"Soerensen also refutes, from anatomical and experimental grounds, the
many objections to Weber's theory of the function of the ossicles. He
would thus attribute to the air-bladder the function of hearing; indeed
in certain species the only reason for the survival of the air-bladder
is that 'the organ is still of acoustic importance; that it acts as a
resonator.' This idea, Soerensen states, is borne out by the anatomical
structure found in _Misgurnus_ and _Chlarias_, which resembles the
celebrated 'Colladon resonator.' This author attributes to the
air-bladder with its 'elastic spring' and various muscular mechanisms
the production of sound as its chief function."

=Origin of the Air-bladder.=--In the more primitive forms, and probably
in the embryos of all species, the air-bladder is joined to the
oesophagus by an air-duct. This duct is lost entirely in the adult
of all or nearly all of the thoracic and jugular fishes, and in some
of the abdominal forms. The lancelets, lampreys, sharks, rays, and
chimaeras have no air-bladder, but in the most primitive forms of true
fishes (Dipnoans and Crossopterygians), having the air-bladder cellular
or lung-like, the duct is well developed, freely admitting the external
air which the fish may rise to the surface to swallow. In most fishes
the duct opens into the oesophagus from the dorsal side, but in the
more primitive forms it enters from the ventral side, like the windpipe
of the higher vertebrates. In some of the Dipnoans the air-bladder
divides into two parts, in further resemblance to the true lungs.

=The Origin of the Lungs.=--The following account of the function of
the air-bladder and of its development and decline is condensed from an
article by Mr. Charles Morris:[5]

"If now we seek to discover the original purpose of this organ, there
is abundant reason to believe that it had nothing to do with swimming.
Certainly the great family of the sharks, which have no bladder, are at
no disadvantage in changing their depth or position in the water. Yet
if the bladder is necessary to any fish as an aid in swimming, why not
to all? And if this were its primary purpose, how shall we explain its
remarkable variability? No animal organ with a function of essential
importance presents such extraordinary modifications in related species
and genera. In the heart, brain, and other organs there is one shape,
position, and condition of greatest efficiency, and throughout the
lower forms we find a steady advance towards this condition. Great
variation, on the other hand, usually indicates that the organ is
of little functional importance, or that it has lost its original
function. Such we conceive to be the case with the air-bladder. The
fact of its absence from some and its presence in other fishes of
closely related species goes far to prove that it is a degenerating
organ; and the same is shown by the fact that it is useless in some
species for the purpose to which it is applied in others. That it had,
at some time in the past, a function of essential importance there
can be no question. That it exists at all is proof of this. But its
modern variations strongly indicate that it has lost this function and
is on the road towards extinction. Larval conditions show that it had
originally a pneumatic duct as one of its essential parts, but this
has in most cases disappeared. The bladder itself has in many cases
partly or wholly disappeared. Where preserved, it seems to be through
its utility for some secondary purpose, such as an aid in swimming or
in hearing. That its evolution began very long ago there can be no
question; and the indications are that it began long ago to degenerate,
through the loss of its primitive function.

"What was this primitive function? In attempting to answer this
question we must first consider the air-bladder in relation to the
fish tribe as a whole. No shark or ray possesses the air-bladder.
In some few sharks, indeed, there is a diverticulum of the pharynx
which may be a rudimentary approach to the air-bladder; but this is
very questionable. The conditions of its occurrence in the main body
of modern fishes, the Teleostean, we have already considered. But in
the most ancient living orders of fishes it exists in an interesting
condition. In every modern Dipnoan, Crossopterygian, and Ganoid the
air-bladder has an effective pneumatic duct. This in the Ganoids
opens into the dorsal side of the oesophagus, but in the Dipnoans and
Crossopterygians, like the windpipe of lung-breathers, it opens into
the ventral side. In the Dipnoans, also survivors from the remote
past, the duct not only opens ventrally into the oesophagus, but the
air-bladder does duty as a lung. Externally it differs in no particular
from an air-bladder; but internally it presents a cellular structure
which nearly approaches that of the lung of the batrachians. There
are three existing representatives of the Dipnoans. One of these, the
Australian lung-fish (_Neoceratodus_) has a single bladder, which,
however, is provided with breathing-pouches having a symmetrical
lateral arrangement. It has no pulmonary artery, but receives branches
from the _arteria coeliaca_. In the other two forms, _Lepidosiren_ and
_Protopterus_, the kindred 'mudfishes' of the Amazon basin and tropical
Africa, the bladder or lung is divided into two lateral chambers, as in
the land animals, and is provided with a separate pulmonary artery.

"The opinion seems to have been tacitly entertained by physiologists
that this employment of the air-bladder by the Dipnoans as a lung
is a secondary adaptation, a side issue from its original purpose.
It is more likely that this is the original purpose, and that its
degeneration is due to the disappearance of the necessity of such
a function. As regards the gravitative employment of the bladder,
the Teleostean fishes, to which this function is confined, are
of comparatively modern origin; while the Dipnoans are surviving
representatives of a very ancient order of fishes, which flourished
in the Devonian age of geology, and in all probability breathed air
then as now; and the Crossopterygians and Ganoids, which approach them
in this particular, are similarly ancient in origin, and were the
ancestors of the Teleosteans. The natural presumption, therefore, is
that the duty which it subserved in the most ancient fishes was its
primitive function.

"The facts of embryology lend strong support to this hypothesis. For
the air-bladder is found to arise in a manner very similar to the
development of the lung. They each begin as an outgrowth from the
fore part of the alimentary tract, the only difference being that the
air-bladder usually rises dorsally and the lung ventrally. The fact
already cited, that the pneumatic duct is always present in the larval
form in fishes that possess a bladder, is equally significant. All the
facts go to show that the introduction of external air into the body
was a former function of the air-bladder, and that the atrophy of the
duct in many cases, and the disappearance of the bladder in others, are
results of the loss of this function.

"Such an elaborate arrangement for the introduction of air into the
body could have, if we may judge from analogy, but one purpose, that
of breathing, to which purpose the muscular and other apparatus
for compressing and dilating the bladder, now seemingly adapted to
gravitative uses, may have been originally applied. The same may
be said of the great development of blood-capillaries in the inner
tunic of the bladder. These may now be used only for the secretion
of gas into its interior, but were perhaps originally employed in
the respiratory secretion of oxygen. In fact all the circumstances
mentioned--the similarity in larval development between the bladder and
lung, the larval existence of the pneumatic duct, the arrangements for
compressing and dilating the bladder, and the capillary vessels on its
inner tunic--point to the breathing of air as its original purpose.

"It is probable that the Ganoid, as well as the Dipnoan, air-bladder is
to some extent still used in breathing. The Dipnoans have both lungs
and gills, and probably breathe with the latter in ordinary cases, but
use their lungs when the inland waters in which they live become thick
and muddy, or are charged with gases from decomposing organic matter.
The Ganoid fishes to some extent breathe the air. In _Polypterus_ the
air-bladder resembles the Dipnoan lung in having lateral divisions and
a ventral connection with the oesophagus, while in _Lepisosteus_ (the
American garpike) it is cellular and lung-like. This fish keeps near
the surface, and may be seen to emit air-bubbles, probably taking in
a fresh supply of air. The American bowfin, or mudfish (_Amia_), has
a bladder of the same lung-like character, and has been seen to come
to the surface, open its jaws widely, and apparently swallow a large
quantity of air. He considers that both _Lepisosteus_ and _Amia_
inhale and exhale air at somewhat regular intervals, resembling in this
the salamanders and tadpoles, 'which, as the gills shrink and the lungs
increase, come more frequently to the surface for air.'

"As the facts stand there is no evident line of demarcation between the
gas-containing bladders of many of the Teleosteans, the air-containing
bladders of the others and the Ganoids, and the lung of the Dipnoans,
and the indications are in favor of their having originally had the
same function, and of this being the breathing of air.

"If now we ask what were the conditions of life under which this
organ was developed, and what the later conditions which rendered
it of no utility as a lung, some definite answer may be given. The
question takes us back to the Devonian and Silurian geological
periods, during which the original development of the bladder
probably took place. In this era the seas were thronged with fishes
of several classes, the Elasmobranchs among others, followed by the
Dipnoi and Crossopterygians. The sharks were without, the Dipnoans
and Crossopterygians doubtless with, an air-bladder--a difference in
organization which was most likely due to some marked difference in
their life-habits. The Elasmobranchs were the monarchs of the seas,
against whose incursions the others put on a thick protective armor,
and probably sought the shallow shore waters, while their foes held
chief possession of the deeper waters without.

"We seem, then, to perceive the lung-bearing fishes, driven by their
foes into bays and estuaries, and the waters of shallow coasts,
ascending streams and dwelling in inland waters. Here two influences
probably acted on them. The waters they dwelt in were often thick
with sediment, and were doubtless in many instances poorly aerated,
rendering gill-breathing difficult. And the land presented conditions
likely to serve as a strong inducement to fishes to venture on shore.
Its plant-life was abundant, while its only animal inhabitants seem to
have been insects, worms, and snails. There can be little doubt that
the active fish forms of that period, having no enemies to fear on the
land, and much to gain, made active efforts to obtain a share of this
vegetable and animal food. Even to-day, when they have numerous foes to
fear, many fishes seek food on the shore, and some even climb trees
for this purpose. Under the conditions of the period mentioned there
was a powerful inducement for them to assume this habit.

"Such conditions must have strongly tended to induce fishes to breathe
the air, and have acted to develop an organ for this purpose. In
addition to the influences of foul or muddy water and of visits to
land may be named that of the drying-out of pools, by which fishes
are sometimes left in the moist mud till the recurrence of rains, or
are even buried in the dried mud during the rainless season. This is
the case with the modern Dipnoi, which use their lungs under such
circumstances. In certain other fresh-water fishes, of the family
Ophiocephalidae, air is breathed while the mud continues soft enough
for the fish to come to the surface, but during the dry period the
animal remains in a torpid state. These fishes have no lungs, but
breathe the air into a simple cavity in the pharynx, whose opening is
partly closed by a fold of the mucous membrane. Other Labyrinthici,
of similar habits, possess a more developed breathing organ. This
is a cavity formed by the walls of the pharynx, in which are thin
laminae, or plates, which undoubtedly perform an oxygenating function.
The most interesting member of this family is _Anabas scandens_, the
climbing perch. In this fish, which not only leaves the water, but is
said to climb trees, the air-breathing organ is greatly developed. The
labyrinthici, moreover, have usually large air-bladders. As regards the
occasional breathing of air by fishes, even in species which do not
leave the water, it is quite common, particularly among fresh-water
species. Cuvier remarks that air is perhaps necessary to every kind of
fish; and that, particularly when the atmosphere is warm, most of our
lacustrine species sport on the surface for no other purpose.

"It is not difficult to draw a hypothetical plan of the development
of the air-bladder as a breathing organ. In the two families of
fishes just mentioned, whose air-bladders indicate that they once
possessed the air-breathing function and have lost it, we perceive
the process of formation of an air-breathing organ beginning over
again under stress of similar circumstances. The larval development of
the air-bladder points significantly in the same direction. In fact
we have strong reason to believe that air-breathing in fishes was
originally performed, as it probably often is now, by the unchanged
walls of the oesophagus. Then these walls expanded inwardly, forming a
simple cavity, partly closed by a fold of membrane, like that of the
Ophiocephalidae. A step further reduced this membranous fold to a narrow
opening, leading to an inner pouch. As the air-breathing function
developed, the opening became a tube, and the pouch a simple lung, with
compressing muscles and capillary vessels. By a continuation of the
process the smooth-walled pouch became sacculated, its surface being
increased by folding into breathing cells. Finally, a longitudinal
constriction divided it into two lateral pouches, such as we find in
the lung of the Dipnoans. This brings us to the verge of the lung of
the amphibians, which is but a step in advance, and from that the line
of progress is unbroken to the more intricate lung of the higher land
animals.

"The dorsal position of the bladder and its duct would be a difficulty
in this inquiry, but for the fact that the duct is occasionally
ventral. This dorsal position may have arisen from the upward pressure
of air in the swimming fish, which would tend to lift the original
pouch. But in the case of fishes which made frequent visits to the
shore new influences must have come into play. The effect of gravity
tended to draw the organ and its duct downward, as we find in the
Crossopterygians and in all the Dipnoans, and its increased use in
breathing required a more extended surface. Through this requirement
came the pouched and cellular lung of the Dipnoans. Of every stage of
the process here outlined examples exist, and there is great reason
to believe that the development of the lung followed the path above
pointed out.

"When the carboniferous era opened there may have been many lung- and
gill-breathing fishes which spent much of their time on land, and some
of which, by a gradual improvement of their organs of locomotion,
changed into batrachians. But with the appearance of the latter, and
of their successors, the reptiles, the relations of the fish to the
land radically changed. The fin, or the simple locomotor organ, of the
Dipnoans could not compete with the leg and foot as organs of land
locomotion, and the fish tribe ceased to be lords of the land, where,
instead of feeble prey, they now found powerful foes, and were driven
back to their native habitat, the water. Nor did the change end here.
In time the waters were invaded by the reptiles, numerous swimming
forms appearing, which it is likely were abundant in the shallower
shore-line of the ocean, while they sent many representatives far out
to sea. These were actively carnivorous, making the fish their prey,
the great mass of whom were doubtless driven into the deeper waters,
beyond the reach of their air-breathing foes.

"In this change of conditions we seem to perceive an adequate cause
for the loss of air-breathing habits in those fishes in which the lung
development had not far progressed. It may indeed have been a leading
influence in the development of the Teleostean or bony fishes, as
it doubtless was in the loss of its primitive function by, and the
subsequent changes of, the air-bladder.

"Such of the Crossopterygians and Dipnoans as survived in their old
condition had to contend with adverse circumstances. Most of them in
time vanished, while their descendants which still exist have lost
in great measure their air-breathing powers, and the Dipnoans, in
which the development of the lung had gone too far for reversal, have
degenerated into eel-like, mud-haunting creatures, in which the organs
of locomotion have become converted into the feeble paddle-like limbs
of Neoceratodus and the filamentary appendages of the other species.

"As regards the presence of a large quantity of oxygen in the bladders
of deep-swimming marine fishes, it not unlikely has a respiratory
purpose, the bladder being, as suggested by Semper, used as a reservoir
for oxygen, to serve the fish when sleeping, or when, from any cause,
not actively breathing. The excess of oxygen is not due to any like
excess in the gaseous contents of sea-water, for the percentage of
oxygen decreases from the surface downward, while that of nitrogen
remains nearly unchanged. In all cases, indeed, the bladder may
preserve a share of its old function, and act as an aid in respiration.
Speaking of this, Cuvier says: 'With regard to the presumed assistance
which the swim-bladder affords in respiration, it is a fact that when a
fish is deprived of that organ, the production of carbonic acid by the
branchiae is very trifling,' thus strongly indicating that the bladder
still plays a part in the oxygenation of the blood.

"Under the hypothesis here presented the process of evolution involved
may be thus summed up. Air-breathing in fishes was originally performed
by the unchanged walls of the oesophagus perhaps at specially vascular
localities. Then the wall folded inward, and a pouch was finally
formed, opening to the air. The pouch next became constricted off,
with a duct of connection. Then the pouch became an air-bladder with
respiratory function, and finally developed into a simple lung. These
air-breathing fishes haunted the shores, their fins becoming converted
into limbs suitable for land locomotion, and in time developed into
the lung- and gill-breathing batrachia, and these in their turn into
the lung-breathing reptilia, the locomotor organs gradually increasing
in efficiency. Of these pre-batrachia we have existing representatives
in the mud-haunting Dipnoi, with their feeble limbs. In the great
majority of the Ganoid fishes the bladder served but a minor purpose
as a breathing organ, the gills doing the bulk of the work. In the
Teleostean descendants of the Ganoids the respiratory function of the
bladder in great measure or wholly ceased, in the majority of cases
the duct closing up or disappearing, leaving the pouch as a closed
internal sac, far removed from its place of origin. In this condition
it served as an aid in swimming, perhaps as a survival of one of its
ancient uses. It gained also in certain cases some connection with the
organ of hearing. But these were makeshift and unimportant functions,
as we may gather from the fact that many fishes found no need for them,
the bladder, in these cases, decreasing in size until too small to be
of use in swimming, and in other cases completely disappearing after
having travelled far from its point of origin. In some other cases,
above cited, the process seems to have begun again, in modern times,
in an eversion of the wall of the oesophagus for respiratory purposes.
The whole process, if I have correctly conceived it, certainly forms a
remarkable organic cycle of development and degeneration, which perhaps
has no counterpart of similarly striking character in the whole range
of organic life."

=The Heart of the Fish.=--The heart of the fish is simple in structure,
small in size, and usually placed far forward, just behind the
branchial cavity, and separated from the abdominal cavity by a sort of
"diaphragm" formed of thickened peritoneum. In certain eels the heart
is remote from the head.

The heart consists of four parts, the sinus venosus, into which the
veins enter, the auricle or atrium, the ventricle, and the arterial
bulb at the base of the great artery which carries the blood to the
gills. Of these parts the ventricle is deepest in color and with
thickest walls. The arterial bulb varies greatly in structure, being in
the sharks, rays, Ganoids, and Dipnoans muscular and provided with a
large number of internal valves, and contracting rhythmically like the
ventricle. In the higher fishes these structures are lost, the walls
of the arterial bulb are not contractile, and the interior is without
valves, except the pair that separate it from the ventricle.

In the lancelet there is no proper heart, the function of the heart
being taken by a contractile blood-vessel situated on the ventral
side of the alimentary canal. In the Dipnoans, which are allied to
the ancestors of the higher vertebrates, there is the beginning of a
division of the ventricle, and sometimes of the auricle, into parts by
a median septum. In the higher vertebrates this septum becomes more
and more specialized, separating auricle and ventricle into right and
left cavities. The blood in the fish is not returned to the heart after
purification, but is sent directly over the body.

=The Flow of Blood.=--The blood in fishes is thin and pale red
(colorless in the lancelet) and with elliptical blood-corpuscles. It
enters the _sinus venosus_ from the head through the jugular vein,
from the kidney and body walls through the cardinal vein, and from
the liver through the hepatic veins. Hence it passes to the auricle
and ventricle, and from the ventricle through the arterial bulb, or
conus arteriosus to the ventral aorta. Thence it flows to the gills,
where it is purified. After passing through the capillaries of the
gill-filaments it is collected in paired arteries from each pair of
gills. These vessels unite to form the dorsal aorta, which extends the
length of the body just below the back-bone. From the dorsal aorta
the subclavian arteries branch off toward the pectoral fins. From a
point farther back arise the mesenteric arteries carrying blood to the
stomach, intestine, liver, and spleen. In the tail the caudal vein
carries blood to the kidneys. These secrete impurities arising from
waste of tissues, after which the blood again passes to the heart
through the _cardinal vein_. From the intestine the blood, charged with
nutritive materials in solution, is carried by the _portal vein_ to
the liver. Here it again passes by the _hepatic sinus_ to the _sinus
venosus_ and the heart.

The details of the circulatory system vary a good deal in the different
groups, and a comparative study of the direction of veins and arteries
is instructive and interesting.

The movement of the blood in fishes is relatively slow, and its
temperature is raised but little above that of the surrounding water.

FOOTNOTES:

[5] The Origin of Lungs: A Chapter in Evolution. American Naturalist,
December, 1892.




CHAPTER VII

THE NERVOUS SYSTEM


=The nerves of the Fish.=--The nervous system in the fish, as in the
higher vertebrates, consists of brain and spinal cord with sensory,
or afferent, and motor, or efferent, nerves. As in other vertebrates,
the nerve substance is divided into gray matter and white matter, or
nerve-cells and nerve-fibres. In the fish, however, the whole nervous
system is relatively small, and the gray matter less developed than in
the higher forms. According to Guenther the brain in the pike (_Esox_)
forms but 1/1305 part of the weight of the body; in the burbot (_Lota_)
about 1/720 part.

The cranium in fishes is relatively small, but the brain does not
nearly fill its cavity, the space between the dura mater, which lines
the skull-cavity, and the arachnoid membrane, which envelops the brain,
being filled with a soft fluid containing a quantity of fat.

=The Brain of the Fish.=--It is most convenient to examine the
fish-brain, first in its higher stages of development, as seen in the
sunfish, striped bass, or perch. As seen from above the brain of a
typical fish seems to consist of five lobes, four of them in pairs, the
fifth posterior to these and placed on the median line. The posterior
lobe is the _cerebellum_, or _metencephalon_, and it rests on the
_medulla oblongata_, the posterior portion of the brain, which is
directly continuous with the spinal cord.

In front of the cerebellum lies the largest pair of lobes, each of
them hollow, the optic nerves being attached to the lower surface.
These are known as the _optic lobes_, or _mesencephalon_. In front of
these lie the two lobes of the cerebrum, also called the hemispheres,
or _prosencephalon_. These lobes are usually smaller than the optic
lobes and solid. In some fishes they are crossed by a furrow, but are
never corrugated as in the brain of the higher animals. In front of
the cerebrum lie the two small olfactory lobes, which receive the large
olfactory nerve from the nostrils. From its lower surface is suspended
the hypophysis or pituitary gland.

[Illustration: FIG. 78.--Brain of a Shark (_Squatina squatina L._).
(After Dean.)

  I.    First cranial nerve (olfactory).
  P.    Prosencephalon (cerebrum).
  E.    Epiphysis.
  T.    Thalamencephalon.
  II.   Second cranial nerve.
  IV.   Fourth cranial nerve.
  V.    Fifth cranial nerve.
  VII.  Seventh cranial nerve.
  V4.   Fourth ventricle.
  M.    Mesencephalon (optic lobes).
  MT.   Metencephalon (medulla).
  EP.   Epencephalon (cerebellum).]

[Illustration: FIG. 79.--Brain of _Chimaera monstrosa_. (After Wilder
per Dean.)]

[Illustration: FIG. 80.--Brain of _Protopterus annectens_. (After
Burckhardt per Dean.)]

In most of the bony fishes the structure of the brain does not differ
materially from that seen in the perch. In the sturgeon, however,
the parts are more widely separated. In the Dipnoans the cerebral
hemispheres are united, while the optic lobe and cerebellum are very
small. In the sharks and rays the large cerebral hemispheres are
usually coalescent into one, and the olfactory nerves dilate into
large ganglia below the nostrils. The optic lobes are smaller than
the hemispheres and also coalescent. The cerebellum is very large,
and the surface of the medulla oblongata is more or less modified or
specialized. The brain of the shark is relatively more highly developed
than that of the bony fishes, although in most other regards the latter
are more distinctly specialized.

=The Pineal Organ.=--Besides the structures noted in other fishes the
epiphysis, or pineal organ, is largely developed in sharks, and traces
of it are found in most or all of the higher vertebrates. In some of
the lizards this epiphysis is largely developed, bearing at its tip a
rudimentary eye. This leaves no doubt that in these forms it has an
optic function. For this reason the structure wherever found has been
regarded as a rudimentary eye, and the "pineal eye" has been called the
"unpaired median eye of chordate" animals.

[Illustration: FIG. 81.--Brain of a Perch, _Perca flavescens_. (After
Dean.)

  R.     Olfactory lobe.
  P.     Cerebrum (prosencephalon).
  E.     Epiphysis.
  M.     Optic lobes (mesencephalon).
  EP.    Cerebellum (epencephalon).
  ML.    Medulla oblongata (metencephalon).
  I.     First cranial nerve.
  II.    Second cranial nerve.
  IV.    Fourth cranial nerve.
  V.     Fifth cranial nerve.
  VII.   Seventh cranial nerve.
  VIII.  Eighth cranial nerve.
  IX.    Ninth cranial nerve.
  X.     Tenth cranial nerve.]

[Illustration: FIG. 82.--_Petromyzon marinus unicolor_ (Dekay). Head of
Lake Lamprey, showing pineal body. (After Gage.)]

It has been supposed that this eye, once possessed by all vertebrate
forms, has been gradually lost with the better development of the
paired eyes, being best preserved in reptiles as "an outcome of the
life-habit which concealed the animal in sand or mud, and allowed the
forehead surface alone to protrude, the median eye thus preserving its
ancestral value in enabling the animal to look directly upward and
backward." This theory receives no support from the structures seen in
the fishes.

In none of the fishes is the epiphysis more than a nervous enlargement,
and neither in fishes nor in amphibia is there the slightest suggestion
of its connection with vision. It seems probable, as suggested by
Hertwig and maintained by Dean that the original function of the pineal
body was a nervous one and that its connection with or development into
a median eye in lizards was a modification of a secondary character.
On consideration of the evidence, Dr. Dean concludes that "the pineal
structures of the true fishes do not tend to confirm the theory that
the epiphysis of the ancestral vertebrates was connected with a median
unpaired eye. It would appear, on the other hand, that both in their
recent and fossil forms the epiphysis was connected in its median
opening with the innervation of the sensory canals of the head. This
view seems essentially confirmed by ontogeny. The fact that three
successive pairs of epiphyseal outgrowths have been noted in the roof
of the thalamencephalon[6] appears distinctly adverse to the theory of
a median eye."[7]

=The Brain of Primitive Fishes.=--The brain of the hagfish differs
widely from that of the higher fishes, and the homologies of the
different parts are still uncertain. The different ganglia are all
solid and are placed in pairs. It is thought that the cerebellum is
wanting in these fishes, or represented by a narrow commissure (_corpus
restiforme_) across the front of the medulla. In the lamprey the brain
is more like that of the ordinary fish.

In the lancelet there is no trace of brain, the band-like spinal cord
tapering toward either end.

=The Spinal Cord.=--The spinal cord extends from the brain to the tail,
passing through the neural arches of the different vertebrae when these
are developed. In the higher fishes it is cylindrical and inelastic.
In a few fishes (headfish, trunkfish) in which the posterior part of
the body is shortened or degenerate, the spinal cord is much shortened,
and replaced behind by a structure called cauda equina. In the headfish
it has shrunk into "a short and conical appendage to the brain." In the
Cyclostomes and chimaera the spinal cord is elastic and more or less
flattened or band-like, at least posteriorly.

=The Nerves.=--The nerves of the fish correspond in general in place
and function with those of the higher animals. They are, however,
fewer in number, both large nerve-trunks and smaller nerves being less
developed than in higher forms.

The _olfactory nerves_, or first pair, extend through the ethmoid bone
to the nasal cavity, which is typically a blind sac with two roundish
openings, but is subject to many variations. The _optic nerves_, or
second pair, extend from the eye to the base of the optic lobes. In
Cyclostomes these nerves run from each eye to the lobe of its own side.
In the bony fishes, or Teleostei, each runs from the eye to the lobe of
the opposite side. In the sharks, rays, chimaeras, and Ganoids the two
optic nerves are joined in a chiasma as in the higher vertebrates.

Other nerves arising in the brain are the third pair, or _nervus
oculorum motorius_, and the fourth pair, _nervus trochlearis_, both
of which supply the muscles of the eye. The fifth pair, _nervus
trigeminus_, and the seventh pair, _nervus facialis_, arise from the
medulla oblongata and are very close together. Their various branches,
sensory and motor, ramify among the muscles and sensory areas of the
head. The sixth pair, _nervus abducens_, passes also to muscles of the
eye, and in sharks to the nictitating membrane or third eyelid.

The eighth pair, _nervus acousticus_, leads to the ear. The ninth
pair, _glosso-pharyngeal_, passes to the tongue and pharynx, and forms
a ganglion connected with the sympathetic system. The tenth pair,
_nervus vagus_, or pneumogastric nerve, arises from strong roots in
the corpus restiforme and the lower part of the medulla oblongata. Its
nerves, motor and sensory, reach the muscles of the gill-cavity, heart,
stomach, and air-bladder, as well as the muscular system and the skin.
In fishes covered with bony plates the skin may be nearly or quite
without sensory nerves. The eleventh pair, _nervus accessorius_, and
twelfth pair, _nervus hypoglossus_, are wanting in fishes.

The spinal nerves are subject to some special modifications, but in
the main correspond to similar structures in higher vertebrates. The
anterior root of each nerve is without ganglionic enlargement and
contains only motor elements. The posterior or dorsal root is sensory
only and widens into a ganglionic swelling near the base.

A sympathetic system corresponding to that in the higher vertebrates is
found in all the Teleostei, or bony fishes, and in the body of sharks
and rays in which it is not extended to the head.

FOOTNOTES:

[6] The thalamencephalon or the interbrain is a name given to the
region of the optic thalami, between the bases of the optic lobes and
cerebrum.

[7] Fishes Recent and Fossil, p. 55.




CHAPTER VIII

THE ORGANS OF SENSE


=The Organs of Smell.=--The sense-organs of the fish correspond in
general to those of the higher vertebrates. The sense of taste is,
however, feeble or wanting, and that of hearing is muffled and without
power of acute discrimination, if indeed it exists at all. According
to Dr. Kingsley (Vert. Zool., p. 75), "recent experiments tend to show
that in fishes the ears are without auditory functions and are solely
organs of equilibration."

The sense of smell resides in the nostrils, which have no relation
to the work of breathing. No fish breathes through its nostrils, and
only in a few of the lowest forms (hagfishes) does the nostril pierce
through the roof of the mouth. In the bony fishes the nostril is a
single cavity, on either side, lined with delicate or fringed membrane,
well provided with blood-vessels, and with nerves from the olfactory
lobe. In most cases each nasal cavity has two external openings. These
may be simple, or the rim of the nostril may be elevated, forming a
papilla or even a long barbel. Either nostril may have a papilla or
barbel, or the two may unite in one structure with two openings or with
sieve-like openings, or in some degenerate types (_Tropidichthys_)
with no obvious openings at all, the olfactory nerves spreading over
the skin of a small papilla. The openings may be round, slit-like,
pore-like, or may have various other forms. In certain families of bony
fishes (_Pomacentridae_, _Cichlidae_, _Hexagrammidae_), there is but one
opening to each nostril. In the sharks, rays, and chimaeras there is
also but one opening on either side and the nostril is large and highly
specialized, with valvular flaps controlled by muscles which are said
to enable them "to scent actively as well as to smell passively."

In the lancelet there is a single median organ supposed to be a
nostril, a small depression at the front of the head, covered by
ciliated membrane. In the hagfish the single median nostril pierces the
roof of the mouth, and is strengthened by cartilaginous rings, like
those of the windpipe. In the lamprey the single median nostril leads
to a blind sac. In the _Barramunda_ (_Neoceratodus_) there are both
external and internal nares, the former being situated just within the
upper lip. In all other fishes there is a nasal sac on either side of
the head. This has usually, but not always, two openings.

There is little doubt that the sense of smell in fishes is relatively
acute, and that the odor of their prey attracts them to it. It is known
that flesh, blood, or a decaying carcass will attract sharks, and other
predatory fish are drawn in a similar manner. At the same time the
strength of this function is yet to be tested by experiments.

[Illustration: FIG. 83.--Dismal Swamp Fish, _Chologaster cornutus_
Agassiz. Supposed ancestor of _Typhlichthys_. Virginia.]

[Illustration: FIG. 84.--Blind Cavefish, _Typhlichthys subterraneus_
Girard. Mammoth Cave, Kentucky.]

=The Organs of Sight.=--The eyes of fishes differ from those of the
higher vertebrates mainly in the spherical form of the crystalline
lens. This extreme convexity is necessary because the lens itself is
not very much denser than the fluid in which the fishes live. The
eyes vary very much in size and somewhat in form and position. They
are larger in fishes living at a moderate depth than in shore fishes
or river fishes. At great depths, as a mile or more, where all light
is lost, they may become aborted or rudimentary, and may be covered
by the skin. Often species with very large eyes, making the most of a
little light or of light from their own luminous spots, will inhabit
the same depths with fishes having very small eyes or eyes apparently
useless for seeing, retained as vestigial structures through heredity.
Fishes which live in caves become also blind, the structures showing
every possible phase of degradation. The details of this gradual loss
of eyes, whether through reversed selection or hypothetically through
inheritance of atrophy produced by disuse, have been given in a number
of memoirs on the blind fishes of the Mississippi Valley by Dr. Carl H.
Eigenmann.

In some fishes the eye is raised on a short, fleshy stalk and can
be moved about at the will of the fish. It is said that the vision
of the pond-skipper, _Periophthalmus_, when hunting insects on the
mud flats of Japan or India is "quite equal to that of a frog." It
is known also that trout possess keen eyesight, and that they show a
marked preference for one sort or another of real or artificial fly.
Nevertheless the vision of fishes in general is probably not very
precise. They apparently notice motion rather than outline, changes
rather than objects, while the extreme curvature of the crystalline
lens would seem to render them all near-sighted.

[Illustration: FIG. 85.--Four-eyed Fish, _Anableps dovii_ Gill.
Tehuantepec, Mexico.]

In the eyes of the fishes there is no lachrymal gland. True eyelids
no fishes possess; the integuments of the head pass over the eye,
becoming transparent as they cross the orbit. In some fishes part of
this integument is thickened, covering the eye fully although still
transparent. This forms the adipose eyelid characteristic of the
mullet, mackerel, and ladyfish. Many of the sharks possess a distinct
nictitating membrane or special eyelid, moved by a set of muscles. The
iris in most fishes surrounds a round pupil without much power of
contraction. It is frequently brightly , red, orange, black,
blue, or green. In fishes, like rays or flounders, which lie on the
bottom, a dark lobe covers the upper part of the pupil--a curtain
to shut out light from above. The cornea is little convex, leaving
small space for aqueous humor. In two genera of fishes, _Anableps_,
_Dialommus_, the cornea is divided by a horizontal partition into two
parts. This arrangement permits these fishes, which swim at the surface
of the water, to see both in and out of the medium. _Anableps_, the
four-eyed fish, is a fresh-water fish of tropical America, which swims
at the surface like a top-minnow, feeding on insects. _Dialommus_ is a
marine blenny from the Panama region, apparently of similar habit.

[Illustration: FIG. 86.--_Ipnops murrayi_ Guenther.]

In one genus of deep-sea fishes, _Ipnops_, the eyes are spread out to
cover the whole upper surface of the head, being modified as luminous
areas. Whether these fishes can see at all is not known.

[Illustration: FIG. 87.--Pond-skipper, _Boleophthalmus chinensis_
(Osbeck). Bay of Tokyo, Japan; from nature. K. Morita. (Eye-stalks
shrunken in preservation.)]

The position of the optic nerves is described in a previous chapter.

In ordinary fishes there is one eye on each side of the head, but in
the flounders, by a distortion of the cranium, both appear on the same
side. This side is turned uppermost as the fish swims in the water or
when it lies on the bottom. This distortion is a matter of development.
The very young flounder swims with its broad axis vertical in the
water, and it has one eye on either side. As soon as it rests on the
bottom it begins to lean to one side. The lower eye changes its axis
and by degrees travels across the face of the fish, part of the bony
interorbital moving with it across to the other side. In some soles it
is said to pass through the substance of the head, reappearing on the
other side. In all species which the writer has examined the cranium
is twisted, the eye moving with the bones; and the frontal bone is
divided, a new orbit being formed by this division. In most northern
flounders the eyes are on the right side in the adult, in tropical
forms more frequently on the left, these distinctions corresponding
with others in the structure of the fish.

In the lowest of the fish-like forms, the lancelet, the eye is simply
a minute pigment-spot situated in the anterior wall of the ventricle
at the anterior end of the central nervous system. In the hagfishes,
which stand next highest in the series, the eye, still incomplete, is
very small and hidden by the skin and muscles. This condition is very
different from that of the blind fishes of the higher groups, in which
the eye is lost through atrophy, because in life in caves or under
rocks the function of seeing is no longer necessary.

=The Organs of Hearing.=--The ear of the typical fish consists of the
labyrinth only, including the vestibule and usually three semicircular
canals, these dilating into sacs which contain one or more large,
loose bones, the ear-stones or otoliths. In the lampreys there are two
semicircular canals, in the hagfish but one. There is no external ear,
no tympanum, and no Eustachian tube. The ear-sac on each side is lodged
in the skull or at the base of the cranial cavity. It is externally
surrounded by bone or cartilage, but sometimes it lies near a
fontanelle or opening in the skull above. In some fishes it is brought
into very close connection with the anterior end of the air-bladder.
The latter organ it is thought may form part of the apparatus for
hearing. The arrangement for this purpose is especially elaborate
in the carp and the catfish families. In these fishes and their
relatives (called _Ostariophysi_) the two vestibules are joined in a
median sac (_sinus impar_) in the substance of the basioccipital. This
communicates with two cavities in the atlas, which again are supported
by two small bones, these resting on a larger one in connection with
the front of the air-bladder. The system of bones is analogous to that
found in the higher vertebrates, but it connects with the air-bladder,
not with an external tympanum. The bones are not homologous with those
of the ear of higher animals, being processes of the anterior vertebrae.
The tympanic chain of higher vertebrates has been thought homologous
with the suspensory of the mandible.

[Illustration: FIG. 88.--Brook Lamprey, _Lampetra wilderi_ Jordan and
Evermann. (After Gage.) Cayuga Lake.]

The otoliths, commonly two in each labyrinth, are usually large, firm,
calcareous bodies, with enamelled surface and peculiar grooves and
markings. Each species has its own form of otolith, but they vary much
in different groups of fishes.

[Illustration: FIG. 89.--European Lancelet, _Branchiostoma lanceolatum_
(Pallas). (After Parker and Haswell.)]

In the Elasmobranchs (sharks and rays) and in the Dipnoans the ear-sac
is enclosed in the cartilaginous substance of the skull. There is a
small canal extending to the surface of the skull, ending sometimes in
a minute foramen. The otoliths in these fishes are soft and chalk-like.

The lancelet shows no trace of an ear. In the cyclostomes, hagfishes,
and lampreys it forms a capsule of relatively simple structure
conspicuous in the prepared skeleton.

The sense of hearing in fishes cannot be very acute, and is at the
most confined to the perception of disturbances in the water. Most
movements of the fish are governed by sight rather than by sound. It
is in fact extremely doubtful whether fishes really hear at all, in
a way comparable to the auditory sense in higher vertebrates. Recent
experiments of Professor G. H. Parker on the killifish tend to show a
moderate degree of auditory sense which grades into the sense of touch,
the tubes of the lateral line assisting in both hearing and touch.
While the killifish responds to a bass-viol string, there may be some
fishes wholly deaf.

=Voices of Fishes.=--Some fishes make distinct noises variously
described as quivering, grunting, grating, or singing. The name
grunt is applied to species of _Haemulon_ and related genera, and
fairly describes the sound these fishes make. The Spanish name ronco
or roncador (grunter or snorer) is applied to several fishes, both
sciaenoid and haemuloid. The noise made by these fishes may be produced
by forcing air from part to part of the complex air-bladder, or it may
be due to grating one on another of the large pharyngeals. The grating
sounds arise, no doubt, from the pharyngeals, while the quivering or
singing sounds arise in the air-bladder. The midshipman, _Porichthys
notatus_, is often called singing fish, from a peculiar sound it emits.
These sounds have not yet been carefully investigated.

=The Sense of Taste.=--It is not certain that fishes possess a sense of
taste, and it is attributed to them only through their homology with
the higher animals. The tongue is without delicate membranes or power
of motion. In some fishes certain parts of the palate or pharyngeal
region are well supplied with nerves, but no direct evidence exists
that these have a function of discrimination among foods. Fishes
swallow their food very rapidly, often whole, and mastication, when it
takes place, is a crushing or cutting process, not one likely to be
affected by the taste of the food.

=The Sense of Touch.=--The sense of touch is better developed among
fishes. Most of them flee from contact with actively moving objects.
Many fishes use sensitive structures as a means of exploring the bottom
or of feeling their way to their food. The barbel or fleshy filament
wherever developed is an organ of touch. In some fishes, barbels are
outgrowths from the nostrils. In the catfish the principal barbel grows
from the rudimentary maxillary bone. In the horned dace and gudgeon the
little barbel is attached to the maxillary. In other fishes barbels
grow from the skin of the chin or snout. In the goatfish and surmullet
the two chin barbels are highly specialized. In _Polymixia_ the chin
barbels are modified _branchiostegals_. In the codfish the single
beard is little developed. In the gurnards and related forms the lower
rays of the pectoral are separate and barbel-like. Detached rays of
this sort are found in the thread-fins (_Polynemidae_), the gurnards
(_Triglidae_), and in various other fishes. Barbels or fleshy flaps are
often developed over the eyes and sometimes on the scales or the fins.

[Illustration: FIG. 90.--Goatfish, _Pseudupeneus maculatus_ (Bloch).
Woods Hole.]

The structure of the lateral line and its probable relation as a
sense-organ is discussed on page 23. It is probable that it is
associated with sense of touch, and hearing as well, the internal ear
being originally "a modified part of the lateral-line system," as shown
by Parker,[8] who calls the skin the lateral line and the ear "three
generations of sense-organs."

The sense of pain is very feeble among fishes. A trout has been known
to bite at its own eye placed on a hook, and similar insensibility
has been noted in the pike and other fishes. "The Greenland shark,
when feeding on the carcass of a whale, allows itself to be repeatedly
stabbed in the head without abandoning its prey." (GUeNTHER.)

FOOTNOTES:

[8] See Parker, on the sense of hearing in fishes, American Naturalist
for March, 1903.




CHAPTER IX

THE ORGANS OF REPRODUCTION


=The Germ-cells.=--In most fishes the germ-cells are produced in large
sacs, ovaries or testes, arranged symmetrically one on either side of
the posterior part of the abdominal cavity. The sexes are generally but
not always similar externally, and may be distinguished on dissection
by the difference between the sperm-cells and the ova. The ovary
with its eggs is more yellow in color and the contained cells appear
granular. The testes are whitish or pinkish, their secretion milk-like,
and to the naked eye not granular.

[Illustration: FIG. 91.--Sword-tail Minnow, male, _Xiphophorus helleri_
Heckel. The anal fin modified as an intromittent organ. Vera Cruz.]

In a very few cases both organs have been found in the same fish, as
in _Serranus_, which is sometimes truly hermaphrodite. All fishes,
however, seem to be normally dioecious, the two sexes in different
individuals. Usually there are no external genital organs, but
in some species a papilla or tube is developed at the end of the
urogenital sinus. This may exist in the breeding season only, as in the
fresh-water lampreys, or it may persist through life as in some gobies.
In the Elasmobranchs, cartilaginous claspers, attached to the ventral
fins in the male, serve as a conduit for the sperm-cells.

=The Eggs of Fishes.=--The great majority of fishes are oviparous, the
eggs being fertilized after deposition. The eggs are laid in gravel or
sand or other places suitable for the species, and the milt containing
the sperm-cells of the male is discharged over or among them in the
water. A very small quantity of the sperm-fluid may impregnate a large
number of eggs. But one sperm-cell can enter a particular egg. In
a number of families the species are ovoviviparous, the eggs being
hatched in the ovary or in a dilated part of the oviduct, the latter
resembling a real uterus. In some sharks there is a structure analogous
to the placenta of higher animals, but not of the same structure or
origin. In the case of viviparous fishes actual copulation takes place
and there is usually a modification of some organ to effect transfer of
the sperm-cells. This is the purpose of the sword-shaped anal fin in
many top-minnows (_Paeciliidae_), the fin itself being placed in advance
of its usual position. In the surf-fishes (_Embiotocidae_) the structure
of part of the anal fin is modified, although it is not used as an
intromittent organ. In the Elasmobranchs, as already stated, large
organs of cartilage (claspers) are developed from the ventral fins.

[Illustration: FIG. 92.--White Surf-fish, viviparous, with young,
_Cymatogaster aggregatus_ Gibbons. San Francisco.]

In some viviparous fishes, as in the rockfishes (_Sebastodes_) and
rosefishes (_Sebastes_), the young are very minute at birth.

[Illustration: FIG. 93.--_Goodea luitpoldi_ (Steindachner). A
viviparous fish from Lake Patzcuaro, Mexico. Family _Paeciliidae_. (After
Meek.)] In others, as the surf-fishes (_Embiotocidae_), they are
relatively large and few in number. In the viviparous sharks, which
constitute the majority of the species of living sharks, the young are
large at birth and prepared to take care of themselves.

[Illustration: FIG. 94.--Egg of _Callorhynchus antarcticus_, the
Bottle-nosed Chimaera. (After Parker and Haswell.)]

The eggs of fishes vary very much in size and form. In those sharks
and rays which lay eggs the ova are deposited in a horny egg-case, in
color and texture suggesting the kelp in which they are laid. The eggs
of the bullhead sharks (_Heterodontus_) are spirally twisted, those of
the cat-sharks (_Scyliorhinidae_) are quadrate with long filaments at
the angles. Those of rays are wheelbarrow-shaped with four "handles."
One egg-case of a ray may sometimes contain several eggs and develop
several young. The eggs of lancelets are small, but those of the
hagfishes are large, ovate, with fibres at each side, each with a
triple hook at tip. The chimaera has also large egg-cases, oblong in
form.

[Illustration: FIG. 95.--Egg of the Hagfish, _Myxine limosa_ Girard,
showing threads for attachment. (After Dean.)]

In the higher fishes the eggs are spherical, large or small according
to the species, and varying in the firmness of their outer walls. All
contain food-yolk from which the embryo in its earlier stages is fed.
The eggs of the eel (_Anguilla_) are microscopic. According to Guenther
25,000 eggs have been counted in the herring, 155,000 in the lumpfish,
3,500,000 in the halibut, 635,200 in the sturgeon, and 9,344,000 in the
cod. Smaller numbers are found in fishes with large ova. The red salmon
has about 3500 eggs, the king salmon about 5200. Where an oviduct
is present the eggs are often poured out in glutinous masses, as in
the bass. When, as in the salmon, there is no oviduct, the eggs lie
separate and do not cohere together. It is only with the latter class
of fishes, those in which the eggs remain distinct, that artificial
impregnation and hatching is practicable. In this regard the value of
the salmon and trout is predominant. In some fishes, especially those
of elongate form, as the needle-fish (_Tylosurus_), the ovary of but
one side is developed.

[Illustration: FIG. 96.--Egg of Port Jackson Shark, _Heterodontus
philippi_ (Lacepede). (After Parker and Haswell.)]

=Protection of the Young.=--In most fishes the parents take no care
of their eggs or young. In some catfishes (_Platystacus_) the eggs
adhere to the under surface of the female. In a kind of pipefish
(_Solenostomus_), a large pouch for retention of the eggs is formed on
the belly of the female. In the sea-horses and pipefishes a pouch is
formed in the skin, usually underneath the tail of the male. Into this
the eggs are thrust, and here the young fishes hatch out, remaining
until large enough to take care of themselves. In certain sea catfishes
(_Galeichthys, Conorhynchos_) the male carries the eggs in his mouth,
thus protecting them from the attacks of other fishes. In numerous
cases the male constructs a rough nest, which he defends against all
intruders, against the female as well as against outside enemies.
The nest-building habit is especially developed in the sticklebacks
(_Gasterosteidae_), a group in which the male fish, though a pygmy in
size, is very fierce in disposition.

In a minnow of Europe (_Rhodeus amarus_) the female is said to deposit
her eggs within the shells of river mussels.

=Sexual Modification.=--In the relatively few cases in which the sexes
are unlike the male is usually the brighter in color and with more
highly developed fins. Blue, red, black, and silvery-white pigment
are especially characteristic of the male, the olivaceous and mottled
coloration of the female. Sometimes the male has a larger mouth, or
better developed crests, barbels, or other appendages. In some species
the pattern of coloration in the two sexes is essentially different.

In various species the male develops peculiar structures not found in
the female, and often without any visible purpose. In the chimaera a
peculiar cartilaginous hook armed with a brush of enamelled teeth at
the tip is developed on the forehead in the male only. In the skates
or true rays (_Raja_) the pectoral fin has near its edge two rows
of stout incurved spines. These the female lacks. In the breeding
season, among certain fishes, the male sometimes becomes much brighter
by the accumulation of bright red or blue pigment accompanied by
black or white pigment cells. This is especially true in the minnows
(_Notropis_), the darters (_Etheostoma_), and other fresh-water species
which spawn in the brooks of northern regions in the spring. In the
minnows and suckers horny excrescences are also developed on head,
body, or fins, to be lost after the deposition of the spawn.

In the salmon, especially those of the Pacific, the adult male becomes
greatly distorted in the spawning season, the jaws and teeth being
greatly elongated and hooked or twisted so that the fish cannot shut
its mouth. The Atlantic salmon and the trout show also some elongation
of the jaws, but not to the same extent.

In those fishes which pair the relation seems not to be permanent, nor
is there anything to be called personal affection among them so far as
the writer has noticed.

There is no evidence that the bright colors or nuptial adornments of
the males are enhanced by sexual selection. In most species the males
deposit the sperm-cells in spawning-grounds without much reference to
the preference of the females. In general the brightest colors are not
found among viviparous fishes. None of the groups in which the males
are showily , while the females are plain, belong to this class.
The brightest colors are found on the individuals most mature or having
greatest vitality.




CHAPTER X

EMBRYOLOGY AND GROWTH OF FISHES


=Segmentation of the Egg.=--The egg of the fish develops only after
fertilization (amphimixis). This process is the union of its nuclear
substance with that of the sperm-cell from the male, each cell carrying
its equal share in the function of heredity. When this process takes
place the egg is ready to begin its segmentation. The eggs of all
fishes are single cells containing more or less food-yolk. The presence
of this food-yolk affects the manner of segmentation in general,
those eggs having the least amount of food-yolk developing most
typically. The simplest of all fish like vertebrates, the lancelet
(_Branchiostoma_) has very small eggs, and in their early development
it passes through stages that are typical for all many-celled animals.
The first stage in development is the simple splitting of the egg
into two halves. These two daughter cells next divide so that there
are four cells; each of these divides, and this division is repeated
until a great number of cells is produced. The phenomenon of repeated
division of the germ-cell is called cleavage, and this cleavage is the
first stage of development in the case of all many-celled animals.
Instead of forming a solid mass the cells arrange themselves in such a
way as to form a hollow ball, the wall being a layer one cell thick.
The included cavity is called the segmentation cavity, and the whole
structure is known as a blastula. This stage also is common to all the
many-celled animals. The next stage is the conversion of the blastula
into a double-walled cup, known as a gastrula by the pushing in of one
side. All the cells of the blastula are very small, but those on one
side are somewhat larger than those of the other, and here the wall
first flattens and then bends in until finally the larger cells come
into contact with the smaller and the segmentation cavity is entirely
obliterated. There is now an inner layer of cells and an outer layer,
the inner layer being known as the endoblast and the outer as the
ectoblast. The cavity of the cup thus formed is the archenteron and
gives rise primarily to the alimentary canal. This third well-marked
stage is called the gastrula stage; and it is thought to occur either
typically or in some modified form in the development of all metazoa,
or many-celled animals. In the lampreys, the Ganoids, and the Dipnoans
the eggs contain a much greater quantity of yolk than those of the
lancelet, but the segmentation resembles that of the lancelet in
that it is complete; that is, the whole mass of the egg divides into
cells. There is a great difference, however, in the size of the cells,
those at the upper pole being much smaller than those at the lower.
In _Petromyzon_ and the Dipnoans blastula and gastrula stages result,
which, though differing in some particulars from the corresponding
stages of the lancelet, may yet readily be compared with them. In
the hagfishes, sharks, rays, chimaeras, and most bony fishes there
is a large quantity of yolk, and the protoplasm, instead of being
distributed evenly throughout the egg, is for the most part accumulated
upon one side, the nucleus being within this mass of protoplasm. When
the food substance or yolk is consumed and the little fish is able
to shift for itself, it leaves the egg-envelopes and is said to be
hatched. The figures on page 135 show some of the stages by which cells
are multiplied and ultimately grouped together to form the little fish.

=Post-embryonic Development.=--In all the fishes the development of the
embryo goes on within the egg long after the gastrula stage is passed,
and until the embryo becomes a complex body, composed of many differing
tissues and organs. Almost all the development may take place within
the egg, so that when the young animal hatches there is necessary
little more than a rapid growth and increase of size to make it a fully
developed mature animal. This is the case with most fishes: a little
fish just hatched has most of the tissues and organs of a full-grown
fish, and is simply a small fish. But in the case of some fishes the
young hatches from the egg before it has reached such an advanced state
of development, and the young looks very different from its parent. It
must yet undergo considerable change before it reaches the structural
condition of a fully developed and fully grown fish. Thus the
development of most fishes is almost wholly embryonic development--that
is, development within the egg or in the body of the mother--while the
development of some of them is to a considerable degree post-embryonic
or larval development. There is no important difference between
embryonic and post-embryonic development. The development is continuous
from egg-cell to mature animal and, whether inside or outside of an
egg, it goes on with a degree of regularity. While certain fishes are
subject to a sort of metamorphosis, the nature of this change is in no
way to be compared with the change in insects which undergo a complete
metamorphosis. In the insects all the organs of the body are broken
down and rebuilt in the process of change. In all fishes a structure
once formed maintains a more nearly continuous integrity although often
considerably altered in form.

=General Laws of Development.=--The general law of development may
be briefly stated as follows: All many-celled animals begin life as
a single cell, the fertilized egg-cell; each animal goes through a
certain orderly series of developmental changes which, accompanied by
growth, leads the animal to change from single-cell to many-celled,
complex form characteristic of the species to which the animal belongs;
this development is from simple to complex structural condition; the
development is the same for all individuals of one species. While all
animals begin development similarly, the course of development in the
different groups soon diverges, the divergence being of the nature of
a branching, like that shown in the growth of a tree. In the free tips
of the smallest branches we have represented the various species of
animals in their fully developed condition, all standing clearly apart
from each other. But in tracing back the development of any kind of
animal we soon come to a point where it very much resembles or becomes
apparently identical with some other kind of animal, and going farther
back we find it resembling other animals in their young condition, and
so on until we come to that first stage of development, that trunk
stage where all animals are structurally alike. Any animal at any stage
in its existence differs absolutely from any other kind of animal, in
this respect: it can develop into only its own kind. There is something
inherent in each developing animal that gives it an identity of its
own. Although in its young stages it may be indistinguishable from some
other species of animal in its young stages, it is sure to come out,
when fully developed, an individual of the same kind as its parents
were or are. The young fish and the young salamander may be alike to
all appearance, but one embryo is sure to develop into a fish, and
the other into a salamander. This certainty of an embryo to become an
individual of a certain kind is called the law of heredity. Viewed in
the light of development, there must be as great a difference between
one egg and another as between one animal and another, for the greater
difference is included in the less.

=The Significance of Facts of Development.=--The significance of the
process of development in any species is yet far from completely
understood. It is believed that many of the various stages in the
development of an animal correspond to or repeat the structural
condition of the animal's ancestors. Naturalists believe that all
animals having a notochord at any stage in their existence are related
to each other through being descended from a common ancestor, the first
or oldest chordate or back-boned animal. In fact it is because all
these chordate animals--the lancelets, lampreys, fishes, batrachians,
the reptiles, the birds, and the mammals--have descended from a common
ancestor that they all develop a notochord, and those most highly
organized replace this by a complete back-bone. It is believed that
the descendants of the first back-boned animal have, in the course
of many generations, branched off little by little from the original
type until there came to exist very real and obvious differences among
the back-boned animals--differences which among the living back-boned
animals are familiar to all of us. The course of development of an
individual animal is believed to be a very rapid and evidently much
condensed and changed recapitulation of the history which the species
or kind of animal to which the developing individual belongs has passed
through in the course of its descent through a long series of gradually
changing ancestors. If this is true, then we can readily understand
why the fish and the salamander and the tortoise and bird and rabbit
are all alike in their earlier stages of development, and gradually
come to differ more and more as they pass through later and later
developmental stages.

=Development of the Bony Fishes.[9]= The mode of development of bony
fishes differs in many and apparently important regards from that of
their nearest kindred, the Ganoids. In their eggs a large amount of
yolk is present, and its relations to the embryo have become widely
specialized. As a rule, the egg of a Teleost is small, perfectly
spherical, and enclosed in delicate but greatly distended membranes.
The germ disc is especially small, appearing on the surface as an
almost transparent fleck. Among the fishes whose eggs float at the
surface during development, as of many pelagic Teleosts, e.g., the
sea-bass, _Centropristes striatus_, the yolk is lighter in specific
gravity than the germ; it is of fluid-like consistency, almost
transparent. In the yolk at the upper pole of the egg an oil globule
usually occurs; this serves to lighten the relative weight of the
entire egg, and from its position must aid in keeping this pole of the
egg uppermost.

[Illustration: FIG. 97.--Development of Sea-bass, _Centropristes
striatus_ (Linnaeus). _a_, egg prior to germination; _b_, germ-disk
after first cleavage; _c_, germ-disk after third cleavage; _d_, embryo
just before hatching. (After H. V. Wilson.)]

In the early segmentation of the germ the first cleavage plane is
established, and the nuclear divisions have taken place for the second;
in the latter the third cleavage has been completed. As in other fishes
these cleavages are vertical, the third parallel to the first. A
segmentation cavity occurs as a central space between the blastomeres,
as it does in the sturgeon and garpike.

In stages of late segmentation the segmentation cavity is greatly
flattened, but extends to the marginal cells of the germ-disk; its
roof consists of two tiers of blastomeres, its floor of a thin film of
the unsegmented substance of the germ; the marginal blastomeres are
continuous with both roof and floor of the cavity, and are produced
into a thin film which passes downward, around the sides of the yolk.
Later the segmentation cavity is still further flattened; its roof
is now a dome-shaped mass of blastomeres; the marginal cells have
multiplied, and their nuclei are seen in the layer of the germ, below
the plane of the segmentation cavity. These are seen in the surface
view of the marginal cells of this stage; they are separated by
cell boundaries only at the sides; below they are continuous in the
superficial down-reaching layer of the germ. The marginal cells shortly
lose all traces of having been separate; their nuclei, by continued
division, spread into the layer of germ flooring the segmentation
cavity, and into the delicate film of germ which now surrounds the
entire yolk. Thus is formed the _periblast_ of the Teleost development,
which from this point onward is to separate the embryo from the yolk;
it is clearly the specialized inner part of the germ, which, becoming
fluid-like, loses its cell-walls, although retaining and multiplying
its nuclei. Later the periblast comes into intimate relations with the
growing embryo; it lies directly against it, and appears to receive
cell increments from it at various regions; on the other hand, the
nuclei of the periblast, from their intimate relations with the yolk,
are supposed to subserve some function in its assimilation.

Aside from the question of periblast, the growth of the blastoderm
appears not unlike that of the sturgeon. From the blastula stage to
that of the early gastrula, the changes have been but slight; the
blastoderm has greatly flattened out as its margins grow downward,
leaving the segmentation cavity apparent. The rim of the blastoderm
has become thickened as the 'germ-ring'; and immediately in front of
the dorsal lip of the blastopore its thickening marks the appearance
of the embryo. The germ-ring continues to grow downward, and shows
more prominently the outline of the embryo; this now terminates at
the head region; while on either side of this point spreads out
tail-ward on either side the indefinite layer of outgrowing mesoderm.
In the next stage the closure of the blastopore is rapidly becoming
completed; in front of it stretches the widened and elongated form of
the embryo. The yolk-plug is next replaced by periblast, the dorsal
lip by the tail-mass, or more accurately the dorsal section of the
germ-rim; the coelenteron under the dorsal lip has here disappeared,
on account of the close approximation of the embryo to the periblast;
its last remnant, the Kupffer's vesicle, is shortly to disappear. The
germ-layers become confluent, but, unlike the sturgeon, the flattening
of the dorsal germ-ring does not permit the formation of a neurenteric
canal.

[Illustration: FIG. 98. Sea-bass, _Centropristes striatus_, natural
size. (From life, by R. W. Shufeldt.)--Page 137.]

The process of the development of the germ-layers in Teleosts appears
as an abbreviated one, although in many of its details it is but
imperfectly known. In the development of the medullary groove, as
an example, the following peculiarities exist: the medullary region
is but an insunken mass of cells without a trace of the groove-like
surface indentation. It is only later, when becoming separate from the
ectoderm, that it acquires its rounded character; its cellular elements
then group themselves symmetrically with reference to a sagittal plane,
where later, by their dissociation, the canal of the spinal cord is
formed. The growth of the entoderm is another instance of specialized
development. In an early stage the entoderm exists in the axial
region, its thickness tapering away abruptly on either side; its lower
surface is closely apposed to the periblast; its dorsal thickening
will shortly become separate as the notochord. In a following stage of
development the entoderm is seen to arch upward in the median line as a
preliminary stage in the formation of the cavity of the gut. Later, by
the approximation of the entoderm-cells in the median ventral line, the
condition is reached where the completed gut-cavity exists.

The formation of the mesoderm in Teleosts is not definitely understood.
It is usually said to arise as a process of 'delamination,' i.e.,
detaching itself in a mass from the entoderm. Its origin is, however,
looked upon generally as of a specialized and secondary character.

The mode of formation of the gill-slit of the Teleost does not differ
from that in other groups; an evagination of the entoderm coming in
contact with an invaginated tract of ectoderm fuses, and at this point
an opening is later established.

The late embryo of the Teleost, though of rounded form, is the more
deeply implanted in the yolk-sac than that of the sturgeon; it is
transparent, allowing notochord, primitive segments, heart, and
sense-organs to be readily distinguished; at about this stage both anus
and mouth are making their appearance.

[Illustration: FIG. 99.--Young Sword-fish, _Xiphias gladius_ (Linnaeus).
(After Luetken.)]

=The Larval Development of Fishes.[10]=--"When the young fish has freed
itself from its egg-membranes it gives but little suggestion of its
adult form. It enters upon a larval existence, which continues until
maturity. The period of change of form varies widely in the different
groups of fishes, from a few weeks' to longer than a year's duration;
and the extent of the changes that the larva undergoes are often
surprisingly broad, investing every organ and tissue of the body, the
immature fish passing through a series of form stages which differ one
from the other in a way strongly contrasting with the mode of growth
of amniotes; since the chick, reptile, or mammal emerges from its
embryonic membranes in nearly its adult form.

[Illustration: FIG. 100.--Sword-fish, _Xiphias gladius_ (Linnaeus).
(After Day.)]

The fish may, in general, be said to begin its existence as a larva as
soon as it emerges from its egg-membranes. In some instances, however,
it is difficult to decide at what point the larval stage is actually
initiated: thus in sharks the excessive amount of yolk material which
has been provided for the growth of the larva renders unnecessary the
emerging from the egg at an early stage; and the larval period is
accordingly to be traced back to stages that are still enclosed in the
egg-membranes. In all cases the larval life may be said to begin when
the following conditions have been fulfilled: the outward form of the
larva must be well defined, separating it from the mass of yolk, its
motions must be active, it must possess a continuous vertical fin-fold
passing dorsally from the head region to the body terminal, and thence
ventrally as far as the yolk region; and the following structures,
characteristic in outward appearance, must also be established: the
sense-organs--eye, ear, and nose--mouth and anus, and one or more
gill-clefts.

[Illustration: FIG. 101.--Larva of the Sail-fish, _Istiophorus_, very
young. (After Luetken.)]

[Illustration: FIG. 102.--Larva of Brook Lamprey, _Lampetra wilderi_,
before transformation, being as large as the adult, toothless, and more
distinctly segmented.]

[Illustration: FIG. 103.--Common Eel. _Anguilla chrisypa_ Rafinesque.
Family _Anguillidae_.]

Among the different groups of fishes the larval changes are brought
about in widely different ways. These larval peculiarities appear at
first of far-reaching significance, but may ultimately be attributed,
the writer believes, to changed environmental conditions, wherein one
process may be lengthened, another shortened. So, too, the changes
from one stage to another may occur with surprising abruptness. As a
rule, it may be said the larval stage is of longest duration in the
Cyclostomes, and thence diminished in length in sharks, lung-fishes,
Ganoids, and Teleosts; in the last-named group a very much curtailed
(i.e., precocious) larval life may often occur.

[Illustration: FIG. 104.--Larva of Common Eel, _Anguilla chrisypa_
(Rafinesque), called _Leptocephalus grassii_. (After Eigenmann.)]

The metamorphoses of the newly hatched Teleost must finally be
reviewed; they are certainly the most varied and striking of all larval
fishes, and, singularly enough, appear to be crowded into the briefest
space of time; the young fish, hatched often as early as on the fourth
day, is then of the most immature character; it is transparent,
delicate, easily injured, inactive; within a month, however, it may
have assumed almost every detail of its mature form. A form hatching
three millimeters in length may acquire the adult form before it
becomes much longer than a centimeter.

[Illustration: FIG. 105.--Larva of Sturgeon, _Acipenser sturio_
(Linnaeus). (After Kupffer, per Dean.)]

[Illustration: FIG. 106.--Larva (called _Tholichthys_) of _Chaetodon
sedentarius_ (Poey). Cuba. (After Luetken,)]

[Illustration: FIG. 107.--Butterfly-fish, _Chaetodon capistratus_
Linnaeus. Jamaica.]

=Peculiar Larval Forms.=--The young fish usually differs from the
adult mainly in size and proportions. The head is larger in the young,
the fins are lower, the appendages less developed, and the body
more slender in the young than in the adult. But to most of these
distinctions there are numerous exceptions, and in some fish there
is a change so marked as to be fairly called a metamorphosis. In
such cases the young fish in its first condition is properly called
a larva. The larva of the lamprey (_Petromyzon_) is nearly blind and
toothless, with slender head, and was long supposed to belong to a
different genus (_Ammocoetes_) from the adult. The larva of sharks and
rays, and also of Dipnoans and Crossopterygians, are provided with
bushy external gills, which disappear in the process of development.
In most soft-rayed fishes the embryonic fringe which precedes the
development of the vertical fins persists for a considerable time.
In many young fishes, especially the _Chaetodontidae_ and their allies
(butterfly-fishes), the young fish has the head armed with broad
plates formed by the backward extension of certain membrane-bones.
In other forms the bones of the head are in the young provided with
long spines or with serrations, which vanish totally with age. Such a
change is noticeable in the swordfish. In this species the production
of the bones of the snout and upper jaw into a long bony sword, or
weapon of offense, takes place only with age. The young fish have jaws
more normally formed, and armed with ordinary teeth. In the headfish
(_Mola mola_) large changes take place in the course of growth, and the
young have been taken for a different type of fishes. Among certain
soft-rayed fishes and eels the young is often developed in a peculiar
way, being very soft, translucent, or band-like, and formed of large
or loosely aggregated cells. These peculiar organisms, long known as
leptocephali, have been shown to be the normal young of fishes when
mature very different. In the ladyfish (_Albula_) Dr. Gilbert has
shown, by a full series of specimens, that in their further growth
these pellucid fishes shrink in size, acquiring greater compactness
of body, until finally reaching about half their maximum length as
larvae. After this, acquiring essentially the form of the adult fish,
they begin a process of regular growth. This leptocephalous condition
is thought by Guenther to be due to arrest of growth in abnormal
individuals, but this is not the case in _Albula_, and it is probably
fully normal in the conger and other eels. In the surf-fishes the
larvae have their vertical fins greatly elevated, much higher than in
the adult, while the body is much more closely compressed. In the
deal-fish (_Trachypterus_) the form of the body and fins changes
greatly with age, the body becoming more elongate and the fins lower.
The differences between different stages of the same fish seem greater
than the differences between distinct species. In fact with this and
with other forms which change with age, almost the only test of species
is found in the count of the fin-rays. So far as known the numbers of
these structures do not change. In the moonfishes (_Carangidae_) the
changes with age are often very considerable. We copy Luetken's figure
of the changes in the genus _Selene_ (fig. 113). Similar changes take
place in _Alectis_, _Vomer_, and other genera.

[Illustration: _Fig. 108._--_Mola mola_ (Linnaeus). Very early larval
stage of the Headfish, called _Centaurus booeps_. (After Richardson.)]

[Illustration: FIG. 109.--_Mola mola_ (Linnaeus). Early larval stage,
called _Molacanthus nummularis_. (After Ryder.)]

[Illustration: FIG. 110.--_Mola mola_ (Linnaeus). Advanced larval stage.
(After Ryder.)]

=The Development of Flounders.=--In the great group of flounders and
soles (_Heterosomata_) the body is greatly compressed and the species
swim on one side or lie flat on the bottom, with one side uppermost.
This upper side is  like the bottom, sand-color, gray, or brown,
while the lower side is mostly white. Both eyes are brought around
to the upper side by a twisting of the cranium and a modification or
division of the frontal bones. When the young flounder is hatched it
is translucent and symmetrical, swimming vertically in the water, with
one eye on either side of the head. After a little the young fish rests
the ventral edge on the bottom. It then leans to one side, and as its
position gradually becomes horizontal the eye on the lower side moves
across with its frontal and other bones to the other side. In most
species it passes directly under the first interneurals of the dorsal
fin. These changes are best observed in the genus _Platophrys_.

=Hybridism.=--Hybridism is very rare among fishes in a state of nature.
Two or three peculiar forms among the snappers (_Lutianus_) in Cuba
seem fairly attributable to hybridism, the single specimen of each
showing a remarkable mixture of characters belonging to two other
common species. Hybrids may be readily made in artificial impregnation
among those fishes with which this process is practicable. Hybrids of
the different salmon or trout usually share nearly equally the traits
of the parent species.

=The Age of Fishes.=--The age of fishes is seldom measured by a
definite period of years. Most of them grow as long as they live, and
apparently live until they fall victims to some stronger species.
It is reputed that carp and pike have lived for a century, but the
evidence needs verification. Some fishes, as the salmon of the Pacific
(_Oncorhynchus_), have a definite period of growth (usually four
years) before spawning. After this act all the individuals die so
far as known. In Japan and China the Ice-fish (_Salanx_), a very
long, slender, transparent fish allied to the trout, may possibly be
annual in habit, all the individuals perhaps dying in the fall to
be reproduced from eggs in the spring. But this alleged habit needs
verification.

[Illustration: FIG. 111.--Headfish (adult), _Mola mola_ (Linnaeus).
Virginia.]

=Tenacity of Life.=--Fishes differ greatly in tenacity of life. In
general, fishes of the deep seas die at once if brought near the
surface. This is due to the reduction of external pressure. The
internal pressure forces the stomach out through the mouth and may
burst the air-bladder and the large blood-vessels. Marine fishes
usually die very soon after being drawn out from the sea.

[Illustration: FIG. 112.--_Albula vulpes_ (Linnaeus). Transformation
of the Ladyfish, from the translucent, loosely compacted larva to the
smaller, firm-bodied young. Gulf of California. (After Gilbert.)]

[Illustration: FIG. 113.--Development of the Horsehead-fish, _Selene
vomer_ (Linnaeus). Family _Carangidae_. (After Luetken.)]

Some fresh-water fishes are very fragile, dying soon in the air, often
with injured air-bladder or blood-vessels. They will die even sooner
in foul water. Other fishes are extremely tenacious of life. The
mud-minnow (_Umbra_) is sometimes ploughed up in the half-dried mud of
Wisconsin prairies. The related Alaskan blackfish (_Dallia_) has been
fed frozen to dogs, escaping alive from their stomachs after being
thawed out. Many of the catfishes (_Siluridae_) will live after lying
half-dried in the dust for hours. The Dipnoan, _Lepidosiren_, lives in
a ball of half-dried mud during the arid season, and certain fishes,
mostly Asiatic, belonging to the group _Labyrinthici_, with accessory
breathing organ can long maintain themselves out of water. Among these
is the China-fish (_Ophiocephalus_), often kept alive in the Chinese
settlements in California and Hawaii. Some fishes can readily endure
prolonged hunger, while others succumb as readily as a bird or a mammal.

[Illustration: FIG. 114.--Ice-fish, _Salanx hyalocranius_ Abbott.
Family _Salangidae_. Tientsin, China.]

[Illustration: FIG. 115.--Alaska Blackfish, _Dallia pectoralis_ (Bean).
St. Michaels, Alaska.]

=The Effects of Temperature on Fish.=--The limits of distribution of
many fishes are marked by changes in temperature. Few marine fishes can
endure any sudden or great change in this regard, although fresh-water
fishes adapt themselves to the seasons. I have seen the cutlass-fish
(_Trichiurus_) benumbed with cold off the coast of Florida while the
temperature was still above the frost-line. Those fishes which are
tenacious of life and little sensitive to changes in climate and food
are most successfully acclimatized or domesticated. The Chinese carp
(_Cyprinus carpio_) and the Japanese goldfish (_Carassius auratus_)
have been naturalized in almost all temperate and tropical river
basins. Within the limits of clear, cold waters most of the salmon
and trout are readily transplanted. But some similar fishes (as the
grayling) are very sensitive to the least change in conditions. Most of
the catfish (_Siluridae_) will thrive in almost any fresh waters except
those which are very cold.

[Illustration: FIG. 116.--Snake-headed China-fish, _Ophiocephalus
barca_. India. (After Day.)]

=Transportation of Fishes.=--The eggs of species of salmon, placed in
ice to <DW44> their development, have been successfully transplanted
to great distances. The quinnat-salmon has been thus transferred
from California to Australia. It has been found possible to stock
rivers and lakes with desirable species, or to restock those in which
the fish-supply has been partly destroyed, through the means of
artificially impregnated eggs.

The method still followed is said to be the discovery of J. L. Jacobi
of Westphalia (about 1760). This process permits the saving of nearly
all the eggs produced by the individuals taken. In a condition of
nature very many of these eggs would be left unfertilized, or be
destroyed by other animals. Fishes are readily kept in captivity
in properly constructed aquaria. Unless injured in capture or
transportation, there are few species outside the deep seas which
cannot adapt themselves to life in a well-constructed aquarium.

=Reproduction of Lost Parts.=--Fishes have little power to reproduce
lost parts. Only the tips of fleshy structures are thus restored after
injury. Sometimes a fish in which the tail has been bitten off will
survive the injury. The wound will heal, leaving the animal with a
truncate body, fin-rays sometimes arising from the scars.

[Illustration: FIG. 117.--Monstrous Goldfish (bred in Japan),
_Carassius auratus_ (Linnaeus). (After Guenther.)]

=Monstrosities among Fishes.=--Monstrosities are rare among fishes
in a state of nature. Two-headed young are frequently seen at
salmon-hatcheries, and other abnormally divided or united young
are not infrequent. Among domesticated species monstrosities are
not infrequent, and sometimes, as in the goldfish, these have been
perpetuated to become distinct breeds or races. Goldfishes with
telescopic eyes and fantastic fins, and with the green coloration
changed to orange, are reared in Japan, and are often seen in other
countries. The carp has also been largely modified, the changes taking
place chiefly in the scales. Some are naked (leather-carp), others
(mirror-carp) have a few large scales arranged in series.

FOOTNOTES:

[9] This account of the normal development of the Teleost fishes is
condensed from Dr. Dean's "Fishes Living and Fossil," in which work the
details of growth in the Teleost are contrasted with those of other
types of fishes.

[10] This paragraph is condensed from Dean's "Fishes Living and Fossil."




CHAPTER XI

INSTINCTS, HABITS, AND ADAPTATIONS


=The Habits of Fishes.=--The habits of fishes can hardly be summarized
in any simple mode of classification. In the usual course of fish-life
the egg is laid in the early spring, in water shallower than that in
which the parents spend their lives. In most cases it is hatched as
the water grows warmer. The eggs of the members of the salmon and cod
families are, however, mostly hatched in cooling waters. The young
fish gathers with others of its species in little schools, feeds on
smaller fishes of other species or of its own, grows and changes until
maturity, deposits its eggs, and the cycle of life begins again, while
the old fish ultimately dies or is devoured.

=Irritability of Animals.=--All animals, of whatever degree of
organization, show in life the quality of irritability or response to
external stimulus. Contact with external things produces some effect
on each of them, and this effect is something more than the mere
mechanical effect on the matter of which the animal is composed. In the
one-celled animals the functions of response to external stimulus are
not localized. They are the property of any part of the protoplasm of
the body. In the higher or many-celled animals each of these functions
is specialized and localized. A certain set of cells is set apart for
each function, and each organ or series of cells is released from all
functions save its own.

=Nerve-cells and Fibres.=--In the development of the individual animal
certain cells from the primitive external layer or ectoblast of the
embryo are set apart to preside over the relations of the creature
to its environment. These cells are highly specialized, and while
some of them are highly sensitive, others are adapted for carrying
or transmitting the stimuli received by the sensitive cells, and
still others have the function of receiving sense-impressions and of
translating them into impulses of motion. The nerve-cells are receivers
of impressions. These are gathered together in nerve-masses or ganglia,
the largest of these being known as the brain, the ganglia in general
being known as nerve-centres. The nerves are of two classes. The one
class, called sensory nerves, extends from the skin or other organ of
sensation to the nerve-centre. The nerves of the other class, motor
nerves, carry impulses to motion.

=The Brain, or Sensorium.=--The brain or other nerve-centre sits
in darkness, surrounded by a bony protecting box. To this main
nerve-centre, or _sensorium_, come the nerves from all parts of the
body that have sensation, the external skin as well as the special
organs of sight, hearing, taste, and smell. With these come nerves
bearing sensations of pain, temperature, muscular effort--all kinds
of sensation which the brain can receive. These nerves are the sole
sources of knowledge to any animal organism. Whatever idea its brain
may contain must be built up through these nerve-impressions. The
aggregate of these impressions constitute the world as the organism
knows it. All sensation is related to action. If an organism is not to
act, it cannot feel, and the intensity of its feeling is related to its
power to act.

=Reflex Action.=--These impressions brought to the brain by the sensory
nerves represent in some degree the facts in the animal's environment.
They teach something as to its food or its safety. The power of
locomotion is characteristic of animals. If they move, their actions
must depend on the indications carried to the nerve-centre from the
outside; if they feed on living organisms, they must seek their food;
if, as in many cases, other living organisms prey on them, they must
bestir themselves to escape. The impulse of hunger on the one hand
and of fear on the other are elemental. The sensorium receives an
impression that food exists in a certain direction. At once an impulse
to motion is sent out from it to the muscles necessary to move the body
in that direction. In the higher animals these movements are more rapid
and more exact. This is because organs of sense, muscles, nerve-fibres,
and the nerve-cells are all alike highly specialized. In the fish the
sensation is slow, the muscular response sluggish, but the method
remains the same. This is simple reflex action, an impulse from the
environment carried to the brain and then unconsciously reflected back
as motion. The impulse of fear is of the same nature. Reflex action is
in general unconscious, but with animals, as with man, it shades by
degrees into conscious action, and into volition or action "done on
purpose."

=Instinct.=--Different animals show differences in method or degree of
response to external influences. Fishes will pursue their prey, flee
from a threatening motion, or disgorge sand or gravel swallowed with
their food. Such peculiarities of different forms of life constitute
the basis of instinct.

Instinct is automatic obedience to the demands of conditions external
to the nervous system. As these conditions vary with each kind of
animal, so must the demands vary, and from this arises the great
variety actually seen in the instincts of different animals. As the
demands of life become complex, so do the instincts. The greater the
stress of environment, the more perfect the automatism, for impulses to
safe action are necessarily adequate to the duty they have to perform.
If the instinct were inadequate, the species would have become extinct.
The fact that its individuals persist shows that they are provided with
the instincts necessary to that end. Instinct differs from other allied
forms of response to external condition in being hereditary, continuous
from generation to generation. This sufficiently distinguishes it from
reason, but the line between instinct and reason and other forms of
reflex action cannot be sharply drawn.

It is not necessary to consider here the question of the origin of
instincts. Some writers regard them as "inherited habits," while
others, with apparent justice, doubt if mere habits or voluntary
actions repeated till they become a "second nature" ever leave a trace
upon heredity. Such investigators regard instinct as the natural
survival of those methods of automatic response which were most useful
to the life of the animal, the individual having less effective methods
of reflex action perishing, leaving no posterity.

=Classification of Instincts.=--The instincts of fishes may be roughly
classified as to their relation to the individual into egoistic and
altruistic instincts.

_Egoistic instincts_ are those which concern chiefly the individual
animal itself. To this class belong the instincts of feeding, those
of self-defense and of strife, the instincts of play, the climatic
instincts, and environmental instincts, those which direct the animal's
mode of life.

_Altruistic instincts_ are those which relate to parenthood and those
which are concerned with the mass of individuals of the same species.
The latter may be called the social instincts. In the former class, the
instincts of parenthood, may be included the instinct of courtship,
reproduction, home-making, nest-building, and care for the young. Most
of these are feebly developed among fishes.

The instincts of feeding are primitively simple, growing complex
through complex conditions. The fish seizes its prey by direct motion,
but the conditions of life modify this simple action to a very great
degree.

The instinct of self-defense is even more varied in its manifestations.
It may show itself either in the impulse to make war on an intruder
or in the desire to flee from its enemies. Among carnivorous forms
fierceness of demeanor serves at once in attack and in defense.

Herbivorous fishes, as a rule, make little direct resistance to their
enemies, depending rather on swiftness of movement, or in some cases on
simple insignificance. To the latter cause the abundance of minnows,
anchovies, and other small or feeble fishes may be attributed, for all
are the prey of carnivorous fishes, which they far exceed in number.

The instincts of courtship relate chiefly to the male, the female
being more or less passive. Among many fishes the male makes himself
conspicuous in the breeding season, spreading his fins, intensifying
his pigmented colors through muscular tension, all this supposedly to
attract the attention of the female. That this purpose is actually
accomplished by such display is not, however, easily proved. In the
little brooks in spring, male minnows can be found with warts on
the nose or head, with crimson pigment on the fins, or blue pigment
on the back, or jet-black pigment all over the head, or with varied
combination of all these. Their instinct is to display all these to
the best advantage, even though the conspicuous hues lead to their own
destruction.

The movements of many migratory animals are mainly controlled by the
impulse to reproduce. Some pelagic fishes, especially flying fishes
and fishes allied to the mackerel, swim long distances to a region
favorable for a deposition of spawn. Some species are known only in the
waters they make their breeding homes, the individuals being scattered
through the wide seas at other times. Many fresh-water fishes, as
trout, suckers, etc., forsake the large streams in the spring,
ascending the small brooks where they can rear their young in greater
safety. Still others, known as anadromous fishes, feed and mature in
the sea, but ascend the rivers as the impulse of reproduction grows
strong. An account of these is given in a subsequent paragraph.

[Illustration: FIG. 118.--Jaws of _Nemichthys avocetta_. Jordan and
Gilbert.]

=Variability of Instincts.=--When we study instincts of animals with
care and in detail, we find that their regularity is much less than has
been supposed. There is as much variation in regard to instinct among
individuals as there is with regard to other characters of the species.
Some power of choice is found in almost every operation of instinct.
Even the most machine-like instinct shows some degree of adaptability
to new conditions. On the other hand, in no animal does reason show
entire freedom from automatism or reflex action. "The fundamental
identity of instinct with intelligence," says Dr. Charles O. Whitman,
"is shown in their dependence upon the same structural mechanism (the
brain and nerves) and in their responsive adaptability."

=Adaptation to Environment.=--In general food-securing structures
are connected with the mouth, or, as in the anglers, are hung as
lures above it; spines of offense and defense, electric organs,
poison-glands, and the like are used in self-protection; the bright
nuptial colors and adornments of the breeding season are doubtfully
classed as useful in rivalry; the egg-sacs, nests, and other structures
or habits may serve to defend the young, while skinny flaps, sand
or weed-like markings, and many other features of mimicry serve as
concessions to the environment.

Each kind of fishes has its own ways of life, fitted to the conditions
of environment. Some species lie on the bottom, flat, as a flounder,
or prone on their lower fins, as a darter or a stone-roller. Some swim
freely in the depths, others at the surface of the depths. Some leap
out of the water from time to time, as the mullet (_Mugil_) or the
tarpon (_Tarpon atlanticus_).

[Illustration: FIG. 119.--Catalina Flying Fish, _Cypsilurus
californicus_ (Cooper). Santa Barbara.]

=Flight of Fishes.=--Some fishes called the flying-fishes sail through
the air with a grasshopper-like motion that closely imitates true
flight. The long pectoral fins, wing-like in form, cannot, however, be
flapped by the fish, the muscles serving only to expand or fold them.
These fishes live in the open sea or open channel, swimming in large
schools. The small species fly for a few feet only, the large ones for
more than an eighth of a mile. These may rise five to twenty feet above
the water.

The flight of one of the largest flying fishes (_Cypsilurus
californicus_) has been carefully studied by Dr. Charles H. Gilbert and
the writer. The movements of the fish in the water are extremely rapid.
The sole motive power is the action under the water of the strong tail.
No force can be acquired while the fish is in the air. On rising from
the water the movements of the tail are continued until the whole body
is out of the water. When the tail is in motion the pectorals seem in
a state of rapid vibration. This is not produced by muscular action
on the fins themselves. It is the body of the fish which vibrates,
the pectorals projecting farthest having the greatest amplitude of
movement. While the tail is in the water the ventral fins are folded.
When the action of the tail ceases the pectorals and ventrals are
spread out wide and held at rest. They are not used as true wings,
but are held out firmly, acting as parachutes, enabling the body to
skim through the air. When the fish begins to fall the tail touches
the water. As soon as it is in the water it begins its motion, and
the body with the pectorals again begins to vibrate. The fish may, by
skimming the water, regain motion once or twice, but it finally falls
into the water with a splash. While in the air it suggests a large
dragon-fly. The motion is very swift, at first in a straight line,
but is later deflected in a curve, the direction bearing little or no
relation to that of the wind. When a vessel passes through a school of
these fishes, they spring up before it, moving in all directions, as
grasshoppers in a meadow.

[Illustration: FIG. 120.--Sand-darter, _Ammocrypta clara_ (Jordan and
Meek). Des Moines River.]

=Quiescent Fishes.=--Some fishes, as the lancelet, lie buried in
the sand all their lives. Others, as the sand-darter (_Ammocrypta
pellucida_) and the hinalea (_Julis gaimard_), bury themselves in the
sand at intervals or to escape from their enemies. Some live in the
cavities of tunicates or sponges or holothurians or corals or oysters,
often passing their whole lives inside the cavity of one animal. Many
others hide themselves in the interstices of kelp or seaweeds. Some
eels coil themselves in the crevices of rocks or coral masses, striking
at their prey like snakes. Some sea-horses cling by their tails to
gulfweed or sea-wrack. Many little fishes (_Gobiomorus_, _Carangus_,
_Psenes_) cluster under the stinging tentacles of the Portuguese
man-of-war or under ordinary jellyfishes. In the tide-pools, whether
rock, coral, or mud, in all regions multitudes of little fishes abound.
As these localities are neglected by most collectors, they have proved
of late years a most prolific source of new species. The tide-pools of
Cuba, Key West, Cape Flattery, Sitka, Unalaska, Monterey, San Diego,
Mazatlan, Hilo, Kailua and Waiahae in Hawaii, Apia and Pago-Pago in
Samoa, the present writer has found peculiarly rich in rock-loving
forms. Even richer are the pools of the promontories of Japan, Hakodate
Head, Misaki, Awa, Izu, Waka, and Kagoshima, where a whole new fish
fauna unknown to collectors in markets and sandy bays has been brought
to light. Some of these rockfishes are left buried in the rock weeds
as the tide flows, lying quietly until it returns. Others cling to the
rocks by ventral suckers, while still others depend for their safety on
their powers of leaping or on their quickness of their movements in the
water. Those of the latter class are often brilliantly , but the
others mimic closely the algae or the rocks. Some fishes live in the sea
only, some prefer brackish-water. Some are found only in the rivers,
and a few pass more or less indiscriminately from one kind of water to
another.

[Illustration: FIG. 121.--Pearl-fish, _Fierasfer acus_ (Linnaeus),
issuing from a _Holothurian_. Coast of Italy. (After Emery.)]

[Illustration: FIG. 122.--Portuguese Man-of-war Fish, _Gobiomorus
gronovii_. Family _Stromateidae_].

=Migratory Fishes.=--The movements of migratory fishes are mainly
controlled by the impulse of reproduction. Some pelagic fishes,
especially those of the mackerel and flying-fish families, swim long
distances to a region favorable for the deposition of spawn. Others
pursue for equal distances the schools of menhaden or other fishes
which serve as their prey. Some species are known mainly in the waters
they make their breeding homes, as in Cuba, Southern California,
Hawaii, or Japan, the individuals being scattered at other times
through the wide seas.

=Anadromous Fishes.=--Many fresh-water fishes, as trout and suckers,
forsake the large streams in the spring, ascending the small brooks
where their young can be reared in greater safety. Still others, known
as _anadromous_ fishes, feed and mature in the sea, but ascend the
rivers as the impulse of reproduction grows strong. Among such fishes
are the salmon, shad, alewife, sturgeon, and striped bass in American
waters. The most remarkable case of the anadromous instinct is found in
the king salmon or quinnat (_Oncorhynchus tschawytscha_) of the Pacific
Coast. This great fish spawns in November, at the age of four years and
an average weight of twenty-two pounds. In the Columbia River it begins
running with the spring freshets in March and April. It spends the
whole summer, without feeding, in the ascent of the river. By autumn
the individuals have reached the mountain streams of Idaho, greatly
changed in appearance, discolored, worn, and distorted. The male is
humpbacked, with sunken scales, and greatly enlarged, hooked, bent, or
twisted jaws, with enlarged dog-like teeth. On reaching the spawning
beds, which may be a thousand miles from the sea in the Columbia, over
two thousand in the Yukon, the female deposits her eggs in the gravel
of some shallow brook. The male covers them and scrapes the gravel
over them. The female salmon does as much as the male in covering the
eggs. Then both male and female drift tail foremost helplessly down the
stream; none, so far as certainly known, ever survive the reproductive
act. The same habits are found in the five other species of salmon in
the Pacific, but in most cases the individuals do not start so early
nor run so far. The blue-back salmon or redfish, however, does not fall
far short in these regards. The salmon of the Atlantic has a similar
habit, but the distance traveled is everywhere much less, and most of
the hook-jawed males drop down to the sea and survive to repeat the
acts of reproduction.

[Illustration: FIG. 123.--Tide-pools of Misaki. The Misaki Biological
Station, from the north side.]

_Catadromous_ fishes, as the true eel (_Anguilla_), reverse this order,
feeding in the rivers and brackish estuaries, apparently finding their
usual spawning-ground in the sea.

[Illustration: FIG. 124.--Squaw-fish, _Ptychocheilus oregonensis_
(Richardson). Columbia River.]

=Pugnacity of Fishes.=--Some fishes are very pugnacious, always
ready for a quarrel with their own kind. The sticklebacks show this
disposition, especially the males. In Hawaii the natives take advantage
of this trait to catch the Uu (_Myripristis murdjan_), a bright
crimson- fish found in those waters. The species lives in
crevices in lava rocks. Catching a live one, the fishermen suspend it
by a string in front of the rocks. It remains there with spread fins
and flashing scales, and the others come out to fight it, when all are
drawn to the surface by a concealed net. Another decoy is substituted
and the trick is repeated until the showy and quarrelsome fishes are
all secured.

In Siam the fighting-fish (_Betta pugnax_) is widely noted. The
following account of this fish is given by Cantor:[11]

"When the fish is in a state of quiet, its dull colors present nothing
remarkable; but if two be brought together, or if one sees its own
image in a looking-glass, the little creature becomes suddenly excited,
the raised fins and the whole body shine with metallic colors of
dazzling beauty, while the projected gill membrane, waving like a black
frill round the throat, adds something of grotesqueness to the general
appearance. In this state it makes repeated darts at its real or
reflected antagonist. But both, when taken out of each other's sight,
instantly become quiet. The fishes were kept in glasses of water, fed
with larvae of mosquitoes, and had thus lived for many months. The
Siamese are as infatuated with the combats of these fish as the Malays
are with their cock-fights, and stake on the issue considerable sums,
and sometimes their own persons and families. The license to exhibit
fish-fights is farmed, and brings a considerable annual revenue to
the king of Siam. The species abounds in the rivulets at the foot
of the hills of Penang. The inhabitants name it 'Pla-kat,' or the
'fighting-fish'; but the kind kept especially for fighting is an
artificial variety cultivated for the purpose."

A related species is the equally famous tree-climber of India (_Anabas
scandens_). In 1797 Lieutenant Daldorf describes his capture of an
_Anabas_, five feet above the water, on the bark of a palm-tree. In the
effort to do this, the fish held on to the bark by its preopercular
spines, bent its tail, inserted its anal spines, then pushing forward,
repeated the operation.

=Fear and Anger in Fishes.=--From an interesting paper by Surgeon
Francis Day[12] on Fear and Anger in Fishes we may make the following
extracts, slightly condensed and with a few slight corrections in
nomenclature. The paper is written in amplification of another by Rev.
S. J. Whitmee, describing the behavior of aquarium fishes in Samoa.

[Illustration: FIG. 125.--Squaw-fish, _Ptychocheilus grandis_ Agassiz.
Running up a stream to spawn, the high water, after a rain, falling,
leaves the fishes stranded. Kelsey Creek, Clear Lake, California, April
29, 1899. (Photograph by O. E. Meddaugh.)--Page 164.]

The means of expression in animals adverted to by Mr. Darwin (excluding
those of the ears, which would be out of place in fishes) are: sounds,
vocally or otherwise produced; the erection of dermal appendages under
the influence of anger or terror, which last would be analogous to
the erection of scales and fin-rays among fishes. Regarding special
expressions, as those of joy, pain, astonishment, etc., we could
hardly expect such so well marked in fishes as in some of the higher
animals, in which the play of the features often affords us an insight
into their internal emotions. Eyes[13] destitute of movable eyelids,
cheeks covered with scales, or the head enveloped in dermal plates, can
scarcely mantle into a smile or expand into a broad grin. We possess,
however, one very distinct expression in fishes which is absent or but
slightly developed in most of the higher animals, namely, change of
color. All are aware that when a fish sickens, its brilliant colors
fade, but less so how its color may be augmented by anger, and a loss
of it be occasioned by depression, the result of being vanquished by a
foe. Some forms also emit sounds when actuated by terror, and perhaps
in times of anger; but of this last I possess no decided proofs.

Similar to the expression of anger in _Betta_ is that of the
three-spined stickleback (_Gasterosteus aculeatus_).[14] After a fight
between two examples, according to Couch, "a strange alteration takes
place almost immediately in the defeated party: his gallant bearing
forsakes him; his gay colors fade away; he becomes again speckled and
ugly; and he hides his disgrace amongst his peaceable companions who
occupy together that part of the tub which their tyrants have not taken
possession of; he is, moreover, for some time the constant object of
his conqueror's persecution."

Fear is shown by fish in many ways. There is not an angler unacquainted
with the natural timidity of fishes, nor a keeper in charge of a
salmon-pass, who does not know how easy it is for poachers to deter the
salmon from venturing along the path raised expressly for his use.

Among the coral reefs of the Andaman Islands I found the little
_Chromis lepisurus_ abundant. As soon as the water was splashed
they appeared to retire for safety to the branching coral, where no
large fish could follow them; so frightened did they become that on
an Andamanese diving from the side of the boat, they at once sought
shelter in the coral, in which they remained until it was removed
from the sea. In Burma I observed, in 1869, that when weirs are not
allowed to stretch across the rivers (which would impede navigation),
the open side as far as the bank is studded with reeds; these, as the
water passes over them, cause vibration, and occasion a curious sound
alarming the fishes, which, crossing to the weired side of the river,
become captured.

Hooker, alluding to gulls, terns, wild geese, and pelicans in the
Ganges Valley, observes: "These birds congregate by the sides of pools
and beat the water with violence, so as to scare the fish, which then
become an easy prey--a fact which was, I believe, first indicated by
Pallas during his residence on the banks of the Caspian Sea."[15]
Fishes, under the influence of terror, dash about with their fins
expanded, and often run into places which must destroy them. Thus
droves and droves of sardines in the east, impelled by the terror
of pursuing sharks, bonitos, and other voracious fishes, frequently
throw themselves on the shores in enormous quantities. Friar Odoric,
who visited Ceylon about 1320, says: "There are fishes in those seas
which come swimming towards the said country in such abundance, that
for a great distance into the sea nothing can be seen but the backs
of fishes, which, casting themselves on the shore, do suffer men for
the space of three days to come, and to take as many of them as they
please, and then they return again into the sea."[16]

Pennant tells us that the river bullhead (_Cottus gobio_) "deposits
its spawn in a hole it forms in the gravel, and quits it with great
reluctance." General Hardwicke tells how the gouramy (_Osphromenus
gouramy_), in the Mauritius, forms a nest amongst the herbage growing
in the shallow water in the sides of tanks. Here the parent continues
to watch the place with the greatest vigilance, driving away any
interloping fish. The amphibious walking-fish of Mysore (_Ophiocephalus
striatus_) appears to make a nest very similar to that of the gouramy,
and over it the male keeps guard; but should he be killed or captured,
the vacant post is filled by his partner. (Colonel Puckle.) When very
young the fishes keep with and are defended by their parents, but so
soon as they are sufficiently strong to capture prey for themselves
they are driven away to seek their own subsistence. (See Fishes of
India, p. 362.) But it is not only these monogamous amphibious fishes
which show an affection for their eggs and also for their fry, but even
the little _Etroplus maculatus_ has been observed to be equally fond of
its ova. "The eggs are not very numerous and are deposited in the mud
at the bottom of the stream, and, when hatched, both parents guard the
young for many days, vigorously attacking any large fish that passes
near them."[17]

Although the proceedings of the members of the marine and estuary genus
of sea-cat (_Tachysurus_) and its allies show not quite so distinctly
signs of affection, still it must be a well-developed instinct which
induces the male to carry about the eggs in its mouth until hatched,
and to remove them in this manner when danger is imminent. I have taken
the ova just ready for the young to come forth out of the mouth and
fauces of the parent (male) fish; and in every animal dissected there
was no trace of food in the intestinal tract.

=Calling the Fishes.=--At many temples in India fishes are called to
receive food by means of ringing bells or musical sounds. Carew, in
Cornwall, is said to have called the gray mullet together by making a
noise like chopping with a cleaver. Lacepede relates that some fishes,
which had been kept in the basins out of the Tuileries for more than a
century, would come when called by their names, and that in many parts
of Germany trout, carp, and tench are summoned to their food by the
sound of a bell. These instances are mostly due to the fishes having
learned by experience that on the hearing certain sounds they may
expect food. But Lacepede mentions that some were able to distinguish
their individual names; and the same occurs in India. Lieutenant
Connolly[18] remarked upon seeing numerous fishes coming to the ghaut
at Sidhnath to be fed when called; and on "expressing our admiration
of the size of the fish, 'Wait,' said a bystander, 'until you have
seen Raghu.' The Brahmin called out his name in a peculiar tone of
voice; but he would not hear. I threw in handful after handful of
ottah (flour) with the same success, and was just leaving the ghaut,
despairing and doubting, when a loud plunge startled me. I thought
somebody had jumped off the bastion of the ghaut into the river, but
was soon undeceived by the general shout of 'Raghu, raghu,' and by
the fishes, large and small, darting away in every direction. Raghu
made two or three plunges, but was so quick in his motions that I was
unable to guess at his species." [It may be said in relation to these
stories quoted by Dr. Day, that they probably belong to the mythology
of fishes. It is very doubtful if fishes are able to make any such
discrimination among sounds in the air.]

=Sounds of Fishes.=--Pallegoix states that in Siam the dog's-tongue
(_Cynoglossus_) is a kind of sole; it attaches itself to the bottom of
boats, and makes a sonorous noise, which is more musical when several
are stuck to the same boat and act in concert (vol. i. p. 193). These
noises can scarcely be due to anger or fear. Sir J. Bowring (vol. ii.
p. 276) also remarks upon having heard this fish, "which sticks to the
bottoms of the boats, and produces a sound something like that of a
jew's-harp struck slowly, though sometimes it increases in loudness,
so as to resemble the full tones and sound of an organ. My men have
pointed me out a fish about four inches long as the author of the
music."

Some years since, at Madras, I (Dr. Day) obtained several specimens
of a fresh-water Siluroid fish (_Macrones vittatus_) which is termed
the "fiddler" in Mysore. I touched one which was on the wet ground, at
which it appeared to become very irate, erecting its dorsal fin, making
a noise resembling the buzzing of a bee. Having put some small carp
into an aquarium containing one of these fishes, it rushed at a small
example, _seized it by the middle of its back_, and shook it like a dog
killing a rat; at this time its barbels were stiffened out laterally
like a cat's whiskers.

Many fish when captured make noises, perhaps due to terror. Thus the
_Carangus hippos_, _Tetraodon_, and others grunt like a hog. Darwin
(Nat. Journ., vol. vii) remarks on a catfish found in the Rio Parana,
and called the armado, which is remarkable for a harsh grating noise
when caught by hook and line; this noise can be distinctly heard when
the fish is beneath the water.

The cuckoo-gurnard (_Trigla pini_) and the maigre (_Pseudosciaena
aquila_) utter sounds when taken out of the water; and herrings,
when the net has been drawn over them, have been observed to do the
same: "this effect has been attributed to an escape of air from the
air-bladder; but no air-bladder exists in the _Cottus_, which makes a
similar noise."

The lesser weaver (_Trachinus_) buries itself in the loose soil at the
bottom of the water, leaving only its head exposed, and awaits its
prey. If touched, it strikes upwards or sideways; and Pennant says it
directs its blows with as much judgment as a fighting-cock. (Yarrell,
vol. i. p. 26.) Fishermen assert that wounds from its anterior dorsal
spines are more venomous than those caused by the spines on its
gill-covers.

As regards fighting, I should suppose that, unless some portion of the
body is peculiarly adapted for this purpose, as the rostrum of the
swordfish, or the spine on the side of the tail in the lancet-fishes,
we must look chiefly to the armature or covering of the jaws for
weapons of offense.

=Lurking Fishes.=--Mr. Whitmee supposes that most carnivorous fish
capture their prey by outswimming them; but to this there are numerous
exceptions; the angler or fishing-frog (_Lophis piscatorius_), "while
crouching close to the ground, by the action of its ventral and
pectoral fins stirs up the sand and mud; hidden by the obscurity thus
produced, it elevates its anterior dorsal spines, moves them in various
directions by way of attraction as a bait, and the small fishes,
approaching either to examine or to seize them, immediately become the
prey of the fisher." (Yarrell.) In India we find a fresh-water Siluroid
(_Chaca lophioides_) which "conceals itself among the mud, from which,
by its lurid appearance and a number of loose filamentous substances
on its skin, it is scarcely distinguishable; and with an immense open
mouth it is ready to seize any small prey that is passing along." (Ham.
Buchanan.) In March, 1868, I obtained a fine example of _Ichthyscopus
lebeck_ (Fishes of India, p. 261), which I placed in water having a bed
of mud; into this it rapidly worked itself, first depressing one side
and then another, until only the top of its head and mouth remained
above the mud, whilst a constant current was kept up through its gills.
It made a noise, half snapping and half croaking, when removed from its
native element.

In the Royal Westminster Aquarium, says Dr. Day, is a live example
of the electric eel (_Electrophorus electricus_) which has in its
electric organs the means of showing when it is affected by anger or
terror. Some consider this curious property is for protection against
alligators: it is certainly used against fishes for the purpose of
obtaining food; but when we remember how, when the Indians drive in
horses and mules to the waters infested by the eels, they immediately
attack them, we must admit that such cannot be for the purpose of
preying upon them, but is due to anger or terror at being disturbed.
(DAY.)

=Carrying Eggs in the Mouth.=--Many catfishes (_Siluridae_) carry their
eggs in the mouth until hatched. The first and most complete account
of this habit of catfishes is that by Dr. Jeffries Wyman, which he
communicated to the Boston Society of Natural History at its meeting on
September 15, 1857. In 1859, in a paper entitled "On Some Unusual Modes
of Gestation," Dr. Wyman published a full account of his observations
as follows, here quoted from a paper on Surinam fishes by Evermann and
Goldsborough:

"Among the Siluroid fishes of Guiana there are several species which,
at certain seasons of the year, have their mouths and branchial
cavities filled either with eggs or young, and, as is believed, for the
purpose of incubation. My attention was first called to this singular
habit by the late Dr. Francis W. Cragin, formerly United States consul
at Paramaribo, Surinam. In a letter dated August, 1854, he says:

"'The eggs you will receive are from another fish. The different
fishermen have repeatedly assured me that these eggs in their nearly
mature state are carried in the mouths of the parent till the young are
relieved by the bursting of the sac. Do you either know or believe this
to be so, and, if possible, where are the eggs conceived and how do
they get into the mouth?'

"In the month of April, 1857, on visiting the market of Paramaribo, I
found that this statement, which at first seemed to be very improbable,
was correct as to the existence of eggs in the mouths of several
species of fish. In a tray of fish which a <DW64> woman offered for
sale, I found the mouths of several filled with either eggs or young,
and subsequently an abundance of opportunities occurred for repeating
the observation. The kinds most commonly known to the colonists,
especially to the <DW64>s, are _jara-bakka_, _njinge-njinge_, _koepra_,
_makrede_, and one or two others, all belonging either to the genus
_Bagrus_ or one nearly allied to it. The first two are quite common in
the market, and I have seen many specimens of them; for the last two I
have the authority of <DW64> fishermen, but have never seen them myself.
The eggs in my collection are of three different sizes, indicating so
many species, one of the three having been brought to me without the
fish from which they were taken.

"The eggs become quite large before they leave the ovaries, and are
arranged in three zones corresponding to three successive broods, and
probably to be discharged in three successive years; the mature eggs
of a jara-bakka 18 inches long measure three-fourths of an inch in
diameter; those of the second zone, one-fourth; and those of the third
are very minute, about one-sixteenth of an inch.

"A careful examination of eight specimens of njinge-njinge about 9
inches long gave the following results:

"The eggs in all instances were carried in the mouths of the males.
This protection, or gestation of the eggs by the males, corresponds
with what has been long noticed with regard to other fishes, as, for
example, _Syngnathus_, where the marsupial pouch for the eggs or
young is found in the males only, and _Gasterosteus_, where the male
constructs the nest and protects the eggs during incubation from the
voracity of the females.

"In some individuals the eggs had been recently laid, in others they
were hatched and the foetus had grown at the expense of some other food
than that derived from the yolk, as this last was not proportionally
diminished in size, and the foetus weighed more than the undeveloped
egg. The number of eggs contained in the mouth was between twenty
and thirty. The mouth and branchial cavity were very much distended,
rounding out and distorting the whole hyoid and branchiostegal region.
Some of the eggs even partially protruded from the mouth. The ova were
not bruised or torn as if they had been bitten or forcibly held by the
teeth. In many instances the foetuses were still alive, though the
parent had been dead for many hours.

"No young or eggs were found in the stomach, although the mouth was
crammed to its fullest capacity.

"The above observations apply to njinge-njinge. With regard to
jarra-bakka, I had but few opportunities for dissection, but in several
instances the same conditions of the eggs were noticed as stated above;
and in one instance, besides some nearly mature foetuses contained in
the mouth, two or three were squeezed apparently from the stomach,
but not bearing any marks of violence or of the action of the gastric
fluid. It is probable that these found their way into that last cavity
after death, in consequence of the relaxation of the sphincter which
separates the cavities of the mouth and the stomach. These facts lead
to the conclusion that this is a mouth gestation, as the eggs are found
there in all stages of development, and even for some time after they
are hatched.

"The question will be very naturally asked, how under such
circumstances these fishes are able to secure and swallow their food. I
have made no observations bearing upon such a question. Unless the food
consists of very minute particles it would seem necessary that during
the time of feeding the eggs should be disgorged. If this supposition
be correct, it would give a very probable explanation of the only fact
which might be considered at variance with the conclusion stated above,
viz., that we have in these fishes a mouth gestation. In the mass
of eggs with which the mouth is filled I have occasionally found the
eggs, rarely more than one or two, of another species. The only way in
which their presence may be accounted for, it seems to me, is by the
supposition that while feeding the eggs are disgorged, and as these
fishes are gregarious in their habits, when the ova are recovered the
stray eggs of another species may be introduced into the mouth among
those which naturally belong there."

One of the earliest accounts of this curious habit which we have seen
is that by Dr. Guenther, referring to specimens of _Tachysurus fissus_
from Cayenne received from Prof. R. Owen:

"These specimens having had the cavity of the mouth and of the gills
extended in an extraordinary manner, I was induced to examine the cause
of it, when, to my great surprise, I found them filled with about
twenty eggs, rather larger than an ordinary pea, perfectly uninjured,
and with the embryos in a forward state of development. The specimens
are males, from 6 to 7 inches long, and in each the stomach was almost
empty.

"Although the eggs might have been put into the mouth of the fish by
their captor, this does not appear probable. On the other hand, it
is a well-known fact that the American Siluroids take care of their
progeny in various ways; and I have no doubt that in this species and
in its allies the males carry the eggs in their mouths, depositing them
in places of safety and removing them when they fear the approach of
danger or disturbance."

=The Unsymmetrical Eyes of Flounders.=--In the two great families of
flounders and soles the head is unsymmetrically formed, the cranium
being twisted and both eyes placed on the same side. The body is
strongly compressed, and the side possessing the eyes is uppermost in
all the actions of the fish. This upper side, whether right or left, is
, while the eyeless side is white or very nearly so.

It is well known that in the very young flounder the body rests
upright in the water. After a little there is a tendency to turn to
one side and the lower eye begins its migration to the other side, the
interorbital bones or part of them moving before it. In most flounders
the eye seems to move over the surface of the head, before the dorsal
fin, or across the axil of its first ray. In the tropical genus
_Platophrys_ the movement of the eye is most easily followed, as the
species reach a larger size than do most flounders before the change
takes place. The larva, while symmetrical, is in all cases transparent.

[Illustration: FIG. 126.]

[Illustration: FIG. 127.

FIGS. 126, 127.--Larval stages of _Platophrys podas_, a flounder of the
Mediterranean, showing the migration of the eye. (After Emery.)]

In a recent study of the migration of the eye in the winter flounder
(_Pseudopleuronectes americanus_) Mr. Stephen R. Williams reaches the
following conclusions:

1. The young of _Limanda ferruginea_ (the rusty dab) are probably in
the larval stage at the same time as those of _Pseudopleuronectes
americanus_ (the winter flounder).

2. The recently hatched fish are symmetrical, except for the relative
positions of the two optic nerves.

3. The first observed occurrence in preparation for metamorphosis in
_P. americanus_ is the rapid resorption of the part of the supraorbital
cartilage bar which lies in the path of the eye.

4. Correlated with this is an increase in distance between the eyes
and the brain, caused by the growth of the facial cartilages.

5. The migrating eye moves through an arc of about 120 degrees.

[Illustration: FIG. 128.--_Platophrys lunatus_ (Linnaeus), the Wide-eyed
Flounder. Family _Pleuronectidae_. Cuba. (From nature by Mrs. H. C.
Nash.)]

6. The greater part of this rotation (three-fourths of it in _P.
americanus_) is a rapid process, taking not more than three days.

7. The anterior ethmoidal region is not so strongly influenced by the
twisting as the ocular region.

[Illustration: FIG. 129.--Young Flounder, just hatched, with
symmetrical eyes. (After S. R. Williams.)]

8. The location of the olfactory nerves (in the adult) shows that the
morphological midline follows the interorbital septum.

9. The cartilage mass lying in the front part of the orbit of the adult
eye is a separate anterior structure in the larva.

10. With unimportant differences, the process of metamorphosis in the
sinistral fish is parallel to that in the dextral fish.

11. The original location of the eye is indicated in the adult by the
direction first taken, as they leave the brain, by those cranial nerves
having to do with the transposed eye.

12. The only well-marked asymmetry in the adult brain is due to the
much larger size of the olfactory nerve and lobe of the ocular side.

13. There is a perfect chiasma.

14. The optic nerve of the migrating eye is always anterior to that of
the other eye.

[Illustration: FIG. 130.--Larval Flounder, _Pseudopleuronectes
americanus_. (After S. R. Williams.)]

[Illustration: FIG. 131.--Larval Flounder, _Pseudopleuronectes
americanus_. (After S. R. Williams.)]

"The why of the peculiar metamorphosis of the _Pleuronectidae_ is an
unsolved problem. The presence or absence of a swim-bladder can have
nothing to do with the change of habit of the young flatfish, for _P.
americanus_ must lose its air-bladder before metamorphosis begins,
since sections showed no evidence of it, whereas in _Lophopsetta
maculata_, 'the windowpane flounder,' the air-sac can often be seen
by the naked eye up to the time when the fish assumes the adult
coloration, and long after it has assumed the adult form.

"Cunningham has suggested that the weight of the fish acting upon the
lower eye after the turning would press it toward the upper side out
of the way. But in all probability the planktonic larva rests on the
sea-bottom little if at all before metamorphosing. Those taken by Mr.
Williams into the laboratory showed in resting no preference for either
side until the eye was near the midline.

"The fact that the change in all fishes is repeated during the
development of each individual fish has been used to support the
proposition that the flatfishes as a family are a comparatively recent
product. They are, on the other hand, comparatively ancient. According
to Zittel flatfishes of species referable to genera living at present,
_Rhombus_ (_Bothus_) and _Solea_, are found in the Eocene deposits.
These two genera are notable in that _Bothus_ is one of the least and
_Solea_ the most unsymmetrical of the _Pleuronectidae_.

[Illustration: FIG. 132.--Face view of recently hatched Flounder.
(After S. R. Williams.)]

"The degree of asymmetry can be correlated with the habit of the
animal. Those fishes, such as the sole and shore-dwelling flounders,
which keep to the bottom are the most twisted representatives of the
family, while the more freely swimming forms, like the sand-dab, summer
flounder, and halibut, are more nearly symmetrical. Asymmetry must be
of more advantage to those fishes which grub in the mud for their food
than to those which capture other fishes; of the latter those which
move with the greatest freedom are the most symmetrical.

"This deviation from the bilateral condition must have come about
either as a 'sport' or by gradual modification of the adults. If by the
latter method--the change proving to be advantageous--selection favored
its appearing earlier and earlier in ontogeny, until it occurred in the
stages of planktonic life. Metamorphosis at a stage earlier than this
would be a distinct disadvantage, because of the lack of the customary
planktonic food at the sea-bottom. At present some forms of selection
are probably continually at work fixing the limit of the period of
metamorphosis by the removal of those individuals which attempt the
transformation at unsuitable epochs; for instance, at the time of
hatching. That there are such individuals is shown by Fullarton, who
figures a fish just hatched 'anticipating the twisting and subsequent
unequal development exhibited by the head of Pleuronectids.' Those
larvae which remain pelagic until better able to compete at the
sea-bottom become the adults which fix the time of metamorphosis on
their progeny." (S. R. WILLIAMS.)

So far as known to the writer, the metamorphosis of flounders always
occurs while the individual is still translucent and swimming at the
surface of the sea before sinking to the bottom.

FOOTNOTES:

[11] Cantor, Catal. Malayan Fishes, 1850, p. 87. Bowring, Siam, p. 155,
gives a similar account of the battles of these fishes.

[12] Francis Day, on Fear and Anger in Fishes, Proc. Zool. Society,
London, Feb. 19, 1878, pp. 214-221.

[13] Couch (Illustrations, etc., p. 305) says: "The faculty of giving
forth brilliant light from the eyes is said to have been observed by
fishermen in the blue shark, as in a cat."

[14] Couch, "British Fishes," 1865, vol. iv. p. 172.

[15] Himalayan Journals, vol. i. p. 80.

[16] Hakluyt, vol. ii. p. 37.

[17] Jerdon, "Madras Journal of Literature and Science," 1849, p. 143.

[18] "Observations on the Past and Present Condition of Onjein,"
Journal of the Asiatic Society of Bengal, vi, p. 820.




CHAPTER XII

ADAPTATIONS OF FISHES


[Illustration: FIG. 133.--Mad-tom, _Schilbeodes furiosus_ Jordan and
Meek. Showing the poisoned pectoral spine. Family _Siluridae_. Neuse
River.]

=Spines of the Catfishes.=--The catfishes or horned pouts (_Siluridae_)
have a strong spine in the pectoral fin, one or both edges of this
being jagged or serrated. This spine fits into a peculiar joint and
by means of a slight downward or forward twist can be set immovably.
It can then be broken more easily than it can be depressed. A slight
turn in the opposite direction releases the joint, a fact known to the
fish and readily learned by the boy. The sharp spine inflicts a jagged
wound. Pelicans which have swallowed the catfish have been known to die
of the wounds inflicted by the fish's spine. When the catfish was first
introduced into the Sacramento, according to Mr. Will S. Green, it
caused the death of many of the native "Sacramento perch" (_Archoplites
interruptus_). This perch (or rather bass) fed on the young catfish,
and the latter erecting their pectoral spines in turn caused the death
of the perch by tearing the walls of its stomach. In like manner the
sharp dorsal and ventral spines of the sticklebacks have been known to
cause the death of fishes who swallow them, and even of ducks. In Puget
Sound the stickleback is often known as salmon-killer.

Certain small catfishes known as stone-cats and mad-toms (_Noturus_,
_Schilbeodes_), found in the rivers of the Southern and Middle Western
States, are provided with special organs of offense. At the base of the
pectoral spine, which is sometimes very jagged, is a structure supposed
by Professor Cope to be a poison gland the nature of which has not yet
been fully ascertained. The wounds made by these spines are exceedingly
painful like those made by the sting of a wasp. They are, however,
apparently not dangerous.

[Illustration: FIG. 134.--Black Nohu, or Poison-fish, _Emmydrichthys
vulcanus_ Jordan. A species with stinging spines, showing resemblance
to lumps of lava among which it lives. Family _Scorpaenidae_. From
Tahiti.]

=Venomous Spines.=--Many species of scorpion-fishes (_Scorpaena_,
_Synanceia_, _Pelor_, _Pterois_, etc.), found in warm seas, as well
as the European weavers (_Trachinus_), secrete poison from under the
skin of each dorsal spine. The wounds made by these spines are very
exasperating, but are not often dangerous. In some cases the glands
producing these poisons form an oblong bag excreting a milky juice, and
placed on the base of the spine.

In _Thalassophryne_, a genus of toad-fishes of tropical America, is
found the most perfect system of poison organs known among fishes.
The spinous armature of the opercle and the two spines of the first
dorsal fin constitute the weapons. The details are known from the
dissections of Dr. Guenther. According to his[19] observations, the
opercle in _Thalassophryne_ "is very narrow, vertically styliform and
very mobile. It is armed behind with a spine eight lines long and of
the same form as the hollow venom-fang of a snake, being perforated at
its base and at its extremity. A sac covering the base of the spine
discharges its contents through the apertures and the canal in the
interior of the spine. The structure of the dorsal spines is similar.
There are no secretory glands imbedded in the membranes of the sacs
and the fluid must be secreted by their mucous membrane. The sacs are
without an external muscular layer and situated immediately below
the thick, loose skin which envelops the spines at their extremity.
The ejection of the poison into a living animal, therefore, can only
be effected as in _Synanceia_, by the pressure to which the sac is
subjected the moment the spine enters another body."

[Illustration: FIG. 135.--Brown Tang, _Teuthis bahianus_ (Ranzani).
Tortugas, Florida.]

=The Lancet of the Surgeon-fish.=--Some fishes defend themselves by
lashing their enemies with their tails. In the tangs, or surgeon-fishes
(_Teuthis_), the tail is provided with a formidable weapon, a
knife-like spine, with the sharp edge directed forward. This spine when
not in use slips forward into a sheath. The fish, when alive, cannot be
handled without danger of a severe cut.

In the related genera, this lancet is very much more blunt and
immovable, degenerating at last into the rough spines of _Balistapus_
or the hair-like prickles of _Monacanthus_.

=Spines of the Sting-ray.=--In all the large group of sting-rays the
tail is provided with one or more large, stiff, barbed spines, which
are used with great force by the animal, and are capable of piercing
the leathery skin of the sting-ray itself. There is no evidence that
these spines bear any specific poison, but the ragged wounds they make
are always dangerous and often end in gangrene. It is possible that the
mucus on the surface of the spine acts as a poison on the lacerated
tissues, rendering the wound something very different from a simple cut.

[Illustration: FIG. 136.--Common Filefish, _Stephanolepis hispidus_
(Linnaeus). Virginia.]

=Protection Through Poisonous Flesh of Fishes.=--In certain groups of
fishes a strange form of self-protection is acquired by the presence in
the body of poisonous alkaloids, by means of which the enemies of the
species are destroyed in the death of the individual devoured.

Such alkaloids are present in the globefishes (_Tetraodontidae_), the
filefishes (_Monacanthus_), and in some related forms, while members
of other groups (_Batrachoididae_) are under suspicion in this regard.
The alkaloids produce a disease known as ciguatera, characterized by
paralysis and gastric derangements. Severe cases of ciguatera with men,
as well as with lower animals, may end fatally in a short time.

The flesh of the filefishes (_Stephanolepis tomentosus_), which the
writer has tested, is very meager and bitter, having a decidedly
offensive taste. It is suspected, probably justly, of being poisonous.
In the globefishes the flesh is always more or less poisonous, that
of _Tetraodon hispidus_, called muki-muki, or death-fish, in Hawaii,
is reputed as excessively so. The poisonous fishes have been lately
studied in detail by Dr. Jacques Pellegrin, of the Museum d'Histoire
Naturelle at Paris. He shows that any species of fish may be poisonous
under certain circumstances, that under certain conditions certain
species are poisonous, and that certain kinds are poisonous more
or less at all times. The following account is condensed from Dr.
Pellegrin's observations.

[Illustration: FIG. 137.--_Tetraodon meleagris_ (Lacepede). Riu Kiu
Islands.]

The flesh of fishes soon undergoes decomposition in hot climates. The
consumption of decayed fish may produce serious disorders, usually
with symptoms of diarrhoea or eruption of the skin. There is in this
case no specific poison, but the formation of leucomaines through
the influence of bacteria. This may take place with other kinds of
flesh, and is known as botulism, or allantiasis. For this disease, as
produced by the flesh of fishes, Dr. Pellegrin suggests the name of
ichthyosism It is especially severe in certain very oily fishes, as the
tunny, the anchovy, or the salmon. The flesh of these and other fishes
occasionally produces similar disorders through mere indigestion. In
this case the flesh undergoes decay in the stomach.

In certain groups (wrasse-fishes, parrot-fishes, etc.) in the tropics,
individual fishes are sometimes rendered poisonous by feeding on
poisonous mussels, holothurians, or possibly polyps, species which
at certain times, and especially in their spawning season, develops
alkaloids which themselves may cause ciguatera. In this case it is
usually the very old or large fishes which are liable to be infected.
In some markets numerous species are excluded as suspicious for this
reason. Such a list is in use in the fish-market of Havana, where the
sale of certain species, elsewhere healthful, or at the most suspected,
was rigidly prohibited under the Spanish regime. A list of these
suspicious fishes has been given by Prof. Poey.

[Illustration: FIG. 138.--The Trigger-fish, _Balistes carolinensis_
Gmelin. New York.]

In many of the eels the serum of the blood is poisonous, but its venom
is destroyed by the gastric juice, so that the flesh may be eaten with
impunity, unless decay has set in. To eat too much of the tropical
morays is to invite gastric troubles, but no true ciguatera. The true
ciguatera is produced by a specific poisonous alkaloid. This is most
developed in the globefishes or puffers (_Tetraodon_, _Spheroides_,
_Tropidichthys_, etc.). It is present in the filefishes (_Monacanthus_,
_Alutera_, etc.), probably in some toad-fishes (_Batrachoides_, etc.),
and similar compounds are found in the flesh of sharks and especially
in sharks' livers.

These alkaloids are most developed in the ovaries and testes, and in
the spawning season. They are also found in the liver and sometimes
elsewhere in the body. In many species otherwise innocuous, purgative
alkaloids are developed in or about the eggs. Serious illness has been
caused by eating the roe of the pike and the barbel. The poison is
less virulent in the species which ascend the rivers. It is also much
less developed in cooler waters. For this reason ciguatera is almost
confined to the tropics. In Havana, Manila, and other tropical ports it
is of frequent occurrence, while northward it is practically unknown
as a disease requiring a special name or treatment. On the coast of
Alaska, about Prince William Sound and Cook Inlet, a fatal disease
resembling ciguatera has been occasionally produced by the eating of
clams.

[Illustration: FIG. 139.--Numbfish, _Narcine brasiliensis_ Henle,
showing _electric cells_. Pensacola, Florida.]

The purpose of the alkaloids producing ciguatera is considered by Dr.
Pellegrin as protective, saving the species by the poisoning of its
enemies. The sickness caused by the specific poison must be separated
from that produced by ptomaines and leucomaines in decaying flesh or
in the oil diffused through it. Poisonous bacteria may be destroyed by
cooking, but the alkaloids which cause ciguatera are unaltered by heat.

It is claimed in tropical regions that the germs of the bubonic plague
may be carried through the mediation of fishes which feed on sewage.
It is suggested by Dr. Charles B. Ashmead that leprosy may be so
carried. It is further suggested that the custom of eating the flesh of
fishes raw almost universal in Japan, Hawaii, and other regions may be
responsible for the spread of certain contagious diseases, in which the
fish acts as an intermediate host, much as certain mosquitoes spread
the germ of malaria and yellow fever.

=Electric Fishes.=--Several species of fishes possess the power to
inflict electric shocks not unlike those of the Leyden jar. This is
useful in stunning their prey and especially in confounding their
enemies. In most cases these electric organs are evidently developed
from muscular substance. Their action, which is largely voluntary,
is in its nature like muscular action. The power is soon exhausted
and must be restored by rest and food. The effects of artificial
stimulation and of poisons are parallel with the effect of similar
agents on muscles.

[Illustration: FIG. 140.--Electric Catfish, _Torpedo electricus_
(Gmelin). Congo River. (Alter Boulenger.)]

In the electric rays or torpedos (_Narcobatidae_) the electric organs
are large honeycomb-like structures, "vertical hexagonal prisms,"
upwards of 400 of them, at the base of the pectoral fins. Each prism
is filled "with a clear trembling jelly-like substance." These fishes
give a shock which is communicable through a metallic conductor, as
an iron spear or the handle of a knife. It produces a peculiar and
disagreeable sensation not at all dangerous. It is said that this
living battery shows all the known qualities of magnetism, rendering
the needle magnetic, decomposing chemical compounds, etc. In the Nile
is an electric catfish (_Torpedo electricus_) having similar powers.
Its electric organ extends over the whole body, being thickest below.
It consists of rhomboidal cells of a firm gelatinous substance.

The electric eel (_Electrophorus electricus_), the most powerful of
electric fishes, is not an eel, but allied rather to the sucker or
carp. It is, however, eel-like in form and lives in rivers of Brazil
and Guiana. The electric organs are in two pairs, one on the back of
the tail, the other on the anal fin. These are made up of an enormous
number of minute cells. In the electric eel, as in the other electric
fishes, the nerves supplying these organs are much larger than those
passing from the spinal cord for any other purpose. In all these cases
closely related species show a no trace of the electric powers.

[Illustration: FIG. 141.--Star-gazer (_Astroscopus guttatus_) settling
in the sand. (From life by R. W. Shufeldt.)]

Dr. Gilbert has described the electric powers of species of star-gazer
(_Astroscopus y-graecum_ and _A. zephyreus_), the electric cells lying
under the naked skin of the top of the head. Electric power is ascribed
to a species of cusk (_Urophycis regius_), but this perhaps needs
verification.

=Photophores or Luminous Organs.=--Many fishes, chiefly of the
deep seas, develop organs for producing light. These are known as
luminous organs, phosphorescent organs, or photophores. These are
independently developed in four entirely unrelated groups of fishes.
This difference in origin is accompanied by corresponding difference in
structure. The best-known type is found in the Iniomi, including the
lantern-fishes and their many relatives. These may have luminous spots,
differentiated areas round or oblong which shine star-like in the
dark. These are usually symmetrically placed on the sides of the body.
They may have also luminous glands or diffuse areas which are luminous,
but which do not show the specialized structure of the phosphorescent
spots. These glands of similar nature to the spots are mostly on the
head or tail. In one genus, _AEthoprora_, the luminous snout is compared
to the headlight of an engine.

[Illustration: FIG. 142.--Headlight Fish, _AEthoprora lucida_ Goode and
Bean. Gulf Stream.]

[Illustration: FIG. 143.--_Corynolophus reinhardti_ (Luetken), showing
luminous bulb (modified after Luetken). Family _Ceratiidae_. Deep sea off
Greenland.]

Entirely different are the photophores in the midshipman or
singing-fish (_Porichthys_), a genus of toad-fishes or _Batrachoididae_.
This species lives near the shore and the luminous spots are outgrowths
from pores of the lateral line.

In one of the anglers (_Corynolophus reinhardti_) the complex bait
is said to be luminous, and luminous areas are said to occur on the
belly of a very small shark of the deep seas of Japan (_Etmopterus
lucifer_). This phenomenon is now the subject of study by one of the
numerous pupils of Dr. Mitsukuri. The structures in _Corynolophus_ are
practically unknown.

[Illustration: FIG. 144.--_Etmopterus lucifer_ Jordan and Snyder.
Misaki, Japan.]

=Photophores in Iniomous Fishes.=--In the _Iniomi_ the luminous organs
have been the subject of an elaborate paper by Dr. R. von Lendenfeld
(Deep-sea Fishes of the Challenger. Appendix B). These he divides
into ocellar organs of regular form or luminous spots, and irregular
glandular organs or luminous areas. The ocellar spots may be on the
scales of the lateral line or on other definite areas. They may be
raised above the surface or sunk below it. They may be simple, with
or without black pigment, or they may have within them a reflecting
surface. They are best shown in the _Myctophidae_ and _Stomiatidae_, but
are found in numerous other families in nearly all soft-rayed fishes of
the deep sea.

The glandular areas may be placed on the lower jaw, on the barbels,
under the gill cover, on the suborbital or preorbital, on the tail, or
they may be irregularly scattered. Those about the eye have usually the
reflecting membrane.

In all these structures, according to Dr. von Lendenfeld, the whole
or part of the organ is glandular. The glandular part is at the base
and the other structures are added distally. The primitive organ was
a gland which produced luminous slime. To this in the process of
specialization greater complexity has been added.

[Illustration: FIG. 145.--_Argyropelecus olfersi_ Cuvier. Gulf Stream.]

The luminous organs of some fishes resemble the supposed original
structure of the primitive photophore, though of course these cannot
actually represent it. The simplest type of photophore now found is in
_Astronesthes_, in the form of irregular glandular luminous patches
on the surface of the skin. There is no homology between the luminous
organs of any insect and those of any fish.

=Photophores of Porichthys.=--Entirely distinct in their origin are
the luminous spots in the midshipman (_Porichthys notatus_), a shore
fish of California. These have been described in detail by Dr. Charles
Wilson Greene (late of Stanford University, now of the University of
Missouri) in the _Journal of Morphology_, xv., p. 667. These are found
on various parts of the body in connection with the mucous pores of
the lateral lines and about the mucous pores of the head. The skin in
_Porichthys_ is naked, and the photophores arise from a modification of
its epidermis. Each is spherical, shining white, and consists of four
parts--the lens, the gland, the reflector, and the pigment. As to its
function Prof. Greene observes:

"I have kept specimens of _Porichthys_ in aquaria at the Hopkins
Seaside Laboratory, and have made numerous observations on them with
an effort to secure ocular proof of the phosphorescence of the living
active fish. The fish was observed in the dark when quiet and when
violently excited, but, with a single exception, only negative results
were obtained. Once a phosphorescent glow of scarcely perceptible
intensity was observed when the fish was pressed against the side of
the aquarium. Then, this is a shore fish and quite common, and one
might suppose that so striking a phenomenon as it would present if
these organs were phosphorescent in a small degree would be observed
by ichthyologists in the field, or by fishermen, but diligent inquiry
reveals no such evidence.

"Notwithstanding the fact that _Porichthys_ has been observed to
voluntarily exhibit only the trace of phosphorescence mentioned
above, still the organs which it possesses in such numbers are beyond
doubt true phosphorescent organs, as the following observations will
demonstrate. A live fish put into an aquarium of sea-water made
alkaline with ammonia water exhibited a most brilliant glow along the
location of the well-developed organs. Not only did the lines of organs
shine forth, but the individual organs themselves were distinguishable.
The glow appeared after about five minutes, remained prominent for a
few minutes, and then for twenty minutes gradually became weaker until
it was scarcely perceptible. Rubbing the hand over the organs was
followed always by a distinct increase in the phosphorescence. Pieces
of the fish containing the organs taken five and six hours after the
death of the animal became luminous upon treatment with ammonia water.

"Electrical stimulation of the live fish was also tried with good
success. The interrupted current from an induction coil was used, one
electrode being fixed on the head over the brain or on the exposed
spinal cord near the brain, and the other moved around on different
parts of the body. No results followed relatively weak stimulation
of the fish, although such currents produced violent contractions of
the muscular system of the body. But when a current strong enough to
be quite painful to the hands while handling the electrodes was used
then stimulation of the fish called forth a brilliant glow of light
apparently from every well-developed photophore. All the lines on
the ventral and lateral surfaces of the body glowed with a beautiful
light, and continued to do so while the stimulation lasted. The single
well-developed organ just back of and below the eye was especially
prominent. No luminosity was observed in the region of the dorsal
organs previously described as rudimentary in structure. I was also
able to produce the same effect by galvanic stimulation, rapidly making
and breaking the current by hand.

[Illustration: FIG. 146.--Luminous organs and lateral line of
Midshipman, _Porichthys notatus_ Girard. Family _Batrachoididae_.
Monterey, California. (After Greene.)]

"The light produced in _Porichthys_ was, as near as could be determined
by direct observation, a white light. When produced by electric
stimulation it did not suddenly reach its maximal intensity, but came
in quite gradually and disappeared in the same way when the stimulation
ceased. The light was not a strong one, only strong enough to enable
one to quite easily distinguish the apparatus used in the experiment.

"An important fact brought out by the above experiment is that an
electrical stimulation strong enough to most violently stimulate the
nervous system, as shown by the violent contractions of the muscular
system, may still be too weak to produce phosphorescence. This fact
gives a physiological confirmation of the morphological result stated
above that no specific nerves are distributed to the phosphorescent
organs.

"I can explain the action of the electrical current in these
experiments only on the supposition that it produces its effect by
direct action on the gland.

[Illustration: FIG. 147.--Cross-section of a ventral phosphorescent
organ of the Midshipman, _Porichthys notatus_ Girard. _l_, lens; _gl_,
gland; _r_, reflector; _bl_, blood; _p_, pigment. (After Greene.)]

"The experiments just related were all tried on specimens of the
fish taken from under the rocks where they were guarding the young
brood. Two specimens, however, taken by hooks from the deeper water
of Monterey Bay, could not be made to show phosphorescence either by
electrical stimulation or by treatment with ammonia. These specimens
did net have the high development of the system of mucous cells of the
skin exhibited by the nesting fish. My observations were, however, not
numerous enough to more than suggest the possibility of a seasonal high
development of the phosphorescent organs.

[Illustration: FIG. 148.--Section of the deeper portion of
phosphorescent organ of _Porichthys notatus_, highly magnified. (After
Greene.)]

"Two of the most important parts of the organ have to do with
the physical manipulation of light--the reflector and the lens,
respectively. The property of the reflector needs no discussion other
than to call attention to its enormous development. The lens cells are
composed of a highly refractive substance, and the part as a whole
gives every evidence of light refraction and condensation. The form
of the lens gives a theoretical condensation of light at a very short
focus. That such is in reality the case, I have proved conclusively
by examination of fresh material. If the fresh fish be exposed to
direct sunlight, there is a reflected spot of intense light from each
phosphorescent organ. This spot is constant in position with reference
to the sun in whatever position the fish be turned and is lost if the
lens be dissected away and only the reflector left. With needles and a
simple microscope it is comparatively easy to free the lens from the
surrounding tissue and to examine it directly. When thus freed and
examined in normal saline, I have found by rough estimates that it
condenses sunlight to a bright point a distance back of the lens of
from one-fourth to one-half its diameter. I regret that I have been
unable to make precise physical developments.

"The literature on the histological structure of known phosphorescent
organs of fishes is rather meager and unsatisfactory. Von Lendenfeld
describes twelve classes of phosphorescent organs from deep-sea
fishes collected by the _Challenger_ expedition. All of these,
however, are greater or less modifications of one type. This type
includes, according to von Lendenfeld's views, three essential parts,
_i.e._, a gland, phosphorescent cells, and a local ganglion. These
parts may have added a reflector, a pigment layer, or both; and all
these may be simple or compounded in various ways, giving rise to
the twelve classes. Blood-vessels and nerves are distributed to the
glandular portion. Of the twelve classes direct ocular proof is given
for one, i.e., ocellar organs of _Myctophum_ which were observed
by Willemoes-Suhm at night to shine 'like a star in the net.' Von
Lendenfeld says that the gland produces a secretion, and he supposes
the light or phosphorescence to be produced either by the 'burning or
consuming' of this secretion by the phosphorescent cells, or else by
some substance produced by the phosphorescent cells. Furthermore, he
says that the phosphorescent cells act at the 'will of the fish' and
are excited to action by the local ganglion.

"Some of these statements and conclusions seem insufficiently grounded,
as, for example, the supposed action of the phosphorescent cells,
and especially the control of the ganglion over them. In the first
place, the relation between the ganglion and the central nervous
system in the forms described by von Lendenfeld is very obscure, and
the structure described as a ganglion, to judge from the figures
and the text descriptions, may be wrongly identified. At least it
is scarcely safe to ascribe ganglionic function to a group of adult
cells so poorly preserved that only nuclei are to be distinguished.
In the second place, no structural character is shown to belong to
the 'phosphorescent cells' by which they may take part in the process
ascribed to them.[20]

"The action of the organs described by him may be explained on other
grounds, and entirely independent of the so-called 'ganglion cells' and
of the 'phosphorescent cells.'

"Phosphorescence as applied to the production of light by a living
animal is, according to our present ideas, a chemical action, _an
oxidation process_. The necessary conditions for producing it are
two--an oxidizable substance that is luminous on oxidation, i.e., a
photogenic substance on the one hand, and the presence of free oxygen
on the other. Every phosphorescent organ must have a mechanism for
producing these two conditions; all other factors are only secondary
and accessory. If the gland of a firefly can produce a substance that
is oxidizable and luminous on oxidation, as shown as far back as
1828 by Faraday and confirmed and extended recently by Watase, it is
conceivable, indeed probable, that phosphorescence in _Myctophum_ and
other deep-sea forms is produced in the same direct way, that is, by
direct oxidation of the secretion of the gland found in each of at
least ten of the twelve groups of organs described by von Lendenfeld.
Free oxygen may be supplied directly from the blood in the capillaries
distributed to the gland which he describes. The possibility of the
regulation of the supply of blood carrying oxygen is analogous to what
takes place in the firefly and is wholly adequate to account for any
'flashes of light' 'at the will of the fish.'

"In the phosphorescent organs of _Porichthys_ the only part the
function of which cannot be explained on physical grounds is the
group of cells called the gland. If the large granular cells of this
portion of the structure produce a secretion, as seems probable from
the character of the cells and their behavior toward reagents, and
this substance be oxidizable and luminous in the presence of free
oxygen, i.e., photogenic, then we have the conditions necessary for
a light-producing organ. The numerous capillaries distributed to the
gland will supply free oxygen sufficient to meet the needs of the
case. Light produced in the gland is ultimately all projected to the
exterior, either directly from the luminous points in the gland or
reflected outward by the reflector, the lens condensing all the rays
into a definite pencil or slightly diverging cone. This explanation
of the light-producing process rests on the assumption of a secretion
product with certain specific characters. But comparing the organ
with structures known to produce such a substance, i.e., the glands
of the firefly or the photospheres of Euphausia, it seems to me
the assumption is not less certain than the assumption that twelve
structures resembling each other in certain particulars have a common
function to that proved for one only of the twelve.

"I am inclined to the belief that whatever regulation of the action of
the phosphorescent organ occurs is controlled by the regulation of the
supply of free oxygen by the blood-stream flowing through the organ;
but, however this may be, the essential fact remains that the organs in
_Porichthys_ are true phosphorescent organs." (GREENE.)

Other species of _Porichthys_ with similar photophores occur in Texas,
Guiana, Panama, and Chile. The name midshipman alludes to these shining
spots, compared to buttons.

[Illustration: FIG. 149.--Sucking-fish, or Pegador, _Leptecheneis
naucrates_ (Linnaeus). Virginia.]

=Globefishes.=--The globefishes (_Tetraodon_, etc.) and the
porcupine-fishes have the surface defended by spines. These fishes have
an additional safeguard through the instinct to swallow air. When one
of these fishes is seriously disturbed it rises to the surface, gulps
air into a capacious sac, and then floats belly upward on the surface.
It is thus protected from other fishes, although easily taken by man.
The same habit appears in some of the frog-fishes (_Antennarius_) and
in the Swell sharks (_Cephaloscyllium_).

The writer once hauled out a netful of globefishes (_Tetraodon
hispidus_) from a Hawaiian lagoon. As they lay on the bank a dog
came up and sniffed at them. As his nose touched them they swelled
themselves up with air, becoming visibly two or three times as large as
before. It is not often that the lower animals show surprise at natural
phenomena, but the attitude of the dog left no question as to his
feeling.

=Remoras.=--The different species of Remora, or shark-suckers, fasten
themselves to the surface of sharks or other fishes and are carried
about by them often to great distances. These fishes attach themselves
by a large sucking-disk on the top of the head, which is a modified
spinous dorsal fin. They do not harm the shark, except possibly to
<DW44> its motion. If the shark is caught and drawn out of the water,
these fishes often instantly let go and plunge into the sea, swimming
away with great celerity.

=Sucking-disks of Clingfishes.=--Other fishes have sucking-disks
differently made, by which they cling to rocks. In the gobies the
united ventrals have some adhesive power. The blind goby (_Typhlogobius
californiensis_) is said to adhere to rocks in dark holes by the
ventral fins. In most gobies the adhesive power is slight. In the
sea-snails (_Liparididae_) and lumpfishes (_Cyclopteridae_) the united
ventral fins are modified into an elaborate circular sucking-disk.
In the clingfishes (_Gobiesocidae_) the sucking-disk lies between
the ventral fins and is made in part of modified folds of the naked
skin. Some fishes creep over the bottom, exploring it with their
sensitive barbels, as the gurnard, surmullet, and goatfish. The suckers
(_Catostomus_) test the bottom with their thick, sensitive lips, either
puckered or papillose, feeding by suction.

[Illustration: FIG. 150.--Clingfish, _Caularchus maeandricus_ (Girard).
Monterey, California.]

=Lampreys and Hagfishes.=--The lampreys suck the blood of other fishes
to which they fasten themselves by their disk-like mouth armed with
rasping teeth.

The hagfishes (_Myxine_, _Eptatretus_) alone among fishes are truly
parasitic. These fishes, worm-like in form, have round mouths, armed
with strong hooked teeth. They fasten themselves at the throats of
large fishes, work their way into the muscle without tearing the skin,
and finally once inside devour all the muscles of the fish, leaving the
skin unbroken and the viscera undisturbed. These fishes become living
hulks before they die. If lifted out of the water, the slimy hagfish
at once slips out and swims quickly away. In gill-nets in Monterey
Bay great mischief is done by hagfish (_Polistotrema stouti_). It is
a curious fact that large numbers of hagfish eggs are taken from the
stomachs of the male hagfish, which seems to be almost the only enemy
of his own species, keeping the numbers in check.

[Illustration: FIG. 151.--Hagfish, _Polistotrema stouti_ (Lockington).]

=The Swordfishes.=--In the swordfish and its relatives, the sailfish
and the spearfish, the bones of the anterior part of the head are
grown together, making an efficient organ of attack. The sword of the
swordfish, the most powerful of these fishes, has been known to pierce
the long planks of boats, and it is supposed that the animal sometimes
attacks the whale. But stories of this sort lack verification.

=The Paddle-fishes.=--In the paddle-fishes (_Polyodon spatula_ and
_Psephurus gladius_) the snout is spread out forming a broad paddle
or spatula. This the animal uses to stir up the mud on the bottoms
of rivers, the small organisms contained in mud constituting food.
Similar paddle-like projections are developed in certain deep-water
Chimaeras (_Harriottia_, _Rhinochimaera_), and in the deep-sea shark,
_Mitsukurina_.

[Illustration: FIG. 152.--Indian Sawfish, _Pristis zysron_ Latham.
River mouths of Hindustan. (After Day.)]

=The Sawfishes.=--A certain genus of rays (_Pristis_, the sawfish) and
a genus of sharks (_Pristiophorus_, the saw-shark), possess a similar
spatula-shaped snout. But in these fishes the snout is provided on
either side with enamelled teeth set in sockets and standing at right
angles with the snout. The animal swims through schools of sardines
and anchovies, strikes right and left with this saw, destroying the
small fishes, who thus become an easy prey. These fishes live in
estuaries and river mouths, _Pristis_ in tropical America and Guinea,
_Pristiophorus_ in Japan and Australia. In the mythology of science,
the sawfish attacks the whale, but in fact the two animals never come
within miles of each other, and the sawfish is an object of danger only
to the tender fishes, the small fry of the sea.

[Illustration: FIG. 153.--Saw-shark, _Pristiophorus japonicus_ Guenther.
Specimen from Nagasaki.]

=Peculiarities of Jaws and Teeth.=--The jaws of fishes are subject
to a great variety of modifications. In some the bones are joined by
distensible ligaments and the fish can swallow other fishes larger than
itself. In other cases the jaws are excessively small and toothless,
at the end of a long tube, so ineffective in appearance that it is a
marvel that the fish can swallow anything at all.

In the thread-eels (_Nemichthys_) the jaws are so recurved that they
cannot possibly meet, and in their great length seem worse than useless.

In some species the knife-like canines of the lower jaw pierce through
the substance of the upper.

In four different and wholly unrelated groups of fishes the teeth are
grown fast together, forming a horny beak like that of the parrot.
These are the Chimaeras, the globefishes (_Tetraodon_), and their
relatives, the parrot-fishes (_Scarus_, etc.), and the stone-wall perch
(_Oplegnathus_). The structure of the beak varies considerably in
these four cases, in accord with the difference in the origin of its
structures. In the globefishes the jaw-bones are fused together, and
in the Chimaeras they are solidly joined to the cranium itself.

=The Angler-fishes.=--In the large group of angler-fishes the first
spine of the dorsal fin is modified into a sort of bait to attract
smaller fishes into the capacious mouth below. This structure is
typical in the fishing-frog (_Lophius_), where the fleshy tip of this
spine hangs over the great mouth, the huge fish lying on the bottom
apparently inanimate as a stone. In other related fishes this spine
has different forms, being often reduced to a vestige, of little value
as a lure, but retained in accordance with the law of heredity. In a
deep-sea angler the bait is enlarged, provided with fleshy streamers
and a luminous body which serves to attract small fishes in the depths.

The forms and uses of this spine in this group constitute a very
suggestive chapter in the study of specialization and ultimate
degradation, when the special function is not needed or becomes
ineffective.

Similar phases of excessive development and final degradation may be
found in almost every group in which abnormal stress has been laid on a
particular organ. Thus the ventral fins, made into a large sucking-disk
in _Liparis_, are lost altogether in _Paraliparis_. The very large
poisoned spines of _Pterois_ become very short in _Aploactis_, the high
dorsal spines of _Citula_ are lost in _Alectis_, and sometimes a very
large organ dwindles to a very small one within the limits of the same
genus. An example of this is seen in the poisoned pectoral spines of
_Schilbeodes_.

=Relation of Number of Vertebrae to Temperature and the Struggle for
Existence.=--One of the most remarkable modifications of the skeleton
of fishes is the progressive increase of the number of vertebrae as
the forms become less specialized, and that this particular form of
specialization is greatest at the equator.[21]

It has been known for some years that in several groups of fishes
(wrasse-fishes, flounders, and "rock-cod," for example) those species
which inhabit northern waters have more vertebrae than those living
in the tropics. Certain arctic flounders, for example, have sixty
vertebrae; tropical flounders have, on the average, thirty. The
significance of this fact is the problem at issue. In science it is
assumed that all facts have significance, else they would not exist. It
becomes necessary, then, to find out first just what the facts are in
this regard.

[Illustration: FIG. 154.--Skeleton of Pike, _Esox lucius_ Linnaeus, a
river fish with many vertebrae.]

Going through the various groups of non-migratory marine fishes we
find that such relations are common. In almost every group the number
of vertebrae grows smaller as we approach the equator, and grows larger
again as we pass into southern latitudes. Taking an average netful of
fishes of different kinds at different places along the coast, the
variation would be evident. At Point Barrow or Cape Farewell or North
Cape a seineful of fishes would perhaps average eighty vertebrae each,
the body lengthened to make room for them; at Sitka or St. Johns or
Bergen, perhaps sixty vertebrae; at San Francisco or New York or St.
Malo, thirty-five; at Mazatlan or Pensacola or Naples, twenty-eight;
and at Panama or Havana or Sierra Leone, twenty-five. Under the equator
the usual number of vertebrae in shore fishes is twenty-four. Outside
tropical and semi-tropical waters this number is the exception. North
of Cape Cod it is virtually unknown.

=Number of Vertebrae.=--The numbers of vertebrae in different groups may
be summarized as follows:

_Lancelets._--Among the lancelets the numbers of segments range from 50
to 80, there being no vertebrae.

_Lampreys._--In this group the number of segments ranges from 100 to
150.

_Elasmobranchs._--Among sharks and skates the usual number of segments
is from 100 to 150 and upwards. In the extinct species as far as known
the numbers are not materially different. The Carboniferous genus,
_Pleuracanthus_, has about 115 vertebrae. The _Chimaeras_ have similar
numbers; _Chimaera monstrosa_ has about 100 in the body and more than as
many more in the filamentous tail.

_Cycliae._--_Palaeospondylus_ has about 85 vertebrae.

_Arthrodires._--There are about 100 vertebrae in _Coccosteus_.

_Dipnoans._--In Protopterus there are upwards of 100 vertebrae, the last
much reduced in size. Figures of _Neoceratodus_ show about 80.

_Crossopterygians._--_Polypterus_ has 67 vertebrae; _Erpetichthys_, 110;
_Undina_, about 85.

_Ganoids._--In this group the numbers are also large--95 in _Amia_,
about 55 in the short-bodied _Microdon_. The Sturgeons all have more
than 100 vertebrae.

=Soft-rayed Fishes.=--Among the _Teleostei_, or bony fishes, those
which first appear in geological history are the _Isospondyli_, the
allies of the salmon and herring. These have all numerous vertebrae,
small in size, and none of them in any notable degree modified or
specialized. They abound in the depths of the ocean, but there are
comparatively few of them in the tropics. The _Salmonidae_ which inhabit
the rivers and lakes of the northern zones have from 60 to 65 vertebrae.
The _Myctophidae, Stomiatidae_, and other deep-sea forms have from 40
upwards in the few species in which the number has been counted. The
group of _Clupeidae_ is nearer the primitive stock of _Isospondyli_ than
the salmon are. This group is essentially northern in its distribution,
but a considerable number of its members are found within the tropics.
The common herring (_Clupea harangus_) ranges farther into the arctic
regions than any other. Its vertebrae are 56 in number. In the shad
(_Alosa sapidissima_), a northern species which ascends the rivers,
the same number is recorded. The sprat (_Clupea sprattus_) and sardine
(_Sardinia pilchardus_), ranging farther south, have from 48 to 50,
while in certain small herrings (_Sardinella_) which are strictly
confined to tropical shores the number is but 40. Allied to the herring
are the anchovies, mostly tropical. The northernmost species, the
common anchovy of Europe (_Engraulis enchrasicolus_), has 46 vertebrae.
A tropical species (_Anchovia browni_) has 41.

There are, however, a few soft-rayed fishes confined to the tropical
seas in which the numbers of vertebrae are still large, an exception
to the general rule. Among these are _Albula vulpes_, the bonefish,
with 70 vertebrae, _Elops saurus_, the ten-pounder, with 72, the tarpon
(_Tarpon atlanticus_), with about 50, and the milkfish, _Chanos
chanos_, with 72.

In a fossil Eocene herring from the Green River shales (_Diplomystus_)
I count 40 vertebrae; in a bass-like fish (_Mioplosus_) from the same
locality 24--these being the usual numbers in the present tropical
members of these groups.

The great family of _Siluridae_, or catfishes, is represented in all
the fresh waters of temperate and tropical America, as well as in the
warmer parts of the Old World. One division of the family, containing
numerous species, abounds on the sandy shores of the tropical seas.
The others are all fresh-water fishes. So far as the vertebrae in
the _Siluridae_ have been examined, no conclusions can be drawn. The
vertebrae in the marine species range from 35 to 50; in the North
American forms, from 37 to 45; and in the South American fresh-water
species, where there is almost every imaginable variation in form and
structure, the numbers range from 28 to 50 or more. The _Cyprinidae_
(carp and minnows), confined to the fresh waters of the northern
hemisphere, and their analogues, the _Characinidae_ of the rivers of
South America and Africa, have also numerous vertebrae, 36 to 50 in most
cases.

In general we may say of the soft-rayed fishes that very few of them
are inhabitants of tropical shores. Of these few, some which are
closely related to northern forms have fewer vertebrae than their
cold-water analogues. In the northern species, the fresh-water species,
and the species found in the deep sea the number of vertebrae is always
large, but the same is true of some of the tropical species also.

=The Flounders.=--In the flounders, the halibut and its relatives,
arctic genera (_Hippoglossus_ and _Atheresthes_), have from 49 to
50 vertebrae. The northern genera (_Hippoglossoides, Lyopsetta_, and
_Eopsetta_) have from 43 to 45; the members of a large semi-tropical
genus (_Paralichthys_) of wide range have from 35 to 41; while the
tropical forms have from 35 to 37.

In the group of turbots and whiffs none of the species really belong
to the northern fauna, and the range in numbers is from 35 to 43. The
highest number, 43, is found in a deep-water species (_Monolene_),
and the next, 40, in species (_Lepidorhombus, Orthopsetta_) which
extend their range well toward the north. Among the plaices, which are
all northern, the numbers range from 35 to 65, the higher numbers,
52, 58, 65, being found in species (_Glyptocephalus_) which inhabit
considerable depths in the arctic seas. The lowest numbers (35) belong
to shore species (_Pleuronichthys_) which range well toward the south.

=Spiny-rayed Fishes.=--Among the spiny-rayed fishes the facts are more
striking. Of these, numerous families are chiefly or wholly confined to
the tropics, and in the great majority of all the species the number
of vertebrae is constantly 24,--10 in the body and 14 in the tail
(10+14). This is true of all or nearly all the _Berycidae_, _Serranidae_,
_Sparidae_, _Sciaenidae_, _Chaetodontidae_, _Haemulidae_, _Gerridae_,
_Gobiidae_, _Acanthuridae_, _Mugilidae_, _Sphyraenidae_, _Mullidae_,
_Pomacentridae_, etc.

In some families in which the process of reduction has gone on to an
extreme degree, as in certain _Plectognath_ fishes, there has been a
still further reduction, the lowest number, 14, existing in the short
inflexible body of the trunkfish (_Ostracion_), in which the vertebral
joints are movable only in the base of the tail. In all these forms the
process of reduction of vertebrae has been accompanied by specialization
in other respects. The range of distribution of these fishes is chiefly
though not quite wholly confined to the tropics.

Thus _Balistes_, the trigger-fish, has 17 vertebrae; _Monacanthus_ and
_Alutera_, foolfishes, about 20; the trunkfish, _Ostracion_, 14; the
puffers, _Tetraodon_ and _Spheroides_, 18; _Canthigaster_, 17; and the
headfish, _Mola_, 17. Among the _Pediculates, Malthe_ and _Antennarius_
have 17 to 19 vertebrae, while in their near relatives, the anglers,
_Lophiidae_, the number varies with the latitude. Thus, in the northern
angler, _Lophius piscatorius_, which is never found south of Cape
Hatteras, there are 30 vertebrae. In a similar species, inhabiting the
north of Japan (_Lophius litulon_), there are 27. In another Japanese
species, ranging farther south, _Lophiomus setigerus_, the vertebrae
are but 19. Yet in external appearance these two fishes are almost
identical. It is, however, a notable fact that some of the deep-water
_Pediculates_, or angling fishes, have the body very short and the
number of vertebrae correspondingly reduced. _Dibranchus atlanticus_,
from a depth of 3600 fathoms, or more than 4 miles, has but 18
vertebrae, and others of its relatives in deep waters show also small
numbers. These soft-bodied fishes are simply animated mouths, with a
feeble osseous structure, and they are perhaps recent offshoots from
some stock which has extended its range from muddy bottom or from
floating seaweed to the depths of the sea.

A very few spiny-rayed families are wholly confined to the northern
seas. One of the most notable of these is the family of viviparous
surf-fishes (_Embiotocidae_), of which numerous species abound on the
coasts of California and Japan, but which enter neither the waters of
the frigid nor of the torrid zone. The surf-fishes have from 32 to
42 vertebrae, numbers which are never found among tropical fishes of
similar appearance or relationship.

The facts of variation with latitude were first noticed among the
_Labridae_. In the northern genera (_Labrus_, _Tautoga_, etc.) there
are 38 to 41 vertebrae; in the semi-tropical genera (_Crenilabrus_,
_Bodianus_, etc.), 30 to 33; in the tropical genera (_Halichoeres_,
_Xyrichthys_, _Thalassoma_, etc.), usually 24.

Equally striking are the facts in the great group of _Pareioplitae_, or
mailed-cheek fishes, composed of numerous families, diverging from each
other in various respects, but agreeing in certain peculiarities of the
skeleton.

Among these fishes the family most nearly related to ordinary fishes is
that of the _Scorpaenidae_ (scorpion-fishes, etc.).

This is a large family containing many species, fishes of local habits,
swarming about the rocks at moderate depths in all zones. The species
of the tropical genera have all 24 vertebrae. Those genera chiefly found
in cooler waters, as in California, Japan, Chile, and the Cape of Good
Hope, have in all their species 27 vertebrae, while in the arctic genera
there are 31.

Allied to the _Scorpaenidae_, but confined to the tropical or
semi-tropical seas, are the _Platycephalidae_, with 27 vertebrae, and
the _Cephalacanthidae_ (flying gurnards), with but 22. In the deeper
waters of the tropics are the _Peristediidae_, with 33 vertebrae, and
extending farther north, belonging as much to the temperate as to the
torrid zone, is the large family of the _Triglidae_ (gurnards) in which
the vertebrae range from 25 to 38.

The family of _Agonidae_ (sea-poachers), with 36 to 40 vertebrae, is
still more decidedly northern in its distribution. Wholly confined
to northern waters is the great family of the _Cottidae_ (sculpins),
in which the vertebrae ascend from 30 to 50. Entirely polar and often
in deep waters are the _Liparididae_ (sea-snails), an offshoot from
the _Cottidae_, with soft, limp bodies, and the vertebrae 35 to 65. In
these northern forms there are no scales, the spines in the fins have
practically disappeared, and only the anatomy shows that they belong to
the group of spiny-rayed fishes. In the _Cyclopteridae_ (lumpfishes),
likewise largely arctic, the body becomes short and thick, the
back-bone inflexible, and the vertebrae are again reduced to 28. In
most cases, as the number of vertebrae increases, the body becomes
proportionally elongate. As a result of this, the fishes of arctic
waters are, for the most part, long and slender, and not a few of them
approach the form of eels. In the tropics, however, while elongate
fishes are common enough, most of them (always excepting the eels)
have the normal number of vertebrae, the greater length being due to
the elongation of their individual vertebrae and not to their increase
in number. Thus the very slender goby, _Gobionellus oceanicus_, has
the same number (25) of vertebrae as its thick-set relative _Gobius
soporator_ or the chubby _Lophogobius cyprinoides_. In the great
group of blenny-like fishes the facts are equally striking. The
arctic species are very slender in form as compared with the tropical
blennies, and this fact, caused by a great increase in the number of
their vertebrae, has led to the separation of the group into several
families. The tropical forms composing the family of _Blenniidae_ have
from 28 to 49 vertebrae, while in the arctic genera the numbers range
from 75 to 100.

Of the true _Blennidae_, which are all tropical or semi-tropical,
_Blennius_ has 28 to 35 vertebrae; _Salarias_, 35 to 38; Lepisoma, 34;
_Clinus_, 49; _Cristiceps_, 40. A fresh-water species of _Cristiceps_
found in Australia has 46. Blennioid fishes in the arctic seas are
_Anarrhichas_, with 76 vertebrae; _Anarrhichthys_, with 100 or more;
_Lumpenus_, 79; _Pholis_, 85; _Lycodes_, 112; _Gymnelis_, 93. _Lycodes_
and _Gymnelis_ have lost all the dorsal spines.

In the cod family (_Gadidae_) the number of vertebrae is usually about
50. The number is 51 in the codfish (_Gadus callarias_), 58 in the
Siberian cod (_Eleginus navaga_), 54 in the haddock (_Melanogrammus
aeglifinus_), 54 in the whiting (_Merlangus merlangus_), 54 in the
coalfish (_Pollachius virens_), 52 in the Alaskan coalfish (_Theragra
chalcogramma_), 51 in the hake (_Merluccius merluccius_). In the burbot
(_Lota lota_), the only fresh-water codfish, 59; in the deep-water ling
(_Molva molva_), 64; in the rocklings (_Gaidropsarus_), 47 to 49. Those
few species found in the Mediterranean and the Gulf of Mexico have
fewer fin-rays and probably fewer vertebrae than the others, but none
of the family enter warm water, the southern species living at greater
depths.

In the deep-sea allies of the codfishes, the grenadiers or rat-tails
(_Macrouridae_), the numbers range from 65 to 80.

=Fresh-water Fishes.=--Of the families confined strictly to the fresh
waters the great majority are among the soft-rayed or physostomous
fishes, the allies of the salmon, pike, carp, and catfish. In all of
these the vertebrae are numerous. A few fresh-water families have their
affinities entirely with the more specialized forms of the tropical
seas. Of these the _Centrarchidae_ (comprising the American fresh-water
sunfish and black bass) have on the average about 30 vertebrae, the
pirate perch 29, and the _Percidae_, perch and darters, etc., 35 to
45, while the _Serranidae_ or sea-bass, the nearest marine relatives
of all these, have constantly 24. The marine family of damsel-fishes
(_Pomacentridae_) have 26 vertebrae, while 30 to 40 vertebrae usually
exist in their fresh-water analogues (or possibly descendants), the
_Cichlidae_, of the rivers of South America and Africa. The sticklebacks
(_Gasterosteidae_), a family of spiny fishes, confined to the rivers and
seas of the north, have from 31 to 41 vertebrae.

=Pelagic Fishes.=--Among the free-swimming or migratory pelagic fishes,
the number of vertebrae is usually greater than among their relatives of
local habits. This fact is most evident among the scombriform fishes,
the allies of the mackerel and tunny. All of these belong properly to
the warm seas, and the reduction of the vertebrae in certain forms has
no evident relation to the temperature, though it seems to be related
in some degree to the habits of the species. Perhaps the retention of
many segments is connected with that strength and swiftness in the
water for which the mackerels are preeminent.

The variations in the number of vertebrae in this group led Dr. Guenther
to divide it into two families, the _Carangidae_ and _Scombridae_.

The _Carangidae_ or _Pampanos_ are tropical shore fishes, local or
migratory to a slight degree. All these have from 24 to 26 vertebrae.
In their pelagic relatives, the dolphins (_Coryphaena_), there are from
30 to 33; in the opah (_Lampris_), 45; in Brama, 42; while the great
mackerel family (_Scombridae_), all of whose members are more or less
pelagic, have from 31 to 50.

The mackerel (_Scomber scombrus_) has 31 vertebrae; the chub mackerel
(_Scomber japonicus_), 31; the tunny (_Thunnus thynnus_), 39; the
long-finned albacore (_Germo alalonga_), 40; the bonito (_Sarda
sarda_), 50; the Spanish mackerel (_Scomberomorus maculatus_), 45.

Other mackerel-like fishes are the cutlass-fishes (_Trichiuridae_),
which approach the eels in form and in the reduction of the fins. In
these the vertebrae are correspondingly numerous, the numbers ranging
from 100 to 160. _Aphanopus_ has 101 vertebrae; _Lepidopus_, 112;
_Trichurus_, 159.

In apparent contradiction to this rule, however, the pelagic family of
swordfishes (_Xiphias_), remotely allied to the mackerels, and with
even greater powers of swimming, has the vertebrae in normal number, the
common swordfish having but 24.

=The Eels.=--The eels constitute a peculiar group of soft-rayed
ancestry, in which everything else has been subordinated to muscularity
and flexibility of body. The fins, girdles, gill-arches, scales, and
membrane bones are all imperfectly developed or wanting. The eel is
perhaps as far from the primitive stock as the most highly "ichthyized"
fishes, but its progress has been of another character. The eel would
be regarded in the ordinary sense as a degenerate type, for its bony
structure is greatly simplified as compared with its ancestral forms,
but in its eel-like qualities it is, however, greatly specialized. All
the eels have vertebrae in great numbers. As the great majority of the
species are tropical, and as the vertebrae in very few of the deep-sea
forms have been counted, no conclusions can be drawn as to the relation
of their vertebrae to the temperature.

It is evident that the two families most decidedly tropical in
their distribution, the morays (_Muraenidae_) and the snake-eels
(_Ophichthyidae_), have diverged farthest from the primitive stock. They
are most "degenerate," as shown by the reduction of their skeleton.
At the same time they are also most decidedly "eel-like," and in
some respects, as in coloration, dentition, muscular development,
most highly specialized. It is evident that the presence of numerous
vertebral joints is essential to the suppleness of body which is the
eel's chief source of power.

So far as known the numbers of vertebrae in eels range from 115 to 160,
some of the deep-sea eels (_Nemichthys_, _Nettastoma_, _Gordiichthys_)
having much higher numbers, in accord with their slender or whip-like
forms.

Among the morays, _Muraena helena_ has 140; _Gymnothorax meleagris_,
120; _G. undulatus_, 130; _G. moringa_, 145; _G. concolor_, 136;
_Echidna catenata_, 116; _E. nebulosa_, 142; _E. zebra_, 135. In other
families the true eel, _Anguilla anguilla_, has 115; the conger-eel,
_Leptocephalus conger_, 156; and _Muraenesox cinereus_, 154.

=Variations in Fin-rays.=--In some families the number of rays in
the dorsal and anal fins is dependent on the number of vertebrae. It
is therefore subject to the same fluctuations. This relation is not
strictly proportionate, for often a variable number of rays with their
interspinal processes will be interposed between a pair of vertebrae.
The myotomes or muscular bands on the sides are usually coincident
with the number of vertebrae. As, however, these and other characters
are dependent on differences in vertebral segmentation, they bear the
same relations to temperature or latitude that the vertebrae themselves
sustain.

Thus in the _Scorpaenidae_, _Sebastes_, and _Sebastolobus_ arctic genera
have the dorsal rays xv, 13, the vertebrae 12+19. The tropical genus
_Scorpaena_ has the dorsal rays xii, 10, the vertebrae 10+14, while the
genus _Sebastodes_ of temperate waters has the intermediate numbers of
dorsal rays xii, 12, and vertebrae 12+15.

=Relation of Numbers to Conditions of Life.=--Fresh-water fishes have
in general more vertebrae than marine fishes of shallow waters. Pelagic
fishes and deep-sea fishes have more than those which live along the
shores, and more than localized or non-migratory forms. To each of
these generalizations there are occasional partial exceptions, but not
such as to invalidate the rule.

The presence of large numbers of vertebrae is noteworthy among those
fishes which swim for long distances, as, for example, many of the
mackerel family. Among such there is often found a high grade of
muscular power, or even of activity, associated with a large number
of vertebrae, these vertebrae being individually small and little
differentiated. For long-continued muscular action of a uniform kind
there would be perhaps an advantage in the low development of the
vertebral column. For muscular alertness, moving short distances with
great speed, the action of a fish constantly on its guard against
enemies or watching for its prey, the advantage would be on the side of
a few vertebrae. There is often a correlation between the free-swimming
habit and slenderness and suppleness of the body, which again is often
dependent on an increase in numbers of the vertebral segments. These
correlations appear as a disturbing element in the problem rather than
as furnishing a clew to its solution. In some groups of fresh-water
fishes there is a reduction in number of vertebrae, not associated with
any degree of specialization of the individual bone, but correlated
with simple reduction in size of body. This is apparently a phenomenon
of degeneration, a survival of dwarfs, where conditions are unfavorable
in full growth.

All these effects should be referable to the same group of causes.
They may, in fact, be combined in one statement. All other fishes now
extant, as well as all fishes existing prior to Cretaceous times, have
a larger number of vertebrae than the marine shore fishes of the tropics
of the present period. There is good reason to believe that in most
groups of spiny-rayed fishes, those with the smaller number of segments
are at once the most highly organized and the most primitive. This is
true among the blennies, the sculpins, the flounders, the perches, and
probably the labroid fishes as well. The present writer once held the
contrary view, that the forms with the higher numbers were primitive,
but the evidence both from comparative anatomy and from palaeontology
seems to indicate that among spiny-rayed fishes the forms most ancient,
most generalized, and most synthetic are those with about 24 vertebrae.
The soft-rayed fishes without exception show larger numbers, and these
are still more primitive. This apparent contradiction is perhaps
explained by Dr. Boulenger's suggestion that the prevalence of the
same number, 24, in the vertebrae of various families of spiny-rayed
fishes is due to common descent, probably from Cretaceous berycoids
having this number. In this theory, perches, sparoids, carangoids,
chaetodonts, labroids, parrot-fishes, gobies, flounders, and sculpins
must be regarded as having a common origin from which all have diverged
since Jurassic times. This view is not at all unlikely and is not
inconsistent with the facts of palaeontology. If this be the case, the
members of these and related families which have larger numbers of
vertebrae must have diverged from the primitive stock. The change has
been one of degeneration, the individual vertebrae being reduced in size
and complexity, with a vegetative increase in their number. At the
same time, the body having the greater number of segments is the more
flexible though the segments themselves are less specialized.

The primitive forms live chiefly along tropical shores, while forms
with increased numbers of vertebrae are found in all other localities.
This fact must be considered in any hypothesis as to the causes
producing such changes. If the development of large numbers be a phase
of degeneration the causes of such degeneration must be sought in the
colder seas, in the rivers, and in the oceanic abysses. What have these
waters in common that the coral reefs, the lava crags, and tide-pools
of the tropics have not?

It is certain that the possession of fewer vertebrae indicates the
higher rank, the greater specialization of parts, even though the many
vertebrae be a feature less primitive. The evolution of fishes is rarely
a movement of progress toward complexity. The time movement in some
groups is accompanied by degradation and loss of parts, by vegetative
repetition of structures, and often by a movement from the fish-form
toward the eel-form. Water life is less exacting than land life,
having less variation of conditions. It is, therefore, less effective
in pushing forward the differentiation of parts. When vertebrae are
few in number each one is relatively larger, its structure is more
complicated, its appendages larger and more useful, and the fins
with which it is connected are better developed. In other words, the
tropical fish is more intensely and compactly a fish, with a better
fish equipment, and in all ways better fitted for the business of a
fish, especially for that of a fish that stays at home.

[Illustration: FIG. 155.--Skeleton of Red Rockfish, _Sebastodes
miniatus_ Jordan and Gilbert. California.]

[Illustration: FIG. 156.--Skeleton of a spiny-rayed fish of the
tropics, _Holacanthus ciliaris_ (Linnaeus).]

In the center of competition no species can afford to be handicapped by
a weak back-bone and redundant vertebrae. Those who are thus weighted
cannot hold their own. They must change or perish.

The conditions most favorable to fish life are among the rocks and
reefs of the tropical seas. About the coral reefs is the center of
fish competition. A coral archipelago is the Paris of fishes. In such
regions is found the greatest variety of surroundings, and therefore
the greatest number of possible adjustments. The struggle is between
fish and fish, not between fishes and hard conditions of life. No form
is excluded from the competition. Cold, darkness, and foul water do not
shut out competitors, nor does any evil influence sap the strength. The
heat of the tropics does not make the sea-water hot. It is never sultry
or laden with malaria.

[Illustration: FIG. 157.--Skeleton of the Cowfish, _Lactophrys
tricornis_ (Linnaeus).]

From conditions otherwise favorable in arctic regions the majority
of competitors are excluded by their inability to bear the cold.
River life is life in isolation. To aquatic animals river life has
the same limitations that island life has to the animals of the land.
The oceanic islands are far behind the continents in the process of
evolution in so far as evolution implies specialization of parts. In a
like manner the rivers are ages behind the seas, so far as progress is
concerned, though through lack of competition the animals in isolation
may be farthest from the original stock.

Therefore the influences which serve as a whole to intensify fish life,
to keep it up to its highest effectiveness, and which tend to rid the
fish of every character or structure it cannot "use in its business,"
are most effective along the shores of the tropics. One phase of this
is the retention of low numbers of vertebrae, or, more accurately, the
increase of stress on each individual bone.

Conversely, as the causes of these changes are still in operation,
we should find that in cold waters, deep waters, dark waters, fresh
waters, and inclosed waters the strain would be less, the relapses to
less complex organization more frequent, the numbers of vertebrae would
be larger, while the individual vertebrae would become smaller, less
complete, and less perfectly ossified.

This in a general way is precisely what we do find in examining the
skeletons of a large variety of fishes.

The cause of the increased numbers of vertebrae in cold waters or
extratropical waters is as yet unknown. Several guesses have been made,
but these can scarcely rise to the level of theories. To ascribe it to
natural selection, as the present writer has done, is to do little more
than to restate the problem.

As a possible tentative hypothesis we may say that the retention of the
higher primitive traits in the tropics is due to continuous selection,
the testing of individuals by the greater variety of external
conditions. The degeneration of extratropical fishes may be due to
isolation and cessation or reversal of selection. Thus fresh waters,
the arctic waters, the oceanic abysses are the "back woods" of fish
life, localities favorable to the retention of primitive simplicity,
equally favorable to subsequent degeneration. Practically all deep-sea
fishes are degenerate descendants of shore fishes of various groups.
Monotony and isolation permit or encourage degeneration of type. Where
the struggle for existence is most intense the higher structures will
be retained or developed. Among such facts as these derived from
natural selection the cause of the relation of temperature to number of
vertebrae must be sought. How the Cretaceous berycoids first acquired
their few vertebrae and the high degree of individual specialization of
these structures we may not know. The character came with the thoracic
ventrals with reduced number of rays, the ctenoid scales, the toothless
maxillary, and other characters which have long persisted in their
subsequent descendants.

An exception to the general rule in regard to the number of vertebrae
is found in the case of the eel. Eels inhabit nearly all seas, and
everywhere they have many vertebrae. The eels of the tropics are at once
more specialized and more degraded. They are better eels than those of
northern regions, but, as the eel is a degraded type, they have gone
farther in the loss of structures in which this degradation consists.

It is not well to push this analogy too far, but perhaps we can find in
the comparison of the tropics and the cities some suggestion as to the
development of the eel.

In the city there is always a class which follows in no degree the
general line of development. Its members are specialized in a wholly
different way. By this means they take to themselves a field which
others have neglected, making up in low cunning what they lack in
humanity or intelligence.

Thus, among fishes, we have in the regions of closest competition this
degenerate and non-fish-like type, lurking in holes among the rocks,
or creeping in the sand; thieves and scavengers among fishes. The
eels thus fill a place otherwise left unfilled. In their way they are
perfectly adapted to the lives they lead. A multiplicity of vertebral
joints is useless to the tropical fish, but to the eel strength and
suppleness are everything. No armature of fin or scale or bone is so
desirable as its power of escaping through the smallest opening. With
the elongation of the body and its increase in flexibility there is a
tendency toward the loss of the paired fins, the ventrals going first,
and afterwards the pectorals. This tendency may be seen in many groups.
Among recent fishes, the blennies, the eel-pouts, and the sea-snails
furnish illustrative examples.

=Degeneration of Structures.=--In the lancelet, which is a primitively
simple organism, the various structures of the body are formed of
simple tissues and in a very simple fashion. It is probable from the
structure of each of these that it has never been very much more
complex. As the individual develops in the process of growth each
organ goes as it were straight to its final form and structure without
metamorphosis or especial alterations by the way. When this type of
development occurs, the organism belongs to a type which is primitively
simple. But there are other forms which in their adult state appear
feeble or simple, in which are found elements of organs of high
complexity. Thus in the sea-snail (_Liparis_), small, weak, with feeble
fins and flabby skin, we find the essential anatomy of the sculpin
or the rosefish. The organs of the latter are there, but each one is
reduced or degenerate, the bones as soft as membranes, the spines
obsolete or buried in the skin. Such a type is said to be degenerate.
It is very different from one primitively simple, and it is likely in
its earlier stages of development to be more complex than when it is
fully grown.

[Illustration: FIG. 158.--Liparid, _Crystallias matsushimae_ (Jordan and
Snyder). Family _Liparididae_. Matsushima Bay, Japan.]

[Illustration: FIG. 159.--Yellow-backed Rockfish, _Sebastichthys
maliger_ Jordan and Gilbert. Sitka, Alaska.]

In the evolution of groups of fishes it is a common feature that
some one organ will be the center of a special stress, in view of
some temporary importance of its function. By the process of natural
selection it will become highly developed and highly specialized.
Some later changes in conditions will render this specialization
useless or even harmful for at least a part of the species possessing
it. The structure then undergoes degeneration, and in many cases it
is brought to a lower estate than before the original changes. An
example of this may be taken from the loricate or mailed-cheek fishes.
One of the primitive members of this group is the rockfish known as
priestfish (_Sebastodes mystinus_). In this fish the head is weakly
armed, covered with ordinary scales. A slight suggestion of cranial
ridges and a slight prolongation of the third suborbital constitute the
chief suggestions of its close affinity with the mailed-cheek fishes.
In other rockfishes the cranial ridges grow higher and sharper. The
third suborbital extends itself farther and wider. It becomes itself
spinous in still others. Finally it covers the whole cheek in a coat of
mail. The head above becomes rough and horny and at last the whole body
also is enclosed in a bony box. But while this specialization reaches
an extraordinary degree in forms like _Agonus_ and _Peristedion_,
it begins to abate with _Cottus_, and thence through _Cottunculus_,
_Psychrolutes_, _Liparis_, and the like, and the mailed cheek finds
its final degradation in _Parliparis_. In this type no spines are
present anywhere, no hard bone, no trace of scales, of first dorsal,
or of ventral fins, and in the soft, limp structure covered with a
fragile, scarf-like skin we find little suggestion of affinity with
the strong rockfish or the rough-mailed _Agonus_. Yet a study of the
skeleton shows that all these loricate forms constitute a continuous
divergent series. The forms figured constitute only a few of the stages
of specialization and degradation which the members of this group
represent.

[Illustration: FIG. 160.--European Sculpin, _Myoxocephalus scorpius_
(Linnaeus). Cumberland Gulf, Arctic America]

[Illustration: FIG. 161.--Sea-raven, _Hemitripterus americanus_
(Gmelin). Halifax, Nova Scotia.]

Some of the features of the habits and development of certain
fresh-water fishes are mentioned in the following chapter.

[Illustration: FIG. 162.--Lumpfish, _Cyclopterus lumpus_ (Linnaeus).
Eastport, Maine.]

The degeneration of the eye of the blind fishes of the caves of the
Mississippi Valley, _Amblyopsis_, _Typhlichthys_, and _Troglichthys_,
have been very fully studied by Dr. Carl H. Eigenmann.

According to his observations

"The history of the eye of _Amblyopsis spelaeus_ may be divided into
four periods:

[Illustration: FIG. 163.--Sleek Sculpin, _Psychrolutes paradoxus_
(Guenther). Puget Sound.]

"(_a_) The first extends from the appearance of the eye till the embryo
is 4-5 mm. long. This period is characterized by a normal palingenic
development, except that the cell division is retarded and there is
very little growth.

[Illustration: FIG. 164.--Agonoid-fish, _Pallasina barbata_
(Steindachner). Port Mulgrave, Alaska.]

"(_b_) The second period extends till the fish is 10 mm. long. It is
characterized by the direct development of the eye from the normal
embryonic stage reached in the first period to the highest stage
reached by the _Amblyopsis_ eye.

[Illustration: FIG. 165.--Blindfish of the Mammoth Cave, _Amblyopsis
spelaeus_ (De Kay). Mammoth Cave, Kentucky.]

"(_c_) The third, from 10 mm. to about 80 or 100 mm. It is
characterized by a number of changes which are positive as contrasted
with degenerative. There are also distinct degenerative processes
taking place during this period.

"(_d_) The fourth, 80-100 mm. to death. It is characterized by
degenerative processes only.

"The eye of _Amblyopsis_ appears at the same stage of growth as in
normal fishes developing normal eyes. The eye grows but little after
its appearance.

"All the developmental processes are retarded and some of them give out
prematurely. The most important, if the last, is the cell division and
the accompanying growth that provide material for the eye.

"The lens appears at the normal time and in the normal way, but its
cells never divide and never lose their embryonic character.

"The lens is first to show degenerative steps and disappears entirely
before the fish is 10 mm. long.

[Illustration: FIG. 166.--Blind Brotula, _Lucifuga subterranea_
(Poey), showing viviparous habit. Joignan Cave, Pinar del Rio, Cuba.
Photographed by Dr. Eigenmann.]

"The optic nerve appears shortly before the fish reaches 5 mm. It does
not increase in size with the growth of the fish and disappears in old
age.

"The scleral cartilages appear when the fish is 10 mm. long; they grow
very slowly, possibly till old age.

"There is no constant ratio between the extent and degree of ontogenic
and phylogenic degeneration.

"The eye is approaching the vanishing point through the route indicated
by the eye of _Troglichthys rosae_.

"There being no causes operative or inhibitive, either within the fish
or in the environment, that are not also operative or inhibitive in
_Chologaster agassizii_, which lives in caves and develops well-formed
eyes, it is evident that the causes controlling the development
are hereditarily established in the egg by an accumulation of such
degenerative changes as are still notable in the later history of the
eye of the adult.

"The foundations of the eye are normally laid, but the superstructure,
instead of continuing the plan with additional material, completes
it out of the material provided for the foundations. The development
of the foundation of the eye is phylogenic; the stages beyond the
foundations are direct."

=Conditions of Evolution among Fishes.=--Dr. Bashford Dean ("Fishes,
Living and Fossil") has the following observations on the processes of
adaptation among fishes:

"The evolution of groups of fishes must accordingly have taken place
during only the longest periods of time. Their aquatic life has
evidently been unfavorable to deep-seated structural changes, or
at least has not permitted these to be perpetuated. Recent fishes
have diverged in but minor regards from their ancestors of the Coal
Measures. Within the same duration of time, on the other hand,
terrestrial vertebrates have not only arisen, but have been widely
differentiated. Among land-living forms the amphibians, reptiles,
birds, and mammals have been evolved, and have given rise to more than
sixty orders.

"The evolution of fishes has been confined to a noteworthy degree
within rigid and unshifting bounds; their living medium, with its
mechanical effects upon fish-like forms and structures, has for ages
been almost constant in its conditions; its changes of temperature and
density and currents have rarely been more than of local importance,
and have influenced but little the survival of genera and species
widely distributed; its changes, moreover, in the normal supply of food
organisms cannot be looked upon as noteworthy. Aquatic life has built
few of the direct barriers to survival, within which the terrestrial
forms appear to have been evolved by the keenest competition.

"It is not, accordingly, remarkable that in their descent fishes are
known to have retained their tribal features, and to have varied from
each other only in details of structure. Their evolution is to be
traced in diverging characters that prove rarely more than of family
value; one form, as an example, may have become adapted for an active
and predatory life, evolving stronger organs of progression, stouter
armoring, and more trenchant teeth; another, closely akin in general
structures, may have acquired more sluggish habits, largely or greatly
diminished size, and degenerate characters in its dermal investiture,
teeth and organs of sense or progression. The flowering out of a
series of fish families seems to have characterized every geological
age, leaving its clearest imprint on the forms which were then most
abundant. The variety that to-day maintains among the families of
bony fishes is thus known to be paralleled among the carboniferous
sharks, the Mesozoic Chimaeroids, and the Palaeozoic lung-fishes and
Teleostomes. Their environment has retained their general characters,
while modelling them anew into forms armored or scaleless, predatory
or defenseless, great, small, heavy, stout, sluggish, light, slender,
blunt, tapering, depressed.

"When members of any group of fishes became extinct, those appear to
have been the first to perish which were the possessors of the greatest
number of widely modified or _specialized_ structures. Those, for
example, whose teeth were adapted for a particular kind of food, or
whose motions were hampered by ponderous size or weighty armoring,
were the first to perish in the struggle for existence; on the other
hand, the forms that most nearly retained the ancestral or tribal
characters--that is, those whose structures were in every way least
extreme--were naturally the best fitted to survive. Thus _generalized_
fishes should be considered those of medium size, medium defenses,
medium powers of progression, omnivorous feeding habits, and wide
distribution, and these might be regarded as having provided the
staples of survival in every branch of descent.

"Aquatic living has not demanded wide divergence from the ancestral
stem, and the divergent forms which may culminate in a profusion of
families, genera, and species do not appear to be again productive of
more generalized groups. In all lines of descent specialized forms
do not appear to regain by regression or degeneration the potential
characters of their ancestral condition. A generalized form is like
potter's clay, plastic in the hands of nature, readily to be converted
into a needed kind of cup or vase; but when thus specialized may never
resume unaltered its ancestral condition: the clay survives; the cup
perishes." (DEAN.)

FOOTNOTES:

[19] Guenther, Introd. to the Study of Fishes, p. 192.

[20] The cells which von Lendenfeld designates 'phosphorescent cells'
have as their peculiar characteristic a large, oval, highly refracting
body imbedded in the protoplasm of the larger end of the clavate cells.
These cells have nothing in common with the structure of the cells of
the firefly known to be phosphorescent in nature. In fact the true
phosphorescent cells are more probably the 'gland-cells' found in ten
of the twelve classes of organs which he describes.

[21] See a more technical paper on this subject entitled "Relations of
Temperature to Vertebrae among Fishes," published in the Proceedings of
the United States National Museum for 1891, pp. 107-120. Still fuller
details are given in a paper contained in the Wilder Quarter-Century
Book, 1893. The substance is also included in Chapter VIII of
foot-notes to Evolution: D. Appleton & Co.




CHAPTER XIII

THE COLORS OF FISHES


=Pigmentation.=--The colors of fishes are in general produced by oil
sacs or pigment cells beneath the epidermis or in some cases beneath
the scales. Certain metallic shades, silvery blue or iridescent,
are produced, not by actual pigment, but, as among insects, by the
deflection of light from the polished skin or the striated surfaces
of the scales. Certain fine striations give an iridescent appearance
through the interference of light.

The pigmentary colors may be divided into two general classes, ground
coloration and ornamentation or markings. Of these the ground color is
most subject to individual or local variation, although usually within
narrow limits, while the markings are more subject to change with age
or sex. On the other hand, they are more distinctive of the species
itself.

=Protective Coloration.=--The ground coloration most usual among
fishes is protective in its nature. In a majority of fishes the back
is olivaceous or gray, either plain or mottled, and the belly white.
To birds looking down into the water, the back is  like the
water itself or like the bottom below it. To fishes in search of prey
from below, the belly is  like the surface of the water or the
atmosphere above it. In any case the darker  upper surface casts
its shadow over the paler lower parts.

In shallow waters or in rivers the bottom is not uniformly .
The fish, especially if it be one which swims close to the bottom, is
better protected if the olivaceous surface is marked by darker cross
streaks and blotches. These give the fish a color resemblance to the
weeds about it or to the sand and stones on which it lies. As a rule,
no fish which lies on the bottom is ever quite uniformly .

[Illustration: FIG. 167.--Garibaldi (scarlet in color), _Hypsypops
rubicunda_ (Girard). La Jolla, San Diego, California.]

In the open seas, where the water seems very blue, blue colors, and
especially metallic shades, take the place of olivaceous gray or green.
As we descend into deep water, especially in the warm seas, red pigment
takes the place of olive. At a moderate depth a large percentage of
the fishes are of various shades of red. Several of the large groupers
of the West Indies are represented by two color forms, a shore form
in which the prevailing shade is olive-green, and a deeper-water
form which is crimson. In several cases an intermediate-color form
also exists which is lemon-yellow. On the coast of California is a
band-shaped blenny (_Apodichthys flavidus_) which appears in three
colors, according to its surroundings, blood-red, grass-green, and
olive-yellow. The red coloration is also essentially protective, for
the region inhabited by such forms is the zone of the rose-red algae.
In the arctic waters, and in lakes where rose-red algae are not found,
the red-ground coloration is almost unknown, although red may appear
in markings or in nuptial colors. It is possible that the red, both of
fishes and algae, in deeper water is related to the effect of water on
the waves of light, but whether this should make fishes red or violet
has never been clearly understood. It is true also that where the red
in fishes ceases violet-black begins.

In the greater depths, from 500 to 4000 fathoms, the ground color in
most fishes becomes deep black or violet-black, sometimes with silvery
luster reflected from the scales, but more usually dull and lusterless.
This shade may be also protective. In these depths the sun's rays
scarcely penetrate, and the fish and the water are of the same apparent
shade, for black coloration is here the mere absence of light.

In general, the markings of various sorts grow less distinct with the
increase of depth. Bright-red fishes of the depths are usually uniform
red. The violet-black fishes of the oceanic abysses show no markings
whatever (luminous glands excepted), and in deep waters there are no
nuptial or sexual differences in color.

Ground colors other than olive-green, gray, brown, or silvery rarely
appear among fresh-water fishes. Marine fishes in the tropics sometimes
show as ground color bright blue, grass-green, crimson, orange-yellow,
or black; but these showy colors are almost confined to fishes of the
coral reefs, where they are often associated with elaborate systems of
markings.

=Protective Markings.=--The markings of fishes are of almost every
conceivable character. They may be roughly grouped as protective
coloration, sexual coloration, nuptial coloration, recognition colors,
and ornamentation, if we may use the latter term for brilliant hues
which serve no obvious purpose to the fish itself.

Examples of protective markings may be seen everywhere. The flounder
which lies on the sand has its upper surface covered with sand-like
blotches, and these again will vary according to the kind of sand it
imitates. It may be true sand or crushed coral or the detritus of lava,
in any case perfectly imitated.

Equally closely will the markings on a fish correspond with rock
surroundings. With granite rocks we find an elaborate series of
granitic markings, with coral rocks another series of shades, and if
red corals be present, red shades of like appearance are found on the
fish. Still another kind of mark indicates rock pools lined with the
red calcareous algae called corallina. Black species are found in lava
masses, grass-green ones among the fronds of ulva, and olive-green
among Sargassum or fucus, the markings and often the form corresponding
to the nature of the algae in which the species makes its home.

[Illustration: FIG. 168.--Gofu, or Poison Fish, _Synanceia verrucosa_
(Linnaeus). Family _Scorpaenidae_. Specimen from Apia, Samoa, showing
resemblance to coral masses, in the clefts of which it lives.]

=Sexual Coloration.=--In many groups of fishes the sexes are
differently . In some cases bright-red, blue, or black markings
characterize the male, the female having similar marks, but less
distinct, and the bright colors replaced by olive, brown, or gray. In a
few cases, however, the female has marks of a totally different nature,
and scarcely less bright than those of the male.

[Illustration: FIG. 169.--Lizard-skipper, _Alticus saliens_ (Forster).
A blenny which lies out of water on lava-rocks, leaping from one to
another with great agility. From nature; specimen from Point Distress,
Tutuila Island, Samoa. (About one-half size.)]

=Nuptial Coloration.=--Nuptial colors are those which appear on the
male in the breeding season only, the pigment afterwards vanishing,
leaving the sexes essentially alike. Such colors are found on most of
the minnows and dace (_Cyprinidae_) of the rivers and to a less degree
in some other fresh-water fishes, as the darters (_Etheostominae_) and
the trout. In the minnows of many species the male in spring has
the skin charged with bright pigment, red, black, or bright silvery,
for the most part, the black most often on the head, the red on the
head and body, and the silvery on the tips of the fins. At the same
time other markings are intensified, and in many species the head and
sometimes the body and fins are covered with warty excrescences. These
shades are most distinct on the most vigorous males, and disappear with
the warty excrescences after the fertilization of the eggs.

[Illustration: FIG. 170.--Blue-breasted Darter, _Etheostoma camurum_
(Cope), the most brilliantly  of American river-fishes.
Cumberland Gap, Tennessee.]

Nuptial colors do not often appear among marine fishes, and in but few
families are the sexes distinguishable by differences in coloration.

=Recognition-marks.=--Under the head of "recognition-marks" may be
grouped a great variety of special markings, which may be conceived to
aid the representatives of a given species to recognize each other.
That they actually serve this purpose is a matter of theory, but the
theory is plausible, and these markings have much in common with the
white tail feathers, scarlet crests,  wing patches, and other
markings regarded as recognition-marks among birds.

Among these are ocelli, black- or blue-ringed with white or yellow,
on various parts of the body; black spots on the dorsal fin; black
spots below or behind the eye; black, red, blue, or yellow spots
variously placed; cross-bars of red or black or green, with or without
pale edges; a blood-red fin or a fin of shining blue among pale ones;
a white edge to the tail; a yellow, blue, or red streamer to the
dorsal fin, a black tip to the pectoral or ventral; a hidden spot
of emerald in the mouth or in the axil; an almost endless variety
of sharply defined markings, not directly protective, which serve
as recognition-marks, if not to the fish itself, certainly to the
naturalist who studies it.

These marks shade off into an equally great variety for which we can
devise no better name than "ornamentation." Some fishes are simply
covered with brilliant spots or bars or reticulations, their nature
and variety baffling description, while no useful purpose seems to be
served by them, unless we stretch still more widely the convenient
theory of recognition-marks.

In many cases the markings change with age, certain bands, stripes, or
ocelli being characteristic of the young and gradually disappearing.
In such cases the same marks will be found permanent in some related
species of less differentiated coloration. In such cases it is safe to
regard them as ancestral.

In case of markings on the fins and of elaborate ornamentation
in general, it is best defined in the oldest and most vigorous
individuals, becoming intensified by degrees. The most brilliantly
 fishes are found about the coral reefs. Here may be found
species of which the ground color is the most intense blue, others
are crimson, grass-green, lemon-yellow, jet-black, and each with a
great variety of contrasted markings. The frontispiece of this volume
shows a series of such fishes drawn from nature from specimens taken
in pools of the great coral reef of Apia in Samoa. These colors are
not protective. The coral masses are mostly plain gray, and the fishes
which lie on the bottom are plain gray also. Nothing could be more
brilliant or varied than the hues of the free-swimming fishes. What
their cause or purpose may be, it is impossible to say. It is certain
that their intense activity and the ease with which they can seek
shelter in the coral masses enable them to defy their enemies. Nature
seems to riot in bright colors where her creatures are not destroyed by
their presence.

=Intensity of Coloration.=--In general, coloration is most intense
and varied in certain families of the tropical shores, and especially
about coral reefs. But in brilliancy of individual markings some
fresh-water fishes are scarcely less notable, especially the darters
(_Etheostominae_) and sunfishes (_Centrarchidae_) of the streams of
eastern North America. The bright hues of these fresh-water fishes
are, however, more or less concealed in the water by the olivaceous
markings and dark blotches of the upper parts.

[Illustration: FIG. 171.--Snake-eels, _Liuranus semicinctus_ (Lay and
Bennett), and _Chlevastes colubrinus_ (Boddaert), from Riu Kiu Islands,
Japan.]

[Illustration: FIG. 172.--Coral Reef at Apia.]

=Coral-reef Fishes.=--The brilliantly  fishes of the tropical
reefs seem, as already stated, to have no need of protective
coloration. They save themselves from their enemies in most cases by
excessive alertness and activity (_Chaetodon_, _Pomacentrus_), or else
by busying themselves in coral sand (_Julis gaimard_), a habit more
frequent than has been suspected. Every large mass of branching coral
is full of lurking fishes, some of them often most brilliantly .

=Fading of Pigments in Spirits.=--In the preservation of specimens most
red and blue pigments fade to whitish, and it requires considerable
care to interpret the traces which may be left of red bands or blue
markings. Yet some blue pigments are absolutely permanent, and
occasionally blood-red pigments persist through all conditions. Black
pigment seldom changes in spirits, and olivaceous markings simply fade
a little without material alteration. It is an important part of the
work of the systematic ichthyologist to learn to interpret the traces
of the faded pigment left on specimens he may have occasion to examine.
In such cases it is more important to trace the markings than to
restore the ground color, as the ground color is at once more variable
with individuals and more constant in large groups.

=Variation in Pattern.=--Occasionally, however, a species is found in
which, other characters being constant, both ground color and markings
are subject to a remarkable range of variation. In such cases the
actual unity of the species is open to serious question. The most
remarkable case of such variation known is found in a West Indian fish,
the vaca, which bears the incongruous name of _Hypoplectrus unicolor_.
In the typical vaca the body is orange with black marks and blue lines,
the fins checkered with orange and blue. In a second form the body
is violet, barred with black, the head with blue spots and bands. In
another form the blue on the head is wanting. In still another the body
is yellow and black, with blue on the head only. In others the fins
are plain orange, without checks, and the body yellow, with or without
blue stripes and spots, and sometimes with spots of black or violet.
In still others the body may be pink or brown, or violet-black, the
fins all yellow, part black or all black. Finally, there are forms deep
indigo-blue in color everywhere, with cross bands of indigo-black, and
these again may have bars of deeper blue on the head or may lack these
altogether. I find, no difference among these fishes except in color,
and no way of accounting for the differences in this regard.

Certain species of puffer (_Tetraodon setosus_, of Panama, and
_Tetraodon nigropunctatus_, of Polynesia) show similar remarkable
variations, being dark gray with white spots, but varying to
indigo-blue, lemon-yellow, or sometimes having coarse blotches of
either. Lemon-yellow varieties of several species are known, and
these may be due to a failure of pigment, a sort of semi-albinism.
True albinos, individuals wholly without pigment, are rare among
fishes. In some cases the markings, commonly black, will be replaced
by a deep crimson which does not fade in alcohol. This change happens
most frequently among the _Scorpaenidae_. An example of this is shown
in the frontispiece of Volume II of this work. The Japanese okose or
poison-fish (_Inimicus_) is black and gray about lava-rocks. In deeper
water among red algae it is bright crimson, the color not fading in
spirits, the markings remaining the same. In still deeper water it is
lemon-yellow.




CHAPTER XIV

THE GEOGRAPHICAL DISTRIBUTION OF FISHES


=Zoogeography.=--Under the head of distribution we consider the
facts of the actual location of species of organisms on the surface
of the earth and the laws by which their location is governed. This
constitutes the subject-matter of the science of zoogeography. In
physical geography we may prepare maps of the earth or of any part of
it, these bringing to prominence the physical features of its surface.
Such maps show here a sea, there a plateau, here a mountain chain,
there a desert, a prairie, a peninsula, or an island. In political
geography the maps show their physical features of the earth as
related to the people who inhabit them and the states or powers which
receive or claim their allegiance. In zoogeography the realms of the
earth are considered in relation to the species or tribes of animals
which inhabit them. Thus series of maps could be drawn representing
those parts of North America in which catfishes or trout or sunfishes
are found in the streams. In like manner the distribution of any
particular fish as the muskallonge or the yellow perch could be shown
on the map. The details of such a map are very instructive, and their
consideration at once raises a series of questions as to the cause
behind each fact. In science it must be supposed that no fact is
arbitrary or meaningless. In the case of fishes the details of the
method of diffusion of species afford matters of deep interest. These
are considered in a subsequent chapter.

The dispersion of animals may be described as a matter of space and
time, the movement being continuous but modified by barriers and
other conditions of environment. The tendency of recent studies in
zoogeography has been to consider the facts of present distribution
as the result of conditions in the past, thus correlating our present
knowledge with the past relations of land and water as shown through
paleontology. Dr. A. E. Ortmann well observes that "Any division of the
earth's surface into zoogeographical regions which starts exclusively
from the present distribution of animals without considering its
origin must always be unsatisfactory." We must therefore consider the
coast-lines and barriers of Tertiary and earlier times as well as those
of to-day to understand the present distribution of fishes.

=General Laws of Distribution.=--The general laws governing the
distribution of all animals are reducible to three very simple
propositions.

Each species of animal is found in every part of the earth having
conditions suitable for its maintenance, unless

(_a_) Its individuals have been unable to reach this region through
barriers of some sort; or,

(_b_) Having reached it, the species is unable to maintain itself,
through lack of capacity for adaptation, through severity of
competition with other forms, or through destructive conditions of
environment; or else,

(_c_) Having entered and maintained itself, it has become so altered
in the process of adaptation as to become a species distinct from the
original type.

=Species Absent through Barriers.=--The absence from the Japanese
fauna of most European or American species comes under the first head.
The pike has never reached the Japanese lakes, though the shade of
the-lotus leaf in the many clear ponds would suit its habits exactly.
The grunt[22] and porgies[23] of our West Indian waters have failed to
cross the ocean and therefore have no descendants in Europe or Asia.

=Species Absent through Failure to Maintain Foothold.=--Of species
under (_b_), those who have crossed the seas and not found lodgement,
we have, in the nature of things, no record. Of the existence of
multitudes of estrays we have abundant evidence. In the Gulf Stream
off Cape Cod are every year taken many young fishes belonging to
species at home in the Bahamas and which find no permanent place in
the New England fauna. In like fashion, young fishes from the tropics
drift northward in the Kuro Shiwo to the coasts of Japan, but never
finding a permanent breeding-place and never joining the ranks of the
Japanese fishes. But to this there have been, and will be, occasional
exceptions. Now and then one among thousands finds permanent lodgement,
and by such means a species from another region will be added to the
fauna. The rest disappear and leave no trace. A knowledge of these
currents and their influence is eventual to any detailed study of the
dispersion of fishes.

The occurrence of the young of many shore fishes of the Hawaiian
Islands as drifting plankton at a considerable distance from the shores
has been lately discovered by Dr. Gilbert. Each island is, in a sense,
a "sphere of influence," affecting the fauna of neighboring regions.

=Species Changed through Natural Selection.=--In the third class,
that of species changed in the process of adaptation, most insular
forms belong. As a matter of fact, at some time or another almost
every species must be in this category, for isolation is a source of
the most potent elements in the initiation and intensification of
the minor differences which separate related species. It is not the
preservation of the most useful features, but of those which actually
existed in the ancestral individuals, which distinguish such species.
Natural selection must include not only the process of the survival
of the fittest, but also the results of the survival of the existing.
This means the preservation through heredity of the traits not of the
species alone, but those of the actual individuals set apart to be the
first in the line of descent in a new environment. In hosts of cases
the persistence of characters rests not on any special usefulness or
fitness, but on the fact that individuals possessing these characters
have, at one time or another, invaded a certain area and populated it.
The principle of utility explains survivals among competing structures.
It rarely accounts for qualities associated with geographical
distribution.

=Extinction of Species.=--The extinction of species may be noted here
in connection with their extension of range. Prof. Herbert Osborn has
recognized five different types of elimination.

1. That extinction which comes from modification or progressive
evolution, a relegation to the past as the result of a transmutation
into more advanced forms. 2. Extinction from changes of physical
environment which outrun the powers of adaptation. 3. The extinction
which results from competition. 4. The extinction from extreme
specialization and limitation to special conditions the loss of which
means extinction. 5. Extinction as a result of exhaustion. As an
illustration of No. 1, we may take almost any species which has a
cognate species on the further side of some barrier or in the tertiary
seas. Thus the trout of the Twin Lakes in Colorado has acquired its
present characters in the place of those brought into the lake by its
actual ancestors. No. 2 is illustrated by the disappearance of East
Indian types (_Zanclus_, _Platax_, _Toxotes_, etc.) in Italy at the
end of the Eocene, perhaps for climatic reasons. Extinction through
competition is shown in the gradual disappearance of the Sacramento
perch (_Archoplitis interruptus_) after the invasion of the river
by catfish and carp. From extreme specialization certain forms have
doubtless disappeared, but no certain case of this kind has been
pointed out among fishes, unless this be the cause of the disappearance
of the Devonian mailed _Ostracophores_ and _Arthrodires_. It is not
likely that any group of fishes has perished through exhaustion of the
stock of vigor.

=Barriers Checking Movement of Marine Fishes.=--The limits of the
distribution of individual species or genera must be found in some sort
of barrier, past or present. The chief barriers which limit marine
fishes are the presence of land, the presence of great oceans, the
differences of temperature arising from differences in latitude, the
nature of the sea bottom, and the direction of oceanic currents. That
which is a barrier to one species may be an agent in distribution to
another. The common shore fishes would perish in deep waters almost as
surely as on land, while the open Pacific is a broad highway to the
albacore or the swordfish.

Again, that which is a barrier to rapid distribution may become an
agent in the slow extension of the range of a species. The great
continent of Asia is undoubtedly one of the greatest of barriers to
the wide movement of species of fish, yet its long shore-line enables
species to creep, as it were, from bay to bay, or from rock to rock,
till, in many cases, the same species is found in the Red Sea and in
the tide-pools or sand-reaches of Japan. In the North Pacific, the
presence of a range of half-submerged volcanoes, known as the Aleutian
and the Kurile Islands, has greatly aided the slow movement of the
fishes of the tide-pools and the kelp. To a school of mackerel or of
flying-fishes these rough islands with their narrow channels might form
an insuperable barrier.

[Illustration: FIG. 173.--Japanese filefish, _Rudarius ercodes_ Jordan
and Snyder. Wakanoura, Japan. Family _Monacanthidae_.]

=Temperature the Central Fact in Distribution.=--It has long been
recognized that the matter of temperature is the central fact in all
problems of geographical distribution. Few species in any group freely
cross the frost-line, and except as borne by oceanic currents, not
many extend their range far into waters colder than those in which
the species is distinctively at home. Knowing the average temperature
of the water in a given region we know in general the types of fishes
which must inhabit it. It is the similarity in temperature and physical
conditions which chiefly explains the resemblance of the Japanese fauna
to that of the Mediterranean or the Antilles. This fact alone must
explain the resemblance of the Arctic and Antarctic faunae, there being
in no case a barrier in the sea that may not some time be crossed. Like
forms lodge in like places.

=Agency of Ocean Currents.=--We may consider again for a moment the
movements of the great currents in the Pacific as agencies in the
distribution of species.

A great current sets to the eastward, crossing the ocean just south
of the equator. It extends past Samoa and passes on nearly to the
coast of Mexico, touching the Galapagos Islands, Clipperton Island,
and especially the Revillagigedos. This may account for the number of
Polynesian species found on these islands, about which they are freely
mixed with immigrants from the mainland of Mexico.

From the Revillagigedos[24] the current moves northward and westward,
passing the Hawaiian Islands and thence onward to the Ladrones. The
absence in Hawaii of most of the characteristic fishes of Polynesia
and Micronesia may be in part due to the long detour made by these
currents, as the conditions of life in these groups of islands are
not very different. Northeast of Hawaii is a great spiral current,
moving with the hands of the watch, forming what is called Fleurieu's
Whirlpool. This does not reach the coast of California. This fact may
help to account for the almost complete distinction in the shore fishes
of Hawaii and California.[25]

No other group of islands in the tropics has a fish fauna so isolated
as that of Hawaii. The genera are largely the ordinary tropical types.
The species are largely peculiar to these islands.

The westward current from Hawaii reaches Luzon and Formosa. It is
deflected to the northward and, joining a northward current from
Celebes, it forms the Kuro Shiwo or Black Stream of Japan, which strews
its tropical species in the rock pools along the Japanese promontories
as far as Tokio. Then, turning into the open sea, it passes northward
to the Aleutian Islands, across to Sitka. Thence it moves southward as
a cold current, bearing Ochotsk-Alaskan types southward as far as the
Santa Barbara Islands, to which region it is accompanied by species of
Aleutian origin. A cold return current seems to extend southward in
Japan, along the east shore perhaps as far as Matsushima. A similar
current in the sea to the west of Japan extends still further to the
southward, to Noto, or beyond.

It is, of course, not necessary that the movements of a species in an
oceanic current should coincide with the direction of the current.
Young fishes, or fresh-water fishes, would be borne along with the
water. Those that dwell within floating bodies of seaweed would go
whither the waters carry the drifting mass. But free-swimming fishes,
as the mackerel or flying-fishes, might as readily choose the reverse
direction. To a free-swimming fish the temperature of the water would
be the only consideration. It is thus evident that a current which to
certain forms would prove a barrier to distribution, to others would be
a mere convenience in movement.

In comparing the Japanese fauna with that of Australia, we find some
trace of both these conditions. Certain forms are perhaps excluded by
cross-currents, while certain others seem to have been influenced only
by the warmth of the water. A few Australian types on the coast of
Chile seem to have been carried over by the cross-currents of the South
Atlantic.

It is fair to say that the part taken by oceanic currents in the
distribution of shore fishes is far from completely demonstrated. The
evidence that they assist in such distribution is, in brief, as follows:

1. The young of shore fishes often swim at the surface.

2. The young of very many tropical fishes drift northward in the Gulf
Stream and the Japanese Kuro Shiwo.

3. The faunal isolation of Hawaii may be correlated with the direction
of the oceanic currents.

=Centers of Distribution.=--We may assume, in regard to any species,
that it has had its origin in or near that region in which it is most
abundant and characteristic. Such an assumption must involve a very
large percentage of error or of doubt, but in considering the mass
of species, it may represent essential truth. In the same fashion we
may regard a genus as being autochthonous or first developed in the
region where it shows the greatest range or variety of species. Those
regions where the greatest number of genera are thus autochthonous may
be regarded as centers of distribution. So far as the marine fishes
are concerned, the most important of these supposed centers are found
in the Pacific Ocean. First of these in importance is the East-Indian
Archipelago, with the neighboring shores of India. Next would come
the Arctic Pacific and its bounding islands, from Japan to British
Columbia. Third in importance in this regard is Australia. Important
centers are found in temperate Japan, in California, the Panama region,
and in New Zealand, Chili, and Patagonia. The fauna of Polynesia is
almost entirely derived from the Indies; and the shore fauna of the Red
Sea, the Bay of Bengal, and Madagascar, so far as genera are concerned,
seems to be not really separable from the Indian fauna generally.

[Illustration: FIG. 174.--Globefish, _Tetraodon setosus_ Rosa Smith.
Clarion Island, Mexico.]

I know of but six genera which may be regarded as autochthonous in the
Red Sea, and nearly all of these are of doubtful value or of uncertain
relation. The many peculiar genera described by Dr. Alcock, from the
dredgings of the _Investigator_ in the Bay of Bengal, belong to the
bathybial or deep-water series, and will all, doubtless, prove to be
forms of wide distribution.

In the Atlantic, the chief center of distribution is the West Indies;
the second is the Mediterranean. On the shores to the northward or
southward of these regions occasional genera have found their origin.
This is true especially of the New England region, the North Sea, the
Gulf of Guinea, and the coast of Argentina. The fish fauna of the North
Atlantic is derived mainly from the North Pacific, the differences
lying mainly in the relative paucity of the North Atlantic. But in
certain groups common to the two regions the migration must have been
in the opposite direction, exceptions that prove the rule.

=Distribution of Marine Fishes.=--The distribution of marine fishes
must be indicated in a different way from that of the fresh-water
forms. The barriers which limit their range furnish also their means
of dispersion. In some cases proximity overbalances the influence of
temperature; with most forms questions of temperature are all-important.

=Pelagic Fishes.=--Before consideration of the coast-lines we may
glance at the differences in vertical distribution. Many species,
especially those in groups allied to the mackerel family, are
pelagic--that is, inhabiting the open sea and ranging widely within
limits of temperature. In this series some species are practically
cosmopolitan. In other cases the genera are so. Each school or group of
individuals has its breeding place, and from the isolation of breeding
districts new species may be conceived to arise. The pelagic types have
reached a species of equilibrium in distribution. Each type may be
found where suitable conditions exist, and the distribution of species
throws little light on questions of distribution of shore fishes. Yet
among these species are all degrees of localization. The pelagic fishes
shade into the shore fishes on the one hand and into the deep-sea
fishes on the other.

=Bassalian Fishes.=--The vast group of bassalian or deep-sea fishes
includes those forms which live below the line of adequate light. These
too are localized in their distribution, and to a much greater extent
than was formerly supposed. Yet as they dwell below the influence
of the sun's rays, zones and surface temperatures are nearly alike
to them, and the same forms may be found in the Arctic or under the
equator. Their differences in distribution are largely vertical, some
living at greater depths than others, and they shade off by degrees
from bathybial into semi-bathybial, and finally into ordinary pelagic
and ordinary shore types. Apparently all of the bassalian fishes are
derived from littoral types, the changes in structure being due to
degeneration of the osseous and muscular systems and of structures not
needed in deep-sea life.

[Illustration: FIG. 175.--Sting-ray, _Dasyatis sabina_ Le Sueur.
Galveston.]

The fishes of the great depths are soft in substance, some of them
blind, some of them with very large eyes, all black in color, and very
many are provided with luminous spots or areas. A large body of species
of fishes are semi-bathybial, inhabiting depths of 20 to 100 fathoms,
showing many of the characters of shore fishes, but far more widely
distributed. Many of the remarkable cases of wide distribution of type
belong to this class. In moderate depths red colors are very common,
corresponding to the zone of red algae, and the colors in both cases are
perhaps determined from the fact that the red rays of light are the
least refrangible.

A certain number of species are both marine and fresh water, inhabiting
estuaries and brackish waters, while some more strictly marine ascend
the rivers to spawn. In none of these cases can any hard and fast line
be drawn, and some groups which are shore fishes in one region will be
represented by semi-bathybial or fluviatile forms in another.[26]

=Littoral Fishes.=--The shore fishes are in general the most highly
specialized in their respective groups, because exposed to the greatest
variety of selecting conditions and of competition. Their distribution
in space is more definite than that of the pelagic and bassalian types,
and they may be more definitely assigned to geographical areas.

=Distribution of Littoral Fishes by Coast-lines.=--Their distribution
is best indicated, not by realms or areas, but as forming four parallel
series corresponding to the four great north and south continental
outlines. Each of these series may be represented as beginning at
the north in the Arctic fauna, practically identical in each of the
four series, actually identical in the two Pacific series. Passing
southward, forms are arranged according to temperature. One by one
in each series, the Arctic types disappear; subarctic, temperate,
and semi-tropical types take their places, giving way in turn to
south-temperate and Antarctic forms. The distribution of these is
modified by barriers and by currents, yet though genera and species may
be different, each isotherm is represented in each series by certain
general types of fishes.

[Illustration: FIG. 176.--Green-sided Darter, _Diplesion blennioides_
Rafinesque. Clinch River. Family _Percidae_.]

Passing southward the two American series, the East Atlantic and the
East Pacific, pass on gradually through temperate to Antarctic types.
These are analogous to those of the Arctic, and in a few cases they
are generally identical. The West Pacific (East Asian) series is not
a continuous line on account of the presence of Australia, the East
Indies, and Polynesia. The irregularities of these regions make a
number of subseries, which break up the simplicity expressed in the
idea of four parallel series. Yet the fauna of Polynesia is strictly
East Indian, modified by the omission or alteration of species,
and that of Australia is Indian at the north, and changes to the
southward much as that of Africa does. In its marine fishes, it does
not constitute a distinct "realm." The East Atlantic (Europe-African)
series follows the same general lines of change as that of the West
Atlantic. It extends, however, only to the South Temperate Zone,
developing no Antarctic elements. The relative shortness of Africa
explains in large degree, as already shown, the similarity between the
tropical elements in the two Old-World series, as the similarity in
tropical elements in the two American series must be due to a former
depression of the connecting Isthmus. The practical unity of the Arctic
marine fauna needs no explanation in view of the present shore lines of
the Arctic Ocean.

=Minor Faunal Areas.=--The minor faunal areas of shore fishes may be
grouped as follows:

East Atlantic.

  Icelandic,
  British,
  Mediterranean,
  Guinean,
  Cape.

West Atlantic.

  Greenlandic,
  New England,
  Virginian,
  Austroriparian,
  Floridian,
  Antillaean,
  Caribbean,
  Brazilian,
  Argentinan,
  Patagonian.

East Pacific.

  Arctic,
  Aleutian,
  Sitkan,
  Californian,
  San Diegan,
  Sinaloan,
  Panamanian,
  Peruvian,
  Revillagigedan,
  Galapagan,
  Chilian,
  Patagonian.

West Pacific.

  Arctic,
  Aleutian,
  Kurile,
  Hokkaido,
  Nippon,
  Chinese,
  East Indian,
  Polynesian,
  Hawaiian,
  Indian,
  Arabian,
  Madagascarian,
  Cape,
  North Australian,
  Tasmanian,
  New Zealand,
  Antarctic.

=Equatorial Fishes Most Specialized.=--In general, the different
types are most highly specialized in equatorial waters. The processes
of specific change, through natural selection or other causes, if
other causes exist, take place most rapidly there and produce most
far-reaching modification. As elsewhere stated, the coral reefs of
the tropics are the centers of fish-life, the cities in fish economy.
The fresh waters, the arctic waters, the deep sea and the open sea
represent forms of ichthyic backwoods, regions where change goes on
more slowly, and in them we find survivals of archaic or generalized
types. For this reason the study in detail of the distribution
of marine fishes of equatorial regions is in the highest degree
instructive.

=Realms of Distribution of Fresh-water Fishes.=--If we consider the
fresh-water fishes alone we may divide the land areas of the earth
into districts and zones not differing fundamentally with those marked
out for mammals and birds. The river basin, bounded by its shores and
the sea at its mouth, shows many resemblances, from the point of view
of a fish, to an island considered as the home of an animal. It is
evident that with fishes the differences in latitude outweigh those of
continental areas, and a primary division into Old World and New World
would not be tenable.

The chief areas of distribution of fresh-water fishes we may indicate
as follows, following essentially the grouping proposed by Dr.
Guenther:[27]

=Northern Zone.=--With Dr. Guenther we may recognize first the _Northern
Zone_, characterized familiarly by the presence of sturgeon, salmon,
trout, whitefish, pike, lamprey, stickleback, and other species of
which the genera and often the species are identical in Europe,
Siberia, Canada, Alaska, and most of the United States, Japan, and
China. This is subject to cross-division into two great districts,
the first Europe-Asiatic, the second North American. These two agree
very closely to the northward, but diverge widely to the southward,
developing a variety of specialized genera and species, and both of
them passing finally by degrees into the Equatorial Zone.

Still another line of division is made by the Ural Mountains in the
Old World and by the Rocky Mountains in the New. In both cases the
Eastern region is vastly richer in genera and species, as well as in
autochthonous forms, than the Western. The reason for this lies in the
vastly greater extent of the river basins of China and the Eastern
United States, as compared with those of Europe or the Californian
region.

[Illustration: FIG. 177.--Japanese Sea-horse, _Hippocampus mohnikei_
Bleeker. Misaki, Japan.]

Minor divisions are those which separate the Great Lake region from
the streams tributary to the Gulf of Mexico; and in Asia, those which
separate China from tributaries of the Caspian, the Black, and the
Mediterranean.

=Equatorial Zone.=--The Equatorial Zone is roughly indicated by the
tropics of Cancer and Capricorn. Its essential feature is that of the
temperature, and the peculiarities of its divisions are caused by
barriers of sea or mountains.

Dr. Guenther finds the best line of separation into two divisions to lie
in the presence or absence of the great group of dace or minnows,[28]
to which nearly half of the species of fresh-water fishes the world
over belong. The entire group, now spread everywhere except in the
Arctic, South America, Australia, and the islands of the Pacific, seems
to have had its origin in India, from which region its genera have
radiated in every direction.

The Cyprinoid division of the Equatorial Zone forms two districts,
the Indian and the African. The Acyprinoid division includes South
America, south of Mexico, and all the islands of the tropical Pacific
lying to the east of Wallace's line. This line, separating Borneo from
Celebes and Bali from Lompoe, marks in the Pacific the western limit
of Cyprinoid fishes, as well as that of monkeys and other important
groups of land animals. This line, recognized as very important in
the distribution of land animals, coincides in general with the ocean
current between Celebes and Papua, which is one of the sources of the
Kuro Shiwo.

In Australia, Hawaii, and Polynesia generally, the fresh-water fishes
are derived from marine types by modification of one sort or another.
In no case, so far as I know, in any island to the eastward of Borneo,
is found any species derived from fresh-water families of either the
Eastern or the Western Continent. Of course, minor subdivisions in
these districts are formed by the contour lines of river basins. The
fishes of the Nile differ from those of the Niger or the Congo, or of
the streams of Madagascar  or Cape Colony, but in all these regions
the essential character of the fish fauna remains the same.

=Southern Zone.=--The third great region, the Southern Zone, is
scantily supplied with fresh-water fishes, and the few it possesses
are chiefly derived from modifications of the marine fauna or from the
Equatorial Zone to the north. Three districts are recognized--Tasmania,
New Zealand, and Patagonia.

=Origin of the New Zealand Fauna.=--The fact that certain peculiar
groups are common to these three regions has attracted the notice of
naturalists. In a critical study of the fish fauna of New Zealand,[29]
Dr. Gill discusses the origin of the four genera and seven species
of fresh-water fishes found in these islands, the principal of these
genera (_Galaxias_) being represented by nearly related species in
South Australia, in Patagonia,[30] the Falkland Islands, and in South
Africa.

According to Dr. Gill, we can account for this anomaly of distribution
only by supposing, on the one hand, that their ancestors were carried
for long distances in some unnatural manner, as (_a_) having been
carried across entombed in ice, or (_b_) being swept by ocean currents,
surviving their long stay in salt water, or else that they were derived
(_c_) from some widely distributed marine type now extinct, its
descendants restricted to fresh water.

On the other hand, Dr. Gill suggests that as "community of type must
be the expression of community of origin," the presence of fishes of
long-established fresh-water types must imply continuity or at least
contiguity of land. The objections raised by geologists to the supposed
land connection of New Zealand and Tasmania do not appear to Dr. Gill
insuperable. It is well known, he says, "that the highest mountain
chains are of comparatively recent geological age. It remains, then,
to consider which is the more probable, (1) that the types now common
in distant regions were distributed in some unnatural manner by the
means referred to, or (2) that they are descendants of forms once
wide-ranging over lands now submerged." After considering questions as
to change of type in other groups, Dr. Gill is inclined to postulate,
from the occurrence of species of the trout-like genus _Galaxias_, in
New Zealand, South Australia, and South America, that "there existed
some terrestrial passage-way between the several regions at a time
as late as the close of the Mesozoic period. The evidence of such a
connection afforded by congeneric fishes is fortified by analogous
representatives among insects, mollusca, and even amphibians. The
separation of the several areas must have occurred little later than
the late Tertiary, inasmuch as the salt-water fishes of corresponding
isotherms found along the coast of the now widely separated lands are
to such a large extent specifically different. In general, change seems
to have taken place more rapidly among marine animals than fresh-water
representatives of the same class."

In this case, when one guess is set against another, it seems to me
that the hypothesis first suggested, rather than the other, lies in
the line of least logical resistance. I think it better to adopt
provisionally some theory not involving the existence of a South
Pacific Antarctic Continent, to account for the distribution of
_Galaxias_. For this view I may give five reasons:

1. There are many other cases of the sort equally remarkable and
equally hard to explain. Among these is the presence of species of
paddle-fish and shovel-nosed sturgeon,[31] types characteristic of
the Mississippi Valley, in Central Asia. The presence of one and only
one of the five or six American species of pike[32] in Europe; of one
of the three species of mud-minnow in Austria,[33] the others being
American. Still another curious case of distribution is that of the
large pike-like trout of the genus _Hucho_, one species (_Hucho hucho_)
inhabiting the Danube, the other (_Hucho blackistoni_) the rivers of
northern Japan. Many such cases occur in different parts of the globe
and at present admit of no plausible explanation.

2. The supposed continental extension should show permanent traces in
greater similarity in the present fauna, both of rivers and of sea. The
other fresh-water genera of the regions in question are different, and
the marine fishes are more different than they could be if we imagine
an ancient shore connection. If New Zealand and Patagonia were once
united other genera than _Galaxias_ would be left to show it.

3. We know nothing of the power of _Galaxias_ to survive submergence in
salt water, if carried in a marine current. As already noticed, I found
young and old in abundance of the commonest of Japanese fresh-water
fishes in the open sea, at a distance from any river. Thus far, this
species, the hakone[34] dace, has not been recorded outside of Japan,
but it might well be swept to Korea or China. Two fresh-water fishes of
Japanese origin now inhabit the island of Tsushima in the Straits of
Korea.

4. The fresh-water fishes of Polynesia show a remarkably wide
distribution and are doubtless carried alive in currents. One
river-goby[35] ranges from Tahiti to the Riu Kiu Islands. Another
species,[36] originally perhaps from Brazil through Mexico, shows an
equally broad distribution.

5. We know that _Galaxias_ with its relatives must have been derived
from a marine type. It has no affinity with any of the fresh-water
families of either continent, unless it be with the Salmonidae. The
original type of this group was marine, and most of the larger species
still live in the sea, ascending streams only to spawn.

When the investigations of geologists show reason for believing in
radical changes in the forms of continents, we may accept their
conclusions. That geological evidence exists which seems to favor
the existence of a former continent, Antarctica, is claimed on high
authority. If this becomes well established we may well explain the
distribution of _Galaxias_ with reference to it. But we cannot, on
the other hand, regard the anomalous distribution of _Galaxias_ alone
constituting proof of shore connection. There can be no doubt that
almost every case of anomalies in the distribution of fishes admits of
a possible explanation through "the slow action of existing causes."

Real causes are always simple when they are once known. All anomalies
in distribution cease to be such when the facts necessary to understand
them are at our disposal.

FOOTNOTES:

[22] _Haemulon._

[23] _Calamus._

[24] Clarion Island and Socorro Island.

[25] A few Mexican shore fishes, _Chaetodon humeralis_, _Galeichthys
dasycephalus_, _Hypsoblennius parvipinnis_, have been wrongly
accredited to Hawaii by some misplacement of labels.

[26] The dragonets (_Callionymus_) are shore fishes of the shallowest
waters in Europe and Asia, but inhabit considerable depths in
tropical America. The sea-robins (_Prionotus_) are shore fishes in
Massachusetts, semi-bathybial fishes at Panama. Often Arctic shore
fishes become semi-bathybial in the Temperate Zone, living in water of
a given temperature. A long period of cold weather will sometimes bring
such to the surface.

[27] "Introduction to the Study of Fishes."

[28] Cyprinidae.

[29] "A Comparison of Antipodal Faunae," 1887.

[30] _Galaxias_, _Neochanna_, _Prototroctes_, and _Retropinna_.

[31] The shovel-nosed sturgeon (_Scaphirynchus_ and _Kessleria_) and
the paddle-fish (_Polyodon_ and _Psephurus_).

[32] _Esox lucius._

[33] _Umrba_, the mud-minnow.

[34] _Leuciscus hakuensis._

[35] _Eleotris fusca._

[36] _Awaous genivittatus._




CHAPTER XV.

ISTHMUS BARRIERS SEPARATING FISH FAUNAS


=The Isthmus of Suez.=--In the study of the effect of the Isthmus of
Suez on the distribution of fishes we may first consider the alleged
resemblance between the fauna of the Mediterranean and that of Japan.
Dr. Guenther claims that the actual identity of genera and species in
these two regions is such as to necessitate the hypothesis that they
have been in recent times joined by a continuous shore-line. This
shore-line, according to Prof. A. Ortmann and others, was not across
the Isthmus of Suez, but farther to the northward, probably across
Siberia.

=The Fish Fauna of Japan.=--For a better understanding of the problem
we may give a brief analysis of the fish fauna of Japan.

The group of islands which constitute the empire of Japan is remarkable
for the richness of its animal life. Its variety in climatic and other
conditions, its nearness to the great continent of Asia and to the
chief center of marine life, the East Indian Islands, its relation to
the warm Black Current or Kuro Shiwo from the south and to the cold
currents from the north, all tend to give variety and richness to the
fauna of its seas. Especially is this true in the group of fishes. In
spite of the political isolation of the Japanese Empire, this fact has
been long recognized and the characteristic types of Japanese fishes
have been well known to naturalists.

At present about 900 species of fishes are known from the four great
islands which constitute Japan proper--Hondo, Hokkaido, Kiusiu, and
Shikoku. About 200 others are known from the volcanic islands to the
north and south. Of these 1100 species, about fifty belong to the fresh
waters. These are all closely allied to forms found on the mainland of
Asia, from which region all of them were probably derived. In general
the same genera appear in China and with a larger range of species.

=Fresh-water Faunas of Japan.=--Two faunal areas of fresh waters may be
fairly distinguished, although broadly overlapping. The northern region
includes the island of Hokkaido and the middle and northern part of the
great island of Hondo. In a rough way, its southern boundary may be
defined by Fuji Yama, and the Bay of Matsushima. It is characterized by
the presence of salmon, trout, and sculpins, and northward by sturgeon
and brook lampreys. The southern area loses by degrees the trout
and other northern fishes, while in its clear waters abound various
minnows, gobies, and the famous ayu, or Japanese dwarf salmon, one of
the most delicate of food fishes. Sculpins and lampreys give place to
minnows, loaches, and chubs. Two genera, a sculpin[37] and a perch,[38]
besides certain minnows and catfishes, are confined to this region
and seem to have originated in it, but, like the other species, from
Chinese stock.

=Origin of Japanese Fresh-water Fishes.=--The question of the origin of
the Japanese river fauna seems very simple. All the types are Asiatic.
While most of the Japanese species are distinct, their ancestors must
have been estrays from the mainland. To what extent river fishes may
be carried from place to place by currents of salt water has never
been ascertained. One of the most widely distributed of Japanese river
fishes is the large hakone dace or chub.[39] This has been repeatedly
taken by us in the sea at a distance from any stream. It would
evidently survive a long journey in salt water. An allied species[40]
is found in the midway island of Tsushima, between Korea and Japan.

=Faunal Areas of Marine Fishes in Japan.=--The distribution of the
marine fishes of Japan is mainly controlled by the temperature of the
waters and the motion of the ocean currents. Five faunal areas may be
more or less clearly recognized, and these may receive names indicating
their scope--Kurile, Hokkaido, Nippon, Kiusiu, Kuro Shiwo, and Riu Kiu.
The first or Kurile district is frankly subarctic, containing species
characteristic of the Ochotsk Sea on the one hand, and of Alaska on
the other. The second or Hokkaido[41] district includes this northern
island and that part of the shore of the main island of Hondo[42]
which lies to the north of Matsushima and Noto. Here the cold northern
currents favor the development of a northern fauna. The herring and the
salmon occupy here the same economic relation as in Norway, Scotland,
Newfoundland, and British Columbia. Sculpins, blennies, rockfish, and
flounders abound of the rocky shores and are seen in all the markets.

South of Matsushima Bay and through the Island Sea as far as Kobe, the
Nippon fauna is distinctly one of the temperate zone. Most of the types
characteristically Japanese belong here, abounding in the sandy bays
and about the rocky islands.

About the islands of Kiusiu and Shikoku, the semi-tropical elements
increase in number and the Kiusiu fauna is less characteristically
Japanese, having much in common with the neighboring shores of China,
while some of the species range northward from India and Java. But
these faunal districts have no sharp barriers. Northern fishes[43]
unquestionably of Alaskan origin range as far south as Nagasaki,
while certain semi-tropical[44] types extend their range northward to
Hakodate and Volcano Bay. The Inland Sea, which in a sense bounds the
southern fauna, serves at the same time as a means of its extension.
While each species has a fairly definite northern or southern limit,
the boundaries of a faunal district as a whole must be stated in the
most general terms.

The well-known boundary called Blackiston's Line, which passes through
the Straits of Tsugaru, between the two great islands of Hondo and
Hokkaido, marks the northern boundary of monkeys, pheasants, and most
tropical and semi-tropical birds and mammals of Japan. But as to the
fishes, either marine or fresh water, this line has no significance.
The northern fresh-water species probably readily cross it; the
southern rarely reach it.

We may define as a fourth faunal area that of the Kuro

Shiwo district itself, which is distinctly tropical and contrasts
strongly with that of the inshore bays behind it. This warm "Black
Current," analogous to our Gulf Stream, has its origin in part from
a return current from the east which passes westward through Hawaii,
in part from a current which passes between Celebes and New Guinea.
It moves northward by way of Luzon and Formosa, touching the east
shores of the Japanese islands Kiusiu and Shikoku, to the main island
of Hondo, flooding the bays of Kagoshima and Kochi, of Waka, Suruga,
and Sagami. The projecting headlands reach out into it and the fauna
of their rock-pools is distinctly tropical as far to the northward as
Tokio.

[Illustration: FIG. 178.--Sacramento Perch, _Archoplites interruptus_
Girard. Family _Centrarchidae_. Sacramento River.]

These promontories of Hondo, Waka, Ise, Izu, Misaki, and Awa have
essentially the same types of fishes as are found on the reefs of
tropical Polynesia. The warmth of the off-shore currents gives the
fauna of Misaki its astonishing richness, and the wealth of life is
by no means confined to the fishes. Corals, crustaceans, worms, and
mollusks show the same generous profusion of species.

A fifth faunal area, closely related to that of the Black Current, is
formed by the volcanic and coral reefs of the Riu Kiu Archipelago. This
fauna, so far as known, is essentially East Indian, the genera and most
of the species being entirely identical with those of the islands about
Java and Celebes.

=Resemblance of the Japanese and Mediterranean Fish Faunas.=--It has
been noted by Dr. Guenther that the fish fauna of Japan bears a marked
resemblance to that of the Mediterranean. This likeness is shown in the
actual identity of genera and species, and in their relation to each
other. This resemblance he proposes to explain by the hypothesis that
at some recent period the two regions, Japan and the Mediterranean,
have been united by a continuous shore-line. The far-reaching character
of this hypothesis demands a careful examination of the data on which
it rests.

The resemblance of the two faunal areas, so far as fishes are
concerned, may be stated as follows: There are certain genera[45] of
shore fishes, tropical or semi-tropical, common to the Mediterranean
and Japan, and wanting to California, Panama, and the West Indies, and
in most cases to Polynesia also. Besides these, certain others found in
deeper water (100 to 200 fathoms) are common to the two areas,[46] and
have been rarely taken elsewhere.

=Significance of Resemblance.=--The significance of these facts can be
shown only by a fuller analysis of the fauna in question, and those
of other tropical and semi-tropical waters. If the resemblances are
merely casual, or if the resemblances are shown by other regions, the
hypothesis of shore continuity would be unnecessary or untenable. It is
tenable if the resemblances are so great as to be accounted for in no
other way.

Of the genera regarded as common, only two[47] or three are represented
in the two regions by identical species, and these have a very wide
distribution in the warm seas. Of the others, nearly all range to
India, to the Cape of Good Hope, to Australia, or to Brazil. They
may have ranged farther in the past; they may even range farther at
present. Not one is confined to the two districts in question. As
equally great resemblances exist between Japan and Australia or Japan
and the West Indies, the case is not self-evident without fuller
comparison. I shall therefore undertake a somewhat fuller analysis
of the evidence bearing on this and similar problems with a view to
the conclusions which may be legitimately drawn from the facts of fish
distribution.

=Differences between Japanese and Mediterranean Fish Faunas.=--We
may first, after admitting the alleged resemblances and others, note
that differences are equally marked. In each region are a certain
number of genera which we may consider as autochthonous. These
genera are represented by many species or by many individuals in the
region of their supposed origin, but are more scantily developed
elsewhere. Such genera in Mediterranean waters are _Crenilabrus_,
_Labrus_, _Spicara_, _Pagellus_, _Mullus_, _Boops_, _Spondyliosoma_,
_Oblata_. None of these occurs in Japan, nor have they any near
relatives there. Japanese autochthonous types, as _Pseudoblennius_,
_Vellitor_, _Duymaeria_, _Anoplus_, _Histiopterus_, _Monocentrus_,
_Oplegnathus_, _Plecoglossus_, range southward to the Indies or to
Australia, but all of them are totally unknown to the Mediterranean.
The multifarious genera of Gobies of Japan show very little resemblance
to the Mediterranean fishes of this family, while blennies, labroids,
scaroids, and scorpaenoids are equally diverse in their forms and
alliances. To the same extent that likeness in faunas is produced by
continuity of means of dispersion is it true that unlikeness is due to
breaks in continuity. Such a break in continuity of coast-line, in the
present case, is the Isthmus of Suez, and the unlikeness in the faunas
is about what we might conceive that such a barrier should produce.

=Sources of Faunal Resemblances.=--There are two main sources of faunal
resemblances: first, the absence of any barriers permitting the actual
mingling of the species; second, the likeness of temperature and shore
configuration on either side of an imperfect barrier. Absolute barriers
do not exist and apparently never have existed in the sea. If the fish
faunas of different regions have mingled in recent times, the fact
would be shown by the presence of the same species in each region. If
the union were of a remote date, the species would be changed, but the
genera might remain identical.

In case of close physical resemblances in different regions, as in the
East Indies and West Indies, like conditions would favor the final
lodgement of like types, but the resemblance would be general, the
genera and species being unlike. Without doubt part of the resemblance
between Japan and the Mediterranean is due to similarity of temperature
and shores. Is that which remains sufficient to demand the hypothesis
of a former shore-line connection?

=Effects of Direction of Shore-line.=--We may first note that a
continuous shore-line produces a mingling of fish faunas only when not
interrupted by barriers due to climate. A north and south coast-line,
like that of the East Pacific, however unbroken, permits great faunal
differences. It is crossed by the different zones of temperature. An
east and west shore-line lies in the same temperature. In all cases
of the kind which now exist on the earth (the Mediterranean, the Gulf
of Mexico, the Caribbean Sea, the shores of India), even species will
extend their range as far as the shore-line goes. The obvious reason
is because such a shore-line rarely offers any important barrier to
distribution, checking dispersion of species. We may, therefore,
consider the age and nature of the Isthmus of Suez and the character of
the faunas it separates.

=Numbers of Genera in Different Faunas.=--For our purposes the
genera must be rigidly defined, a separate name being used in case
of each definable difference in structure. The wide-ranging genera
of the earlier systematists were practically cosmopolitan, and their
geographical distribution teaches us little. On the other hand, when
we come to the study of geological distribution, the broad definition
of the genus is the only one usually available. The fossil specimens
are always defective. Minor characters may be lost past even the
possibility of a guess, and only along broad lines can we achieve the
classification of the individual fossil.

Using the modern definition of genus, we find in Japan 483 genera of
marine fishes; in the Red Sea, 225; in the Mediterranean, 231. In New
Zealand 150 are recorded; in Hawaii, 171; 357 from the West Indies, 187
from the Pacific coast of tropical America, 300 from India, 450 from
the East-Indian islands, and 227 from Australia.

Of the 483 genera ascribed to Japan, 156 are common to the
Mediterranean also, 188 to the West Indies and Japan, 169 to the
Pacific coast of the United States and Mexico. With Hawaii Japan shares
90 genera, with New Zealand 62; 204 are common to Japan and India, 148
to Japan and the Red Sea, most of these being found in India also. Two
hundred genera are common to Japan and Australia.

From this it is evident that Japan and the Mediterranean have much in
common, but apparently not more than Japan shares with other tropical
regions. Japan naturally shows most likeness to India, and next to this
to the Red Sea. Proportionately less is the resemblance to Australia,
and the likeness to the Mediterranean seems much the same as that to
the West Indies or to the Pacific coast of America.

But, to make these comparisons just and effective, we should consider
not the fish fauna as a whole; we should limit our discussion solely
to the forms of equatorial origin. From the fauna of Japan we may
eliminate all the genera of Alaskan-Aleutian origin, as these could not
be found in the other regions under comparison. We should eliminate
all pelagic and all deep-sea forms, for the laws which govern the
distribution of these are very different from those controlling the
shore fishes, and most of the genera have reached a kind of equilibrium
over the world.

=Significance of Rare Forms.=--We may note also, as a source of
confusion in our investigation, that numerous forms found in Japan
and elsewhere are very rarely taken, and their real distribution is
unknown. Some of these will be found to have, in some unexpected
quarter, their real center of dispersion. In fact, since these pages
were written, I have taken in Hawaii representatives of three[48]
genera which I had enumerated as belonging chiefly to Japan and the
West Indies. Numerous other genera common to the two regions have
since been obtained by Dr. Gilbert. Such species may inhabit oceanic
plateaus, and find many halting places in their circuit of the tropical
oceans. We have already discovered that Madeira, St. Helena, Ascension,
and other volcanic islands constitute such halting places. We shall
find many more such, when the deeper shore regions are explored,
the region between market-fishing and the deep-sea dredgings of the
_Challenger_ and the _Albatross_. In some cases, no doubt, these forms
are verging on extinction and a former wide distribution has given
place to isolated colonies.

The following table shows the contents, so far as genera are
concerned, of those equatorial areas in which trustworthy catalogues
of species are accessible. It includes only those fishes of stationary
habit living in less than 200 fathoms. It goes without saying that
considerable latitude must be given to these figures, to allow for
errors, omissions, uncertainties, and differences of opinion.

=Distribution of Shore Fishes.=--

                   _A. Japan and the Mediterranean._

  Genera[49] chiefly confined to these regions                         2
  Genera of wide distribution                                         77
                                                                    ----
    Total of common genera                                            79
    Total in both regions                                            399
  Genera above included, found in all equatorial regions              55
  Genera[50] found in most equatorial regions                         11
  Genera more or less restricted                                      13
                                                                    ----
                                                                      79

                      _B. Japan and the Red Sea._

  Genera[51] chiefly confined to these two regions                     2
  Genera of wide distribution                                        109
                                                                    ----
    Total genera common                                              111
    Total in both regions                                            424

                         _C. Japan and Hawaii._

  Genera chiefly confined to these regions                             3
  Genera of wide distribution                                         79
                                                                    ----
    Total genera common                                               82
    Total in both regions                                            396

                       _D. Japan and Australia._

  Genera chiefly confined to these regions                            13
  Genera of wide distribution (chiefly East Indian)                  122
                                                                    ----
    Total genera common                                              135
    Total in both regions                                            533

                         _E. Japan and Panama._

  Genera chiefly confined to these regions                             2
  Genera of wide distribution                                         89
                                                                    ----
    Total genera common                                               91
    Total in both regions                                            499

                    _F. Japan and the West Indies._

  Genera chiefly confined to these regions                             5
  Genera of wide distribution                                        108
                                                                    ----
    Total genera common                                              113
    Total in both regions                                            520

                _G. The Mediterranean and the Red Sea._

  Genera confined to the Suez region                                   0
  Genera of wide distribution (chiefly Indian)                        40
                                                                    ----
    Total genera common                                               40
    Total in both regions                                            295

                _H. West Indies and the Mediterranean._

  Genera chiefly confined to the equatorial Atlantic                  11
  Genera of wide distribution                                         59
                                                                    ----
    Total                                                             70
    Total in both regions                                            373

                      _I. West Indies and Panama._

  Genera chiefly confined to equatorial America                       68
  Genera of wide distribution                                        101
                                                                    ----
    Total genera common                                              169
    Total in equatorial America                                      376

                        _J. Hawaii and Panama._

  Genera chiefly confined to the regions in question                   3
  Genera of wide distribution                                         74
                                                                    ----
    Total genera common                                               77
    Total in both regions                                            323

                    _K. Hawaii and the East Indies._

  Genera chiefly confined to Hawaii                                    4
  Genera of wide distribution in the equatorial Pacific              123
  Genera confined to Hawaii and the West Indies                        1

                               _Summary._

  Genera (shore fishes only) in the Mediterranean
  Sea.                                                               144
  Genera in the Red Sea                                              191
  Genera in India                                                    280
  Genera in Japan (exclusive of northern forms)                      334
  Genera in Australia                                                344
  Genera in New Zealand                                              108
  Genera in Hawaii                                                   144
  Genera about Panama                                                256
  Genera in West Indies                                              299

=Extension of Indian Fauna.=--From the above tables it is evident that
the warm-water fauna of Japan, as well as that of Hawaii, is derived
from the great body of the fauna of the East Indies and Hindostan; that
the fauna of the Red Sea is derived in the same way; that the fauna
of the Mediterranean bears no especial resemblance to that of Japan,
rather than to other elements of the East Asiatic fauna in similar
conditions of temperature, and no greater than is borne by either to
the West Indies; that the faunas of the sides of the Isthmus of Suez
have relatively little in common, while those of the two sides of the
Isthmus of Panama show large identity of genera, although few species
are common to the two sides. Of the 255 genera recorded from the Panama
region, 179, or over 70 per cent., are also in the West Indies, while
68, or more than 30 per cent. of the number, are limited to the two
regions in question.

=The Isthmus of Suez as a Barrier to Distribution.=--With the aid of
the above table we may examine further the relation of the fauna of
Japan to that of the Mediterranean. If a continuity of shore-line
once existed, it would involve the obliteration of the Isthmus. With
free connection across this isthmus the fauna of the Red Sea must
have been once practically the same as that of the Mediterranean.
The present differences must be due to later immigrations to one or
the other region, or to the extinction of species in one locality
or the other, through some kind of unfitness. In neither region is
there evidence of extensive immigration from the outside. The present
conditions of water and temperature differ a little, but not enough
to explain the difference in faunae. The Red Sea is frankly tropical
and its fauna is essentially Indian, much the same, so far as genera
are concerned, as that of southern Japan. The Mediterranean is at most
not more than semi-tropical and its fishes are characteristically
European. Its tropical forms belong rather to Guinea than to the East
Indies. With the Red Sea the Mediterranean has very little in common,
not so much, for example, as has Hawaii. Forty genera of shore fishes
(and only fifty of all fishes) are identical in the two regions, the
Mediterranean and the Red Sea. Of those, every one is a genus of wide
distribution, found in nearly all warm seas. Of shore fishes, only one
genus in seven is common to the two regions. Apparently, therefore, we
cannot assume a passage across the Isthmus of Suez within the lifetime
of the present genera. Not one of the types alleged to be peculiar to
Japan and the Mediterranean is thus far known in the Red Sea. Not one
of the characteristically abundant Mediterranean types[52] crosses the
Isthmus of Suez, and the distinctive Red Sea and Indian types[53] are
equally wanting in the Mediterranean. The only genera which could have
crossed the Isthmus are certain shallow-water or brackish-water forms,
sting-rays, torpedoes, sardines, eels, and mullets, widely diffused
through the East Indies and found also in the Mediterranean. The former
channel, if one ever existed, had, therefore, much the same value in
distribution of species as the present Suez Canal.

=Geological Evidence of Submergence of the Isthmus of Suez.=--Yet, from
geological data, there is strong evidence that the Isthmus of Suez was
submerged in relatively recent times. The recognized geological maps
of the Isthmus show that a broad area of post-Pliocene or Pliocene
deposits constitutes the Isthmus and separates the nummulitic hills
of Suez from their fellows about thirty miles to the eastward. The
northern part of the Isthmus is alluvium from the Nile, and its western
part is covered with drifting sands. The Red Sea once extended farther
north than now and the Mediterranean farther to the southeast. Assuming
the maps to be correct, the Isthmus must have been open water in the
late Pliocene or post-Pliocene times.

Admitting this as a fact, the difference in the fish fauna would seem
to show that the waters over the submerged area were so shallow that
the rock-loving forms did not and could not cross it. Moreover, the
region was very likely overspread with silt-bearing fresh waters from
the Nile. To such fishes as _Chaetodon_, _Holocentrus_, _Thalassoma_
of the Red Sea, or to _Crenilabrus_, _Boops_, and _Zeus_ of the
Mediterranean, such waters would form a barrier as effective as the
sand-dunes of to-day.

=Conclusions as to the Isthmus of Suez.=--We are led, therefore, to
these conclusions:

1. There is no evidence derivable from the fishes of the recent
submergence of the Isthmus of Suez.

2. If the Isthmus was submerged in Pliocene or post-Pliocene times,
the resultant channel was shallow and muddy, so that ordinary marine
fishes or fishes of rock bottoms or of deep waters did not cross it.

3. It formed an open water to brackish-water fishes only.

4. The types common to Japan and the Mediterranean did not enter either
region from the other by way of the Red Sea.

5. As most of these are found also in India or Australia or both, their
dispersion was probably around the south coast of Africa or by the Cape
of Good Hope.

6. In view of the fact that numerous East Indian genera, as _Zanclus_,
_Enoplosus_, _Toxotes_, _Ephippus_, _Platax_, _Teuthis_, _Acanthurus_
(_Monoceros_), _Myripristis_ occur in the Eocene rocks of Tuscany,
Syria, and Switzerland, we may well suppose that an open waterway
across Africa then existed. Perhaps these forms were destroyed
in European waters by a wave of glacial cold, perhaps after the
Miocene. As our knowledge of the Miocene fish faunae of Europe is
still imperfect, we cannot locate accurately the period of their
disappearance. About half the species found in the Eocene of Italy
belong to existing genera, and these genera are almost all now
represented in the Indian fauna, and those named above with others are
confined to it.

The study of fishes alone furnishes no adequate basis for mapping
the continental masses of Tertiary times. The known facts in regard
to their distribution agree fairly with the provisional maps lately
published by Dr. Ortmann (Bull. Philos. Soc., XLI). In the Eocene
map (Fig. 179) the Mediterranean extends to the northward of Arabia,
across to the mouth of the Ganges. This extension would account for the
tropical, Eocene, and Miocene fish fauna of Southern Europe.

=The Cape of Good Hope as a Barrier to Fishes.=--The fishes of the Cape
of Good Hope are not well enough known for close comparison with those
of other regions. Enough is known of the Cape fauna to show its general
relation to those of India and Australia. The Cape of Good Hope lies in
the South Temperate Zone. It offers no absolutely impassable barrier
to the tropical fishes from either side. It bears a closer relation to
either the Red Sea or the Mediterranean than they bear to each other.
It is, therefore, reasonable to conclude that the transfer of tropical
shore fishes of the Old World between the Atlantic and Pacific, in
recent times, has taken place mainly around the southern point of
Africa. To pelagic and deep-sea fishes the Cape of Good Hope has
offered no barrier whatever. To ordinary fishes it is an obstacle, but
not an impassable one. This the fauna itself shows. It has, however,
not been passed by many tropical species, and by these only as the
result of thousands of years of struggle and point-to-point migration.

=Relations of Japan to Mediterranean Explainable by Present
Conditions.=--We may conclude that the resemblance of the Mediterranean
fish fauna to that of Japan or India is no more than might be expected,
even had the present contour of the continents been permanent for the
period of duration of the present genera and species. An open channel
in recent times would have produced much greater resemblances than
actually exist.

=The Isthmus of Panama as a Barrier to Distribution.=--Conditions in
some regards parallel with those of the Isthmus of Suez exist in but
one other region--the Isthmus of Panama. Here the first observers were
very strongly impressed by the resemblance of forms. Nearly half the
genera found on the two sides of this isthmus are common to both sides.
Taking those of the Pacific shore for first consideration, we find
that three-fourths of the genera of the Panama fauna occur in the West
Indies as well.

This identity is many times greater than that existing at the Isthmus
of Suez. Moreover, while the Cape of Good Hope offers no impassable
barrier to distribution, the same is not true of the southern part of
South America. The subarctic climate of Cape Horn has doubtless formed
a complete check to the movements of tropical fishes for a vast period
of geologic time.

=Unlikeness of Species on the Shores of the Isthmus of Panama.=--But,
curiously enough, this marked resemblance is confined chiefly to the
genera and does not extend to the species on the two shores.

Of 1400 species of fishes recorded from tropical America north of the
Equator, only about 70 are common to the two coasts. The number of
shore fishes common is still less. In this 70 are included a certain
number of cosmopolitan types which might have reached either shore from
the Old World.

[Illustration: FIG. 179.--Map of the Continents, Eocene time. (After
Ortmann.)]

A few others invade brackish or fresh waters and may possibly have
found their way, in one way or another, across the Isthmus of
Nicaragua. Of fishes strictly marine, strictly littoral, and not known
from Asia or Polynesia, scarcely any species are left as common to the
two sides. This seems to show that no waterway has existed across the
Isthmus within the lifetime, whatever that may be, of the existing
species. The close resemblance of genera shows apparently with almost
equal certainty that such a waterway has existed, and within the
period of existence of the groups called genera. How long a species
of fish may endure unchanged no one knows, but we know that in this
regard great differences must exist in different groups. Assuming that
different species crossed the Isthmus of Panama in Miocene times, we
should not be surprised to find that a few remain to all appearances
unchanged; that a much larger number have become "representative"
species, closely related forms retaining relations to the environment
to those of the parent form, and, finally, that a few species have been
radically altered.

This is exactly what has taken place at the Isthmus of Panama with
the marine shore fishes. Curiously enough, the movement of genera
seems to have been chiefly from the Atlantic to the Pacific. Certain
characteristic genera[54] of the Panama region have not passed over to
the Pacific. On the other hand, most of the common genera[55] show a
much larger number of species on the Atlantic side. This may be held to
show their Atlantic origin.

Of the relatively small number of genera which Panama has received from
Polynesia[56] few have crossed the Isthmus to appear in the West Indian
fauna.

=Views of Earlier Writers on the Fishes of the Isthmus of Panama.=--The
elements of the problem at Panama may be better understood by a glance
at the results of previous investigations.

In 1869 Dr. Guenther, after enumerating the species examined by him
from Panama, reaches the conclusion that nearly one-third of the marine
fishes on the two shores of tropical America will be found to be
identical. He enumerates 193 such species as found on the two coasts;
59 of these, or 31 per cent. of the total, being actually identical.
From this he infers that there must have been, at a comparatively
recent date, a depression of the Isthmus and intermingling of the two
faunas.[57]

=Catalogue of Fishes of Panama.=--In an enumeration of the fishes of
the Pacific coast in 1885,[58] the present writer showed that Dr.
Guenther's conclusions were based on inadequate data.

In my list 407 species were recorded from the Pacific coast of tropical
America--twice the number enumerated by Dr. Guenther. Of these 71
species, or 17-1/2 per cent., were found also in the Atlantic. About
800 species are known from the Caribbean and adjacent shores, so
that out of the total number of 1,136 species but 71, or 6 per cent.
of the whole, are common to the two coasts. This number does not
greatly exceed that of the species common to the West Indies and the
Mediterranean, or even the West Indies and Japan. It is to be noted
also that the number 71 is not very definitely ascertained, as there
must be considerable difference of opinion as to the boundaries of
species, and the actual identity in several cases is open to doubt.

This discrepancy arises from the comparatively limited representation
of the two faunas at the disposal of Dr. Guenther. He enumerates 193
marine or brackish-water species as found on the two coasts, 59 of
which are regarded by him as specifically identical, this being 31 per
cent. of the whole. But in 30 of these 59 cases I regard the assumption
of complete identity as erroneous, so that taking the number 193 as
given I would reduce the percentage to 15. But these 193 species form
but a fragment of the total fauna, and any conclusion based on such
narrow data is certain to be misleading.

Of the 71 identical species admitted in our list, several (_e.g._,
_Mola_, _Thunnus_) are pelagic fishes common to most warm seas.

Still others (_e.g._, _Trachurus_, _Carangus_, _Diodon_ sp.) are
cosmopolitan in the tropical waters. Most of the others (_e.g._,
_Gobius_, _Gerres_, _Centropomus_, _Galeichthys_ sp., etc.) often
ascend the rivers of the tropics, and we may account for their
diffusion, perhaps, as we account for the dispersion of fresh-water
fishes on the Isthmus, on the supposition that they may have crossed
from marsh to marsh at some time in the rainy season.

In very few cases are representatives of any species from opposite
sides of the Isthmus exactly alike in all respects. These differences
in some cases seem worthy of specific value, giving us "representative
species" on the two sides. In other cases the distinctions are very
trivial, but in most cases they are appreciable, especially in fresh
specimens.

Further, I expressed the belief that "fuller investigation will not
increase the proportion of common species. If it does not, the two
faunas show no greater resemblance than the similarity of physical
conditions on the two sides would lead us to expect." This similarity
causes the same types of fishes to persist on either side of the
Isthmus while through isolation or otherwise these have become
different as species.

This conclusion must hold so far as species are concerned, but the
resemblance of the genera on the sides has a significance of its own.

In 1880[59] Dr Guenther expressed his views in still stronger language,
claiming a still larger proportion of the fishes of tropical America to
be identical on the two sides of the continent. He concluded that "with
scarcely any exceptions the genera are identical, and of the species
found on the Pacific side, nearly one-half have proved to be the same
as those of the Atlantic. The explanation of this fact has been found
in the existence of communications between the two oceans by channels
and straits which must have been open till within a recent period. The
isthmus of Central America was then partially submerged, and appeared
as a chain of islands similar to that of the Antilles; but as the
reef-building corals flourished chiefly north and east of these islands
and were absent south and west of them, reef fishes were excluded
from the Pacific shores when the communications were destroyed by the
upheaval of land."

=Conclusions of Evermann and Jenkins.=--This remark led to a further
discussion of the subject on the part of Dr. B. W. Evermann and Dr. O.
P. Jenkins. From their paper on the fishes of Guaymas[60] I make the
following quotations:

"The explorations since 1885 have resulted (1) in an addition of about
100 species to one or other of the two faunas; (2) in showing that at
least two species that were regarded as identical on the two shores[61]
are probably distinct; and (3) in the addition of but two species to
those common to both coasts.[62]

"All this reduces still further the percentage of common species.

"Of the 110 species obtained by us, 24, or less than 21 per cent.,
appear to be common to both coasts. Of these 24 species, at least 16,
from their wide distribution, would need no hypothesis of a former
waterway through the Isthmus to account for their presence on both
sides. They are species fully able to arrive at the Pacific shores of
the Americas from the warm seas west. It thus appears that not more
than eight species, less than 8 per cent. of our collection, all of
which are marine species, require any such hypothesis to account for
their occurrence on both coasts of America. This gives us, then, 1,307
species that should properly be taken into account when considering
this question, not more than 72 of which, or 5.5 per cent., seem to be
identical on the two coasts. This is very different from the figures
given by Dr. Guenther in his 'Study of Fishes.'

"Now, if from these 72 species, admitted to be common to both coasts,
we subtract the 16 species of wide distribution--so wide as to keep
them from being a factor in this problem--we have left but 56 species
common to the two coasts that bear very closely upon the waterway
hypothesis. _This is less than 4.3 per cent. of the whole number._

"But the evidence obtained from a study of other marine life of that
region points to the same conclusion.

"In 1881, Dr. Paul Fischer discussed the same question in his 'Manual
de Conchyliologie,' pp. 168, 169, in a section on the Molluscan Fauna
of the Panamic Province, and reached the same general conclusions. He
says: 'Les naturalistes Americians se sont beaucoup preeoccupes des
especes de Panama qui paraissent identiques avec celles des Antilles,
ou qui sont representatives. P. Carpenter estime qu'il en existe 35.
Dans la plupart des cas, l'identite absolue n'a pu etre constantee
et on a trouve quelques caracteres distinctifs, ce qui n'a rien
d'etonnant, puisque dans l'hypothese d'une origine commune, les deux
races pacifique et atlantique sont separee depuis la periode Miocene.
Voici un liste de ces especes representatives ou identiques.' Here
follows a list of 20 species. 'Mais ces formes semblables,' he says,
'constituent un infime minorite (3 per cent.).'

"These facts have a very important bearing upon certain geological
questions, particularly upon the one concerning the cold of the Glacial
period.

"In Dr. G. Frederick Wright's recent book, 'The Ice Age in North
America,' eight different theories as to the cause of the cold are
discussed. The particular theory which seems to him quite reasonable
is that one which attributes the cold as due to a change of different
parts of the country, and a depression of the Isthmus of Panama is
one of the important changes he considers. He says: 'Should a portion
of the Gulf Stream be driven through a depression across the Isthmus
of Panama into the Pacific, and an equal portion be diverted from the
Atlantic coast of the United States by an elevation of the sea-bottom
between Florida and Cuba, the consequences would necessarily be
incalculably great, so that the mere existence of such a possible cause
for great changes in the distribution of moisture over the northern
hemisphere is sufficient to make one hesitate before committing himself
unreservedly to any other theory; at any rate, to one which has not for
itself independent and adequate proof.'

"In the appendix to the same volume Mr. Warren Upham, in discussing
the probable causes of glaciation, says: 'The quaternary uplifts of
the Andes and Rocky Mountains and of the West Indies make it nearly
certain that the Isthmus of Panama has been similarly elevated during
the recent epoch.... It may be true, therefore, that the submergence
of this isthmus was one of the causes of the Glacial period, the
continuation of the equatorial oceanic currents westward into the
Pacific having greatly diminished or wholly diverted the Gulf Stream,
which carries warmth from the tropics to the northern Atlantic and
northwestern Europe.'

[Illustration: FIG. 180.--_Caulophryne jordani_ Goode and Bean, a
deep-sea fish of the Gulf Stream. Family _Ceratiidae_.]

[Illustration: FIG. 181.--_Exerpes asper_ Jenkins and Evermann, a fish
of the rock-pools, Guaymas, Mexico. Family _Blenniidae_.]

"Any _very_ recent means by which the fishes could have passed readily
from one side to the other would have resulted in making the fish
faunas of the two shores practically identical; but the time that has
elapsed since such a waterway could have existed has been long enough
to allow the fishes of the two sides to become _practically distinct_.
That the mollusks of the two shores are almost wholly distinct, as
shown by Dr. Fischer, is even stronger evidence of the remoteness of
the time when the means of communication between the two oceans could
have existed, for 'species' among the mollusks are probably more
persistent than among fishes.

"Our present knowledge, therefore, of the fishes of tropical America
justifies us in regarding the fish faunas of the two coasts as being
essentially distinct, and believing that there has not been, at any
comparatively recent time, any waterway through the Isthmus of Panama."

It is thus shown, I think, conclusively, that the Isthmus of Panama
could not have been depressed for any great length of time in a recent
geological period.

=Conclusions of Dr. Hill.=--These writers have not, however, considered
the question of generic identity. To this we may find a clue in the
geological investigations of Dr. Robert T. Hill.

In a study of "The Geological History of the Isthmus of Panama and
Portions of Costa Rica," Dr. Hill uses the following language:

"By elimination we have concluded that the only period of time since
the Mesozoic within which communication between the seas could have
taken place is the Tertiary period, and this must be restricted to
the Eocene and Oligocene epochs of that period. The paleontologic
evidence upon which such an opening can be surmised at this period
is the occurrence of a few California Eocene types in the Atlantic
sides of the tropical American barrier, within the ranges of latitude
between Galveston (Texas) and Colon, which are similar to others found
in California. There are no known structural data upon which to locate
the site of this passage, but we must bear in mind, however, that this
structure has not been completely explored.

"Even though it was granted that the coincidence of the occurrence of a
few identical forms on both sides of the tropical American region, out
of the thousands which are not common, indicates a connection between
the two seas, there is still an absence of any reason for placing this
connection at the Isthmus of Panama, and we could just as well maintain
that the locus thereof might have been at some other point in the
Central American region.

"The reported fossil and living species common to both oceans are
littoral forms, which indicate that if a passage existed it must have
been of a shallow and ephemeral character.

"There is no evidence from either a geologic or a biologic standpoint
for believing that the oceans have ever communicated across the
Isthmian regions since Tertiary time. In other words, there is no
evidence for these later passages which have been established upon
hypothetical data, especially those of Pleistocene time.

"The numerous assertions, so frequently found in literature, that the
two oceans have been frequently and recently connected across the
Isthmus, and that the low passes indicative of this connection still
exist, may be dismissed at once and forever and relegated to the domain
of the apocryphal. A few species common to the waters of both oceans in
a predominantly Caribbean fauna of the age of the Claiborne epoch of
the Eocene Tertiary is the only paleontologic evidence in any time upon
which such a connection may be hypothesized.

"There has been a tendency in literature to underestimate the true
altitude of the isthmian passes, which, while probably not intentional,
has given encouragement to those who think that this Pleistocene
passage may have existed. Maack has erroneously given the pass at 186
feet. Dr. J. W. Gregory states 'that the summit of the Isthmus at one
locality is 154 feet and in another 287 feet in height.' The lowest
isthmian pass, which is not a summit, but a drainage col, is 287-295
feet above the ocean.

"If we could lower the isthmian region 300 feet at present, the waters
of the two oceans would certainly commingle through the narrow Culebra
Pass. But the Culebra Pass is clearly the headwater col of two streams,
the Obispo flowing into the Chagres, and the Rio Grande flowing into
the Pacific, and has been cut by fluviatile action, and not by marine
erosion, out of a land mass which has existed since Miocene time. Those
who attempt to establish Pleistocene interoceanic channels through this
pass on account of its present low altitude must not omit from their
calculations the restoration of former rock masses which have been
removed by the general levelling of the surface by erosion."

[Illustration: FIG. 182.--_Xenocys jessiae_ Jordan and Bollman.
Galapagos Islands. Family _Lutianidae_.]

In conclusion, Dr. Hill asserts that "there is considerable evidence
that a land barrier in the tropical region separated the two oceans as
far back in geologic history as Jurassic time, and that that barrier
continued throughout the Cretaceous period. The geological structure
of the Isthmus and Central American regions, so far as investigated,
when considered aside from the paleontology, presents no evidence by
which the former existence of a free communication of oceanic waters
across the present tropical land barriers can be established. The
paleontologic evidence indicates the ephemeral existence of a passage
at the close of the Eocene period. All lines of inquiry--geologic,
paleontologic, and biologic--give evidence that no connection has
existed between the two oceans since the close of the Oligocene. This
structural geology is decidedly opposed to any hypothesis by which the
waters of the two oceans could have been connected across the regions
in Miocene, Pliocene, Pleistocene, or recent times."

=Final Hypothesis as to Panama.=--If we assume the correctness of Dr.
Hill's conclusions, they may accord in a remarkable degree with the
actual facts of the distribution of the fishes about the Isthmus. To
account for the remarkable identity of genera and divergence of species
I may suggest the following hypothesis:

During the lifetime of most of the present species, the Isthmus has not
been depressed. It was depressed in or before Miocene time, during the
lifetime of most of the present genera. We learn from other sources
that few of the extant species of fishes are older than the Pliocene.
Relatively few genera go back to the Eocene, and most of the modern
families appear to begin in the Eocene or later Cretaceous. In general
the Miocene may be taken as the date of the origin of modern genera.
The channel formed across the Isthmus was relatively shallow, excluding
forms inhabiting rocky bottoms at considerable depths. It was wide
enough to permit the infiltration from the Caribbean Sea of numerous
species, especially of shore fishes of sandy bays, tide pools, and
brackish estuaries. The currents set chiefly to the westward, favoring
the transfer of Atlantic rather than Pacific types.

[Illustration: FIG. 183.--Channel Catfish, _Ictalurus punctatus_
(Rafinesque). Illinois River. Family _Siluridae_.]

Since the date of the closing of this channel the species left on the
two sides have been altered in varying degrees by the processes of
natural selection and isolation. The cases of actual specific identity
are few, and the date of the establishment as species, of the existing
forms, is subsequent to the date of the last depression of the Isthmus.

We may be certain that none of the common genera ever found their way
around Cape Horn. Most of them disappear to the southward, along the
coasts of Brazil and Peru.

While local oscillations, involving changes in coast-lines, have
doubtless frequently taken place and are still going on, the past and
present distribution of fishes does not alone give adequate data for
their investigation.

Further, it goes without saying that we have no knowledge of the
period of time necessary to work specific changes in a body of species
isolated in an alien sea. Nor have we any data as to the effect on
a given fish fauna of the infiltration of many species and genera
belonging to another. All such forces and results must be matters of
inference.

The present writer does not wish to deny that great changes have
taken place in the outlines of continents in relatively recent times.
He would, however, insist that the theory of such changes must be
confirmed by geological evidence, and evidence from groups other
than fishes, and that likeness in separated fish faunas may not be
conclusive.

[Illustration: FIG. 184.--Drawing the net on the beach of Hilo, Hawaii.
Photograph by Henry W. Henshaw.]

FOOTNOTES:

[37] _Rheopresbe._

[38] _Bryttosus._

[39] _Leuciscus hakuensis_ Guenther.

[40] _Leuciscus jouyi._

[41] Formerly, but no longer, called Yeso in Japan.

[42] Called Nippon on foreign maps, but not so in Japan, where Nippon
means the whole empire.

[43] _Pleuronichthys cornutus_, _Hexogrammos otakii_, etc.

[44] As _Halichoeres_, _Tetrapturus_, _Callionymus_, _Ariscopus_, etc.

[45] Of these, the principal ones are _Oxystomus_, _Myrus_, _Pagrus_,
_Sparus_, _Macrorhamphosus_, _Cepola_, _Callionymus_, _Zeus_,
_Uranoscopus_, _Lepidotrigla_, _Chelidonichthys_.

[46] Among these are _Beryx_, _Helicolenus_, _Lotella_, _Nettastoma_,
_Centrolophus_, _Hoplostethus_, _Aulopus_, _Chlorophthalmus_,
_Lophotes_.

[47] _Beryx_, _Hoplostethus_.

[48] _Antigonia_, _Etelis_, _Emmelichthys_.

[49] _Lepadogaster_, _Myrus_; _Lophotes_, thus far recorded from
Japan, the Mediterranean, and the Cape of Good Hope, is bassalian and
of unknown range. _Beryx_, _Trachichthys_, _Hoplostethus_, etc., are
virtually cosmopolitan as well as semi-bassalian.

[50] In this group we must place _Cepola_, _Callionymus_, _Pagrus_,
_Sparus_, _Beryx_, _Zeus_, all of which have a very wide range in
Indian waters.

[51] _Cryptocentrus_, _Asterropteryx_. The range of neither of these
genera of small shore fishes is yet well known.

[52] As _Crenilabrus_, _Labrus_, _Symphodus_, _Pagellus_,
_Spondyliosoma_, _Sparisoma_.

[53] As _Chaetodon_, _Lethrinus_, _Monotaxis_, _Glyphisodon_, etc.

[54] _Hoplopagrus_, _Xenichthys_, _Xenistius_, _Xenocys_,
_Microdesmus_, _Cerdale_, _Cratinus_, _Azevia_, _Microlepidotus_,
_Orthostoechus_, _Isaciella_, etc.

[55] _Haemulon_, _Anisotremus_, _Gerres_, _Centropomus_, _Galeichthys_,
_Hypoplectrus_, _Mycteroperca_, _Ulaema_, _Stellifer_, _Micropogon_,
_Bodianus_, _Microspathodon_.

[56] Among these are perhaps _Teuthis_ (_Acanthurus_), _Ilisha_,
_Salarias_, _Myripristis_, _Thalassoma_. Some such which have not
crossed the Isthmus are _Cirrhitus_, _Sectator_, _Sebastopsis_, and
_Lophiomus_.

[57] "Fishes of Central America," 1869, 397.

[58] _Proc. U. S. Nat. Mus._, 1885, 393.

[59] Introduction to the "Study of Fishes," 1880, p. 280.

[60] _Proc. U. S. Nat. Mus._, 1891, pp. 124-126.

[61] _Citharichthys spilopterus_ and _C. gilberti_.

[62] _Haemulon steindachneri_ and _Gymnothorax castaneus_ of the west
coast probably being identical with _H. schranki_ and _Gymnothorax
funebris_ of the east coast.




CHAPTER XVI

DISPERSION OF FRESH-WATER FISHES[63]


=Dispersion of Fishes.=--The methods of dispersion of fishes may be
considered apart from the broader topic of distribution or the final
results of such dispersion. In this discussion we are mainly concerned
with the fresh-water fishes, as the methods of distribution of marine
fishes through marine currents and by continuity of shore and water
ways are all relatively simple.

=The Problem of Oatka Creek.=--When I was a boy and went fishing in
the brooks of western New York, I noticed that the different streams
did not always have the same kinds of fishes in them. Two streams in
particular in Wyoming County, not far from my father's farm, engaged in
this respect my special attention. Their sources are not far apart, and
they flow in opposite directions, on opposite sides of a low ridge--an
old glacial moraine, something more than a mile across. The Oatka Creek
flows northward from this ridge, while the East Coy runs toward the
southeast on the other side of it, both flowing ultimately into the
same river, the Genesee.

It does not require a very careful observer to see that in these two
streams the fishes are not quite the same. The streams themselves
are similar enough. In each the waters are clear and fed by springs.
Each flows over gravel and clay, through alluvial meadows, in many
windings, and with elms and alders "in all its elbows." In both streams
we were sure of finding trout,[64] and in one of them the trout are
still abundant. In both we used to catch the brook chub,[65] or, as
we called it, the "horned dace"; and in both were large schools of
shiners[66] and of suckers.[67] But in every deep hole, and especially
in the millponds along the East Coy Creek, the horned pout[68] swarmed
on the mucky bottoms. In every eddy, or in the deep hole worn out at
the root of the elm-trees, could be seen the sunfish,[69] strutting
in green and scarlet, with spread fins keeping intruders away from
its nest. But in the Oatka Creek were found neither horned pout nor
sunfish, nor have I ever heard that either has been taken there. Then
besides these nobler fishes, worthy of a place on every schoolboy's
string, we knew by sight, if not by name, numerous smaller fishes,
darters[70] and minnows,[71] which crept about in the gravel on the
bottom of the East Coy, but which we never recognized in the Oatka.

There must be a reason for differences like these, in the streams
themselves or in the nature of the fishes. The sunfish and the horned
pout are home-loving fishes to a greater extent than the others which
I have mentioned; still, where no obstacles prevent, they are sure to
move about. There must be, then, in the Oatka some sort of barrier,
or strainer, which keeping these species back permits others more
adventurous to pass; and a wider knowledge of the geography of the
region showed that such is the case. Farther down in its course, the
Oatka falls over a ledge of rock, forming a considerable waterfall at
Rock Glen. Still lower down its waters disappear in the ground, sinking
into some limestone cavern or gravel-bed, from which they reappear,
after some six miles, in the large springs at Caledonia. Either of
these barriers might well discourage a quiet-loving fish; while the
trout and its active associates have some time passed them, else we
should not find them in the upper waters in which they alone form the
fish fauna. This problem is a simple one; a boy could work it out, and
the obvious solution seems to be satisfactory.

=Generalizations as to Dispersion.=--Since those days I have been a
fisherman in many waters,--not an angler exactly, but one who fishes
for fish, and to whose net nothing large or small ever comes amiss; and
wherever I go I find cases like this.

We do not know all the fishes of America yet, nor all those well
that we know by sight; still this knowledge will come with time and
patience, and to procure it is a comparatively easy task. It is also
easy to ascertain the more common inhabitants of any given stream. It
is difficult, however, to obtain negative results which are really
results. You cannot often say that a species does not live in a
certain stream. You can only affirm that you have not yet found it
there, and you can rarely fish in any stream so long that you can find
nothing that you have not taken before. Still more difficult is it to
gather the results of scattered observations into general statements
regarding the distribution of fishes. The facts may be so few as to be
misleading, or so numerous as to be confusing, and the few writers who
have taken up this subject in detail have found both these difficulties
to be serious. Whatever general propositions we may maintain must be
stated with the modifying clause of "other things being equal"; and
other things are never quite equal. The saying that "Nature abhors a
generalization" is especially applicable to all discussions of the
relations of species to environment.

Still less satisfactory is our attempt to investigate the causes on
which our partial generalizations depend,--to attempt to break to
pieces the "other things being equal" which baffle us in our search
for general laws. The same problems, of course, come up on each of
the other continents and in all groups of animals or plants; but most
that I shall say will be confined to the question of the dispersion
of fishes in the fresh waters of North America. The broader questions
of the boundaries of faunae and of faunal areas I shall bring up only
incidentally.

=Questions Raised by Agassiz.=--Some of the problems to be solved were
first noticed by Prof. Agassiz in 1850, in his work on Lake Superior.
Later (1854), in a paper on the fishes of the Tennessee River,[72] he
makes the following statement:

"The study of these features [of distribution] is of the greatest
importance, inasmuch as it may eventually lead to a better
understanding of the intentions implied in this seemingly arbitrary
disposition of animal life....

"There is still another very interesting problem respecting the
geographical distribution of our fresh-water animals which may be
solved by the further investigation of the fishes of the Tennessee
River. The water-course, taking the Powell, Clinch, and Holston Rivers
as its head waters, arises from the mountains of Virginia in latitude
37 deg.; it then flows S.W. to latitude 34 deg. 25', when it turns W. and N.W.,
and finally empties into the Ohio, under the same latitude as its
source in 37 deg..

[Illustration: FIG. 185.--Horned Dace, _Semotilus atromaculatus_
(Mitchill). Aux Plaines River, Ills. Family _Cyprinidae_.]

"The question now is this: Are the fishes of this water system the
same throughout its extent? In which case we should infer that water
communication is the chief condition of geographical distribution of
our fresh-water fishes. Or do they differ in different stations along
its course? And if so, are the differences mainly controlled by the
elevation of the river above the level of the sea, or determined by
climatic differences corresponding to differences of latitude? We
should assume that the first alternative was true if the fishes of the
upper course of the river differed from those of the middle and lower
courses in the same manner as in the Danube, from its source to Pesth,
where this stream flows nearly for its whole length under the same
parallel. We would, on the contrary, suppose the second alternative
to be well founded if marked differences were observed between the
fish of such tracts of the river as do not materially differ in their
evolution above the sea, but flow under different latitudes. Now, a few
collections from different stations along this river, like that sent
me by Dr. Newman from the vicinity of Huntsville, would settle at once
this question, not for the Tennessee River alone, but for most rivers
flowing under similar circumstances upon the surface of the globe.
Nothing, however, short of such collections, compared closely with one
another, will furnish a reliable answer.... Whoever will accomplish
this survey will have made a highly valuable contribution to our
knowledge."

=Conclusions of Cope.=--Certain conclusions were also suggested by
Prof. Cope in his excellent memoir on the fishes of the Alleghany
region[73] in 1868. From this paper I make the following quotations:

"The distribution of fresh-water fishes is of special importance to the
questions of the origin and existence of species in connection with the
physical conditions of the waters and of the land. This is, of course,
owing to the restricted nature of their habitat and the impossibility
of their making extended migrations. With the submergence of land
beneath the sea, fresh-water fish are destroyed in proportion to the
extent of the invasion of salt water, while terrestrial vertebrates can
retreat before it. Hence every inland fish fauna dates from the last
total submergence of the country.

"Prior to the elevation of a given mountain chain, the courses of the
rivers may generally have been entirely different from their later
ones. Subsequent to this period, they can only have undergone partial
modifications. As subsequent submergences can rarely have extended to
the highlands where such streams originate, the fishes of such rivers
can only have been destroyed so far as they were unable to reach those
elevated regions, and preserve themselves from destruction from salt
water by sheltering themselves in mountain streams. On the other hand,
a period of greater elevation of the land, and of consequent greater
cold, would congeal the waters and cover their courses with glaciers.
The fishes would be driven to the neighborhood of the coast, though no
doubt in more southern latitudes a sufficient extent of uncongealed
fresh waters would flow by a short course into the ocean, to preserve
from destruction many forms of fresh-water fishes. Thus, through many
vicissitudes, the fauna of a given system of rivers has had opportunity
of uninterrupted descent, from the time of the elevation of the
mountain range, in which it has its sources....

"As regards the distinction of species in the disconnected basins of
different rivers, which have been separated from an early geologic
period, if species occur which are common to any two or more of them,
the supporter of the theory of distinct creations must suppose that
such species have been twice created, once for each hydrographic basin,
or that waters flowing into the one basin have been transferred to
another. The developmentalist, on the other hand, will accept the last
proposition, or else suppose that time has seen an identical process
and similar result of modification in these distinct regions.

[Illustration: FIG. 186.--Chub of the Great Basin, _Leuciscus lineatus_
(Girard). Heart Lake, Yellowstone Park. Family _Cyprinidae_.]

"Facts of distribution in the eastern district of North America are
these. Several species of fresh-water fishes occur at the same time
in many Atlantic basins from the Merrimac or from the Hudson to the
James, and throughout the Mississippi Valley, and in the tributaries of
the Great Lakes. On the other hand, the species of each river may be
regarded as pertaining to four classes, whose distribution has direct
reference to the character of the water and the food it offers: first,
those of the tide-waters, of the river channels, bayous, and sluggish
waters near them, or in the flat lands near the coast; second, those of
the river channels of its upper course, where the currents are more
distinct; third, those of the creeks of the hill country; fourth, those
of the elevated mountain streams which are subject to falls and rapids."

In the same paper Prof. Cope reaches two important general conclusions,
thus stated by him:

"I. That species not generally distributed exist in waters on different
sides of the great watershed.

"II. That the distribution of the species is not governed by the outlet
of the rivers, streams having similar discharges (Holston and Kanawha,
Roanoke and Susquehanna) having less in common than others having
different outlets (Kanawha, or Susquehanna and James).

[Illustration: FIG. 187.--Butterfly-sculpin, _Melletes papilio_ Bean, a
fish of the rock-pools. St. Paul, Pribilof Islands.]

"In view of the first proposition, and the question of the origin of
species, the possibility of an original or subsequent mingling of the
fresh waters suggests itself as more probable than that of distinct
origin in the different basins."

=Questions Raised by Cope.=--Two questions in this connection are
raised by Prof. Cope. The first question is this: "Has any destruction
of the river faunae taken place since the first elevation of the
Alleghanies, when the same species were thrown into waters flowing in
opposite directions?" Of such destruction by submergence or otherwise,
Prof. Cope finds no evidence. The second question is, "Has any means
of communication existed, at any time, but especially since the last
submergence, by which the transfer of species might occur?" Some
evidence of such transfer exists in the wide distribution of certain
species, especially those which seek the highest streamlets in the
mountains; but except to call attention to the cavernous character
of the Subcarboniferous and Devonian limestones, Prof. Cope has made
little attempt to account for it.

Prof. Cope finally concludes with this important generalization:

"It would appear, from the previous considerations, that the
distribution of fresh-water fishes is governed by laws similar to those
controlling terrestrial vertebrates and other animals, in spite of the
seemingly confined nature of their habitat."

=Views of Guenther.=--Dr. Guenther[74] has well summarized some of the
known facts in regard to the manner of dispersion of fishes:

"The ways in which the dispersal of fresh-water fishes has been
affected were various. They are probably all still in operation,
but most work so slowly and imperceptibly as to escape direct
observation; perhaps they will be more conspicuous after science and
scientific inquiry shall have reached a somewhat greater age. From the
great number of fresh-water forms which we see at this present day
acclimatized in, gradually acclimatizing themselves in, or periodically
or sporadically migrating into, the sea, we must conclude that under
certain circumstances salt water may cease to be a barrier at some
period of the existence of fresh-water species, and that many of them
have passed from one river through salt water into another. Secondly,
the headwaters of some of the grandest rivers, the mouths of which are
at opposite ends of the continents which they drain, are sometimes
distant from each other a few miles only. The intervening space may
have been easily bridged over for the passage of fishes by a slight
geological change affecting the level of the watershed or even by
temporary floods; and a communication of this kind, if existing for a
limited period only, would afford the ready means of an exchange of
a number of species previously peculiar to one or the other of these
river or lake systems. Some fishes provided with gill-openings so
narrow that the water moistening the gills cannot readily evaporate,
and endowed, besides, with an extraordinary degree of vitality, like
many Siluroids (_Chlarias_, _Callichthys_), eels, etc., are enabled to
wander for some distance over land, and may thus reach a water-course
leading them thousands of miles from their original home. Finally,
fishes or their ova may be accidentally carried by water-spouts, by
aquatic birds or insects, to considerable distances."

=Fresh-water Fishes of North America.=--We now recognize about six
hundred species[75] of fishes as found in the fresh waters of North
America, north of the Tropic of Cancer, these representing thirty-four
of the natural families. As to their habits, we can divide these
species rather roughly into the four categories proposed by Prof. Cope,
or, as we may call them,

(1) Lowland fishes; as the bowfin,[76] pirate-perch,[77] large-mouthed
black bass,[78] sunfishes, and some catfishes.

(2) Channel-fishes; as the channel catfish,[79] the mooneye,[80]
garpike,[81] buffalo-fishes,[82] and drum.[83]

(3) Upland fishes; as many of the darters, shiners, and suckers, and
the small-mouthed black bass.[84]

(4) Mountain-fishes; as the brook trout and many of the darters and
minnows.

To these we may add the more or less distinct classes of (5) lake
fishes, inhabiting only waters which are deep, clear, and cold, as
the various species of whitefish[85] and the Great Lake trout;[86]
(6) anadromous fishes, or those which run up from the sea to spawn in
fresh waters, as the salmon,[87] sturgeon,[88] shad,[89] and striped
bass;[90] (7) catadromous fishes, like the eel,[91] which pass down to
spawn in the sea; and (8) brackish-water fishes, which thrive best in
the debatable waters of the river-mouths, as most of the sticklebacks
and the killifishes.

As regards the range of species, we have every possible gradation
from those which seem to be confined to a single river, and are rare
even in their restricted habitat, to those which are in a measure
cosmopolitan,[92] ranging everywhere in suitable waters.

=Characters of Species.=--Still, again, we have all degrees of
constancy and inconstancy in what we regard as the characters of a
species. Those found only in a single river-basin are usually uniform
enough; but the species having a wide range usually vary much in
different localities. Such variations have at different times been
taken to be the indications of as many different species. Continued
explorations bring to light, from year to year, new species; but the
number of new forms now discovered each year is usually less than the
number of recognized species which are yearly proved to be untenable.
Four complete lists of the fresh-water fishes of the United States
(north of the Mexican boundary) have been published by the present
writer. That of Jordan and Copeland,[93] published in 1876, enumerates
670 species. That of Jordan[94] in 1878 contains 665 species, and that
of Jordan and Gilbert[95] in 1883, 587 species. That of Jordan and
Evermann[96] in 1898 contains 585 species, although upwards of 130 new
species were detected in the twenty-two years which elapsed between
the first and the last list. Additional specimens from intervening
localities are often found to form connecting links among the nominal
species, and thus several supposed species become in time merged
in one. Thus the common channel catfish[97] of our rivers has been
described as a new species not less than twenty-five times, on account
of differences real or imaginary, but comparatively trifling in value.

Where species can readily migrate, their uniformity is preserved;
but whenever a form becomes localized its representatives assume some
characters not shared by the species as a whole. When we can trace,
as we often can, the disappearance by degrees of these characters,
such forms no longer represent to us distinct species. In cases where
the connecting forms are extinct, or at least not represented in
collections, each form which is apparently different must be regarded
as a distinct species.

The variations in any type become, in general, more marked as we
approach the tropics. The genera are represented, on the whole, by
more species there, and it would appear that the processes of specific
change go on more rapidly under the easier conditions of life in the
Torrid Zone.

We recognize now in North America twenty-five distinct species of
fresh-water catfishes,[98] although nearly a hundred (93) nominal
species of these fishes have been from time to time described. But
these twenty-five species are among themselves very closely related,
and all of them are subject to a variety of minor changes. It
requires no strong effort of the imagination to see in them all the
modified descendants of some one species of catfish, not unlike our
common "bullhead,"[99] an immigrant probably from Asia, and which
has now adjusted itself to its surroundings in each of our myriad of
catfish-breeding streams.

=Meaning of Species.=--The word "species," then, is simply a term of
convenience, including such members of a group similar to each other as
are tangibly different from others, and are not known to be connected
with these by intermediate forms. Such connecting links we may suppose
to have existed in all cases. We are only sure that they do not now
exist in our collections, so far as these have been carefully studied.

When two or more species of any genus now inhabit the same
waters, they are usually species whose differentiation is of long
standing,--species, therefore, which can be readily distinguished
from one another. When, on the other hand, we have "representative
species,"--closely related forms, neither of which is found within the
geographical range of the other,--we can with some confidence look
for intermediate forms where the territory occupied by the one bounds
that inhabited by the other. In very many such cases the intermediate
forms have been found; and such forms are considered as subspecies of
one species, the one being regarded as the parent stock, the other
as an offshoot due to the influences of different environment. Then,
besides these "species" and "subspecies," groups more or less readily
recognizable, there are varieties and variations of every grade, often
too ill-defined to receive any sort of name, but still not without
significance to the student of the origin of species. Comparing a
dozen fresh specimens of almost any kind of fish from any body of
water with an equal number from somewhere else, one will rarely fail
to find some sort of differences,--in size, in form, in color. These
differences are obviously the reflex of differences in the environment,
and the collector of fishes seldom fails to recognize them as such;
often it is not difficult to refer the effect to the conditions. Thus
fishes from grassy bottoms are darker than those taken from over
sand, and those from a bottom of muck are darker still, the shade of
color being, in some way not well understood, dependent on the color
of the surroundings. Fishes in large bodies of water reach a larger
size than the same species in smaller streams or ponds. Fishes from
foul or sediment-laden waters are paler in color and slenderer in form
than those from waters which are clear and pure. Again, it is often
true that specimens from northern waters are less slender in body
than those from farther south; and so on. Other things being equal,
the more remote the localities from each other, the greater are these
differences.

[Illustration: FIG. 188.--_Scartichthys enosimae_ Jordan and Snyder, a
fish of the rock-pools of the sacred island of Enoshima, Japan. Family
_Blenniidae_.]

In our fresh-water fishes each species on an average has been described
as new from three to four times, on account of minor variations, real
or supposed. In Europe, where the fishes have been studied longer and
by more different men, upwards of six or eight nominal species have
been described for each one that is now considered distinct.

=Special Creation Impossible.=--It is evident, from these and other
facts, that the idea of a separate creation for each species of fishes
in each river-basin, as entertained by Agassiz, is wholly incompatible
with our present knowledge of the specific distinctions or of the
geographical distribution of fishes. This is an unbroken gradation in
the variations from the least to the greatest,--from the peculiarities
of the individual, through local varieties, geographical subspecies,
species, sub-genera, genera, families, super-families, and so on, until
all fish-like vertebrates are included in a single bond of union.

=Origin of American Species of Fishes.=--It is, however, evident that
not all American types of fishes had their origin in America, or even
first assumed in America their present forms. Some of these are perhaps
immigrants from northern Asia, where they still have their nearest
relatives. Still others are evidently modified importations from the
sea; and of these some are very recent immigrants, land-locked species
which have changed very little from the parent stock.

The problems of analogous variation or parallelism without homology
are very often met with among fishes. In shallow, swift brooks in all
lands there are found small fishes which hug the bottom--large-finned,
swift of movement, with speckled coloration, and with the air-bladder
reduced in size. In the eastern United States these fishes are darters,
dwarf perches; in northern India they are catfishes; in Japan, gobies
or loaches; in Canada, sculpins; in South America, characins. Members
of various groups may be modified to meet the same conditions of
life. Being modified to look alike, the thought of mutual affinity
is naturally suggested, but in such cases the likeness is chiefly
external. The internal organs show little trace of such modifications.
The inside of an animal tells what it really is, the outside where
it has been. In other words, it is the external characters which are
most readily affected by the environment. Throughout all groups of
animals and plants, there are large branches similarly affected by
peculiarities of conditions.

This is the basis of the law of "Adaptive Radiation." Prof. H. F.
Osborn thus states this law:

"It is a well-known principle of zoological evolution that an isolated
region, if large and sufficiently varied in its topography, soil,
climate, and vegetation, will give rise to a diversified fauna
according to the law of adaptive radiation from primitive and central
types. Branches will spring off in all directions to take advantage of
every possible opportunity of securing food. The modifications which
animals undergo in this adaptive radiation are largely of mechanical
nature; they are limited in number and kind by hereditary stirp or
germinal influences, and thus result in the independent evolution of
similar types in widely separated regions under the law of parallelism
or homoplasy."

FOOTNOTES:

[63] This chapter and the next are in substance reprinted from an essay
published by the present writer in a volume called Science Sketches. A.
C. McClurg & Co., Chicago.

[64] _Salvelinus fontinalis_ Mitchill.

[65] _Semotilus atromaculatus_ Mitchill.

[66] _Notropis cornutus_ Rafinesque.

[67] _Catostomus commersoni_ (Lacepede).

[68] _Ameiurus melas_ Rafinesque.

[69] _Eupomotis gibbosus_ Linnaeus.

[70] _Etheostoma flabellare_ Rafinesque.

[71] _Rhinichthys atronasus_ Mitchill.

[72] On Fishes from Tennessee River, Alabama. American Journal of
Science and Arts, xvii., 2d series, 1854, p. 26.

[73] On the Distribution of Fresh-water Fishes in the Alleghany Region
of Southwestern Virginia. Journ. Acad. Nat. Sci., Phila., 1868, pp.
207-247.

[74] Introduction to the Study of Fishes, 1880, p. 211.

[75] The table below shows approximately the composition of the
fresh-water fish fauna of Europe, as compared with that of North
America north of the Tropic of Cancer.

  Families.                                    Europe.    N. America.

  Lamprey               _Petromyzonidae_       3 species.   8 species.
  Paddle-fish           _Polyodontidae_       --    "       1    "
  Sturgeon              _Acipenseridae_       10    "       6    "
  Garpike               _Lepisosteidae_       --    "       3    "
  Bowfin                _Amiidae_             --    "       1    "
  Mooneye               _Hiodontidae_         --    "       3    "
  Herring               _Clupeidae_            2    "       5    "
  Gizzard-shad          _Dorosomidae_         --    "       1    "
  Salmon                _Salmonidae_          12    "      28    "
  Characin              _Characinidae_        --    "       1    "
  Carp                  _Cyprinidae_          61    "     230    "
  Loach                 _Cobiridae_            3    "      --    "
  Sucker                _Catostomidae_        --    "      51    "
  Catfish               _Siluridae_            1    "      25    "
  Trout-perch           _Percopsidae_         --    "       2    "
  Blindfish             _Amblyopsidae_        --    "       6    "
  Killifish             _Cyprinodontidae_      3    "      52    "
  Mud-minnow            _Umbridae_             1    "       2    "
  Pike                  _Esocidae_             1    "       5    "
  Alaska blackfish      _Dalliidae_           --    "       1    "
  Eel                   _Anguillidae_          2    "       1    "
  Stickleback           _Gasterosteidae_       3    "       7    "
  Silverside            _Atherinidae_          2    "       2    "
  Pirate perch          _Aphredoderidae_      --    "       1    "
  Elassoma              _Elassomidae_         --    "       2    "
  Sunfish               _Centrarchidae_       --    "      37    "
  Perch                 _Percidae_            11    "      72    "
  Bass                  _Serranidae_           1    "       4    "
  Drum                  _Sciaenidae_           --    "       1    "
  Surf-fish             _Embiotocidae_        --    "       1    "
  Cichlid               _Cichlidae_           --    "       2    "
  Goby                  _Gobiidae_             2    "       6    "
  Sculpin               _Cottidae_             2    "      21    "
  Blenny                _Blenniidae_           3    "      --    "
  Cod                   _Gadidae_              1    "       1    "
  Flounder              _Pleuronectidae_       1    "      --    "
  Sole                  _Soleidae_             1    "       1    "

Total: Europe, 21 families; 126 species. North America, 34 families;
590 species. A few new species have been added since this enumeration
was made.

According to Dr. Guenther (Guide to the Study of Fishes, p. 243), the
total number of species now known from the temperate regions of Asia
and Europe is about 360. The fauna of India, south of the Himalayas,
is much more extensive, numbering 625 species. This latter fauna bears
little resemblance to that of North America, being wholly tropical in
its character.

[76] _Amia calva_ Linnaeus.

[77] _Aphredoderus sayanus_ Gilliams.

[78] _Micropterus salmoides_ Lacepede.

[79] _Ictalurus punctatus_ Rafinesque.

[80] _Hiodon tergisus_ Le Sueur.

[81] _Lepisosteus osseus_ Linnaeus.

[82] _Ictiobus bubalus_, _cyprinella_, etc.

[83] _Aplodinotus grunniens_ Rafinesque.

[84] _Micropterus dolomieu_ Lacepede.

[85] _Coregonus clupeiformis_, _Argyrosomus artedi_, etc.

[86] _Cristivomer namaycush_ Walbaum.

[87] _Salmo salar_ Linnaeus.

[88] _Acipenser sturio_ and other species.

[89] _Alosa sapidissima_ Wilson.

[90] _Roccus lineatus_ Bloch.

[91] _Anguilla chrysypa_ Raf.

[92] Thus the chub-sucker (_Erimyzon sucetta_) in some of its varieties
ranges everywhere from Maine to Dakota, Florida, and Texas; while a
number of other species are scarcely less widely distributed.

[93] Check List of the Fishes of the Fresh Waters of North America,
by David S. Jordan and Herbert E. Copeland. Bulletin of the Buffalo
Society of Natural History, 1876, pp. 133-164.

[94] A Catalogue of the Fishes of the Fresh Waters of North America.
Bulletin of the United States Geological Survey, 1878, pp. 407-442.

[95] A Catalogue of the Fishes Known to Inhabit the Waters of
North America North of the Tropic of Cancer. Annual Report of the
Commissioner of Fish and Fisheries for 1884 and 1885.

[96] Check List of the Fishes of North and Middle America. Report of
the U. S. Commissioner of Fisheries for 1895.

[97] _Ictalurus punctatus_ Rafinesque.

[98] _Siluridae._

[99] _Ameiurus nebulosus._




CHAPTER XVII

BARRIERS TO DISPERSION OF RIVER FISHES


=The Process of Natural Selection.=--We can say, in general, that in
all waters not absolutely uninhabitable there are fishes. The processes
of natural selection have given to each kind of river or lake species
of fishes adapted to the conditions of life which obtain there. There
is no condition of water, of bottom, of depth, of speed of current, but
finds some species with characters adjusted to it. These adjustments
are, for the most part, of long standing; and the fauna of any single
stream has as a rule been produced by immigration from other regions
or from other streams. Each species has an ascertainable range of
distribution, and within this range we may be reasonably certain to
find it in any suitable waters.

[Illustration: FIG. 189.--Slippery-dick or Doncella, _Halichoeres
bivittatus_ Bloch, a fish of the coral reefs, Key West. Family
_Labridae_.]

But every species has beyond question some sort of limit to its
distribution, some sort of barrier which it has never passed in all
the years of its existence. That this is true becomes evident when we
compare the fish fauna of widely separated rivers. Thus the Sacramento,
Connecticut, Rio Grande, and St. John's Rivers have not a single
species in common; and with one or two exceptions, not a species is
common to any two of them. None of these[100] has any species peculiar
to itself, and each shares a large part of its fish fauna with the
water-basin next to it. It is probably true that the faunas of no two
distinct hydrographic basins are wholly identical, while on the other
hand there are very few species confined to a single one. The supposed
cases of this character, some twenty in number, occur chiefly in the
streams of the South Atlantic States and of Arizona. All of these need,
however, the confirmation of further exploration. It is certain that in
no case has an entire river fauna[101] originated independently from
the divergence into separate species of the descendants of a single
type.

The existence of boundaries to the range of species implies, therefore,
the existence of barriers to their diffusion. We may now consider these
barriers and in the same connection the degree to which they may be
overcome.

=Local Barriers.=--Least important to these are the barriers which may
exist within the limits of any single basin, and which tend to prevent
a free diffusion through its waters of species inhabiting any portion
of it. In streams flowing southward, or across different parallels of
latitude, the difference in climate becomes a matter of importance. The
distribution of species is governed very largely by the temperature
of the water. Each species has its range in this respect,--the
free-swimming fishes, notably the trout, being most affected by it;
the mud-loving or bottom fishes, like the catfishes, least. The
latter can reach the cool bottoms in hot weather, or the warm bottoms
in cold weather, thus keeping their own temperature more even than
that of the surface of the water. Although water communication is
perfectly free for most of the length of the Mississippi, there is a
material difference between the faunae of the stream in Minnesota and
in Louisiana. This difference is caused chiefly by the difference
in temperature occupying the difference in latitude. That a similar
difference in longitude, with free water communication, has no
appreciable importance, is shown by the almost absolute identity of
the fish faunae of Lake Winnebago and Lake Champlain. While many large
fishes range freely up and down the Mississippi, a majority of the
species do not do so, and the fauna of the upper Mississippi has more
in common with that of the tributaries of Lake Michigan than it has
with that of the Red River or the Arkansas. The influence of climate
is again shown in the paucity of the fauna of the cold waters of Lake
Superior, as compared with that of Lake Michigan. The majority of
our species cannot endure the cold. In general, therefore, cold or
Northern waters contain fewer species than Southern waters do, though
the number of individuals of any one kind may be greater. This is shown
in all waters, fresh or salt. The fisheries of the Northern seas are
more extensive than those of the tropics. There are more fishes there,
but are far less varied in kind. The writer once caught seventy-five
species of fishes in a single haul of the seine at Key West, while on
Cape Cod he obtained with the same net but forty-five species in the
course of a week's work. Thus it comes that the angler, contented with
many fishes of few kinds, goes to Northern streams to fish, while the
naturalist goes to the South.

[Illustration: FIG. 190.--_Peristedion miniatum_ Goode and Bean, a
deep-red  fish of the depths of the Gulf Stream.]

But in most streams the difference in latitude is insignificant,
and the chief differences in temperature come from differences in
elevation, or from the distance of the waters from the colder source.
Often the lowland waters are so different in character as to produce
a marked change in the quality of their fauna. These lowland waters
may form a barrier to the free movements of upland fishes; but
that this barrier is not impassable is shown by the identity of the
fishes in the streams[102] of the uplands of middle Tennessee with
those of the Holston and French Broad. Again, streams of the Ozark
Mountains, similar in character to the rivers of East Tennessee, have
an essentially similar fish fauna, although between the Ozarks and the
Cumberland range lies an area of lowland bayous, into which such fishes
are never known to penetrate. We can, however, imagine that these
upland fishes may be sometimes swept down from one side or the other
into the Mississippi, from which they might ascend on the other side.
But such transfers certainly do not often happen. This is apparent
from the fact that the two faunas[103] are not quite identical, and in
some cases the same species are represented by perceptibly different
varieties on one side and the other. The time of the commingling of
these faunae is perhaps now past, and it may have occurred only when the
climate of the intervening regions was colder than at present.

The effect of waterfalls and cascades as a barrier to the diffusion
of most species is self-evident; but the importance of such obstacles
is less, in the course of time, than might be expected. In one way or
another very many species have passed these barriers. The falls of the
Cumberland limit the range of most of the larger fishes of the river,
but the streams above it have their quota of darters and minnows. It
is evident that the past history of the stream must enter as a factor
into this discussion, but this past history it is not always possible
to trace. Dams or artificial waterfalls now check the free movement of
many species, especially those of migratory habits; while conversely,
numerous other species have extended their range through the agency of
canals.[104]

Every year fishes are swept down the rivers by the winter's floods;
and in the spring, as the spawning season approaches, almost every
species is found working its way up the stream. In some cases, notably
the Quinnat salmon[105] and the blue-back salmon,[106] the length of
these migrations is surprisingly great. To some species rapids and
shallows have proved a sufficient barrier, and other kinds have been
kept back by unfavorable conditions of various sorts. Streams whose
waters are always charged with silt or sediment, as the Missouri,
Arkansas, or Brazos, do not invite fishes; and even the occasional
floods of red mud such as disfigure otherwise clear streams, like
the Red River or the Colorado (of Texas), are unfavorable. Extremely
unfavorable also is the condition which obtains in many rivers of the
Southwest, as, for example, the Red River, the Sabine, and the Trinity,
which are full from bank to bank in winter and spring, and which
dwindle to mere rivulets in the autumn droughts.

=Favorable Waters have Most Species.=--In general, those streams which
have conditions most favorable to fish life will be found to contain
the greatest number of species. Such streams invite immigration; and
in them the struggle for existence is individual against individual,
species against species, and not a mere struggle with hard conditions
of life. Some of the conditions most favorable to the existence in any
stream of a large number of species of fishes are the following, the
most important of which is the one mentioned first: Connection with
a large hydrographic basin; a warm climate; clear water; a moderate
current; a bottom of gravel (preferably covered by a growth of weeds);
little fluctuation during the year in the volume of the stream or in
the character of the water.

Limestone streams usually yield more species than streams flowing
over sandstone, and either more than the streams of regions having
metamorphic rocks. Sandy bottoms usually are not favorable to fishes.
In general, glacial drift makes a suitable river bottom, but the higher
temperature usual in regions beyond the limits of the drift gives
to certain Southern streams conditions still more favorable. These
conditions are all well realized in the Washita River in Arkansas, and
in various tributaries of the Tennessee, Cumberland, and Ohio; and in
these, among American streams, the greatest number of species has been
recorded.

The isolation and the low temperature of the rivers of New England have
given to them a very scanty fish fauna as compared with the rivers of
the South and West. This fact has been noticed by Professor Agassiz,
who has called New England a "zoological island."[107]

In spite of the fact that barriers of every sort are sometimes crossed
by fresh-water fishes, we must still regard the matter of freedom of
water communication as the essential one in determining the range of
most species. The larger the river basin, the greater the variety of
conditions likely to be offered in it, and the greater the number of
its species. In case of the divergence of new forms by the processes
called "natural selection," the greater the number of such forms which
may have spread through its waters; the more extended any river basin,
the greater are the chances that any given species may sometimes find
its way into it; hence the greater the number of species that actually
occur in it, and, freedom of movement being assumed, the greater the
number of species to be found in any one of its affluents.

Of the six hundred species of fishes found in the rivers of the United
States, about two hundred have been recorded from the basin of the
Mississippi. From fifty to one hundred of these species can be found in
any one of the tributary streams of the size, say, of the Housatonic
River or the Charles. In the Connecticut River there are but about
eighteen species permanently resident; and the number found in the
streams of Texas is not much larger, the best known of these, the Rio
Colorado, having yielded but twenty-four species.

The waters of the Great Basin are not rich in fishes, the

[Illustration: FIG. 191.--Ancient Outlet of Lake Bonneville, Great Salt
Lake, in Idaho. (Photograph by Prof. J. M. Aldrich.)] species now
found being evidently an overflow from the Snake River when in late
glacial times it drained Lake Bonneville. This postglacial lake once
filled the present basin of the Great Salt Lake and Utah Lake, its
outlet flowing northwest from Ogden into Snake River. The same fishes
are now found in the upper Snake River and the basins of Utah Lake and
of Sevier Lake. In the same fashion Lake Lahontan once occupied the
basin of Nevada, the Humboldt and Carson sinks, with Pyramid Lake.
Its drainage fell also into the Snake River, and its former limits
are shown in the present range of species. These have almost nothing
in common with the group of species inhabiting the former drainage of
Lake Bonneville. Another postglacial body of water, Lake Idaho, once
united the lakes of Southeastern Oregon. The fauna of Lake Idaho, and
of the lakes Malheur, Warner, Goose, etc., which have replaced it, is
also isolated and distinctive. The number of species now known from
this region of these ancient lobes is about 125. This list is composed
almost entirely of a few genera of suckers,[108] minnows,[109] and
trout.[110] None of the catfishes, perch, darters, or sunfishes,
moon-eyes, pike, killifishes, and none of the ordinary Eastern types of
minnows[111] have passed the barrier of the Rocky Mountains.

West of the Sierra Nevada the fauna is still more scanty, only about
seventy species being enumerated. This fauna, except for certain
immigrants[112] from the sea, is of the same general character as that
of the Great Basin, though most of the species are different. This
latter fact would indicate a considerable change, or "evolution,"
since the contents of the two faunae were last mingled. There is a
considerable difference between the fauna of the Columbia and that
of the Sacramento. The species which these two basins have in common
are chiefly those which at times pass out into the sea. The rivers of
Alaska contain but few species, barely a dozen in all, most of these
being found also in Siberia and Kamchatka. In the scantiness of its
faunal list, the Yukon agrees with the Mackenzie River, and with Arctic
rivers generally.

There can be no doubt that the general tendency is for each species to
extend its range more and more widely until all localities suitable
for its growth are included. The various agencies of dispersal which
have existed in the past are still in operation. There is apparently
no limit to their action. It is probable that new "colonies" of one
species or another may be planted each year in waters not heretofore
inhabited by such species. But such colonies become permanent only
where the conditions are so favorable that the species can hold
its own in the struggle for food and subsistence. That the various
modifications in the habitat of certain species have been caused by
human agencies is of course too well known to need discussion here.

=Watersheds.=--We may next consider the question of watersheds, or
barriers which separate one river basin from another.

Of such barriers in the United States, the most important and most
effective is unquestionably that of the main chain of the Rocky
Mountains. This is due in part to its great height, still more to its
great breadth, and most of all, perhaps, to the fact that it is nowhere
broken by the passage of a river. But two species--the red-throated or
Rocky Mountain trout[113] and the Rocky Mountain whitefish[114]--are
found on both sides of it, at least within the limits of the United
States; while many genera, and even several families, find in it either
an eastern or a western limit to their range. In a few instances
representative species, probably modifications or separated branches
of the same stock, occur on opposite sides of the range, but there
are not many cases of correspondence even thus close. The two faunas
are practically distinct. Even the widely distributed red-spotted or
"dolly varden" trout[115] of the Columbia River and its affluents does
not cross to the east side of the mountains, nor does the Montana
grayling[116] ever make its way to the West. In Northern Mexico,
however, numerous Eastern river fishes have crossed the main chain of
the Sierra Madre.

=How Fishes Cross Watersheds.=--It is easy to account for this
separation of the faunae; but how shall we explain the almost universal
diffusion of the whitefish and the trout in suitable waters on both
sides of the dividing ridge? We may notice that these two are the
species which ascend highest in the mountains, the whitefish inhabiting
the mountain pools and lakes, the trout ascending all brooks and
rapids in search of their fountainheads. In many cases the ultimate
dividing ridge is not very broad, and we may imagine that at some time
spawn or even young fishes may have been carried across by birds or
other animals, or by man, or more likely by the dash of some summer
whirlwind. Once carried across in favorable circumstances, the species
might survive and spread.

The following is an example of how such transfer of species may be
accomplished, which shows that we need not be left to draw on the
imagination to invent possible means of transit.

=The Suletind.=--There are few watersheds in the world better defined
than the mountain range which forms the "back-bone" of Norway. I
lately climbed a peak in this range, the Suletind. From its summit I
could look down into the valleys of the Laera and the Baegna, flowing in
opposite directions to opposite sides of the peninsula. To the north of
the Suletind is a large double lake called the Sletningenvand. The maps
show this lake to be one of the chief sources of the westward-flowing
river Laera. This lake is in August swollen by the melting of the
snows, and at the time of my visit it was visibly the source of both
these rivers. From its southeastern side flowed a large brook into
the valley of the Baegna, and from its southwestern corner, equally
distinctly, came the waters which fed the Laera. This lake, like
similar mountain ponds in all northern countries, abounds in trout;
and these trout certainly have for part of the year an uninterrupted
line of water communication from the Sognefjord on the west of Norway
to the Christianiafjord on the southeast,--from the North Sea to the
Baltic. Part of the year the lake has probably but a single outlet
through the Laera. A higher temperature would entirely cut off the
flow into the Baegna, and a still higher one might dry up the lake
altogether. This Sletningenvand, with its two outlets on the summit
of a sharp watershed, may serve to show us how other lakes, permanent
or temporary, may elsewhere have acted as agencies for the transfer of
fishes. We can also see how it might be that certain mountain fishes
should be so transferred while the fishes of the upland waters may
be left behind. In some such way as this we may imagine that various
species of fishes have attained their present wide range in the Rocky
Mountain region; and in similar manner perhaps the Eastern brook
trout[117] and some other mountain species[118] may have been carried
across the Alleghanies.

=The Cassiquiare.=--Professor John C. Branner calls my attention to
a marshy upland which separates the valley of the La Plata from that
of the Amazon, and which permits the free movement of fishes from
the Paraguay River to the Tapajos. It is well known that through the
Cassiquiare River the Rio <DW64>, another branch of the Amazon, is
joined to the Orinoco River. It is thus evident that almost all the
waters of eastern South America form a single basin, so far as the
fishes are concerned.

As to the method of transfer of the trout from the Columbia to the
Missouri, we are not now left in doubt.

=Two-Ocean Pass.=--To this day, as the present writer and later
Evermann and Jenkins[119] have shown, the Yellowstone and Snake Rivers
are connected by two streams crossing the main divide of the Rocky
Mountains from the Yellowstone to the Snake across Two-Ocean Pass.

Prof. Evermann has described the locality as follows:

"Two-Ocean Pass is a high mountain meadow, about 8,200 feet above
the sea and situated just south of the Yellowstone National Park, in
longitude 110 deg. 10' W., latitude 44 deg. 3' N. It is surrounded on all sides
by rather high mountains except where the narrow valleys of Atlantic
and Pacific creeks open out from it. Running back among the mountains
to the northward are two small canyons down which come two small
streams. On the opposite is another canyon down which comes another
small stream. The extreme length of the meadow from east to west is
about a mile, while the width from north to south is not much less. The
larger of the streams coming in from the north is Pacific Creek, which,
after winding along the western side of the meadow, turns abruptly
westward, leaving the meadow through a narrow gorge. Receiving numerous
small affluents, Pacific Creek soon becomes a good-sized stream, which
finally unites with Buffalo Creek a few miles above where the latter
stream flows into Snake River.

"Atlantic Creek was found to have two forks entering the pass. At the
north end of the meadow is a small wooded canyon down which flows the
North Fork. This stream hugs the border of the flat very closely. The
South Fork comes down the canyon on the south side, skirting the brow
of the hill a little less closely than does the North Fork. The two,
coming together near the middle of the eastern border of the meadow,
form Atlantic Creek, which after a course of a few miles flows into the
Upper Yellowstone. But the remarkable phenomena exhibited here remain
to be described.

"Each fork of Atlantic Creek, just after entering the meadow, divides
as if to flow around an island, but the stream toward the meadow,
instead of returning to the portion from which it had parted, continues
its westerly course across the meadow. Just before reaching the western
border the two streams unite and then pour their combined waters into
Pacific Creek; thus are Atlantic and Pacific creeks united and a
continuous waterway from the Columbia via Two-Ocean Pass to the Gulf of
Mexico is established.

"Pacific Creek is a stream of good size long before it enters the pass,
and its course through the meadow is in a definite channel, but not so
with Atlantic Creek. The west bank of each fork is low and the stream
is liable to break through anywhere and thus send part of its water
across to Pacific Creek. It is probably true that one or two branches
always connect the two creeks under ordinary conditions, and that
following heavy rains or when the snows are melting, a much greater
portion of the water of Atlantic Creek crosses the meadow to the other
side.

[Illustration: FIG. 192.--Silver Surf-fish (viviparous),
_Hypocritichthys analis_ (Agassiz). Monterey.]

"Besides the channels already mentioned, there are several more or
less distinct ones that were dry at the time of our visit. As already
stated, the pass is a nearly level meadow covered with a heavy growth
of grass and many small willows one to three feet high. While it is
somewhat marshy in places it has nothing of the nature of a lake about
it. Of course, during wet weather the small springs at the borders of
the meadow would be stronger, but the important facts are that there
is no lake or even marsh there and that neither Atlantic nor Pacific
Creek has its rise in the meadow. Atlantic Creek, in fact, comes into
the pass as two good-sized streams from opposite directions and leaves
it by at least four channels, thus making an island of a considerable
portion of the meadow. And it is certain that there is, under ordinary
circumstances, a continuous waterway through Two-Ocean Pass of such a
character as to permit fishes to pass easily and readily from Snake
River over to the Yellowstone, or in the opposite direction. Indeed,
it is quite possible, barring certain falls in the Snake River, for a
fish so inclined, to start at the mouth of the Columbia, travel up that
great river to its principal tributary, the Snake, thence on through
the long, tortuous course of that stream, and, under the shadows of the
Grand Teton, enter the cold waters of Pacific Creek, by which it could
journey on up to the very crest of the great continental divide,--to
Two-Ocean Pass; through this pass it may have a choice of two routes
to Atlantic Creek, in which the down-stream journey is begun. Soon it
reaches the Yellowstone, down which it continues to Yellowstone Lake,
then through the lower Yellowstone out into the turbid waters of the
Missouri; for many hundred miles it may continue down this mighty river
before reaching the Father of Waters, which will finally carry it to
the Gulf of Mexico--a wonderful journey of nearly 6,000 miles, by far
the longest possible fresh-water journey in the world.

"We found trout in Pacific Creek at every point where we examined it.
In Two-Ocean Pass we found trout in each of the streams and in such
positions as would have permitted them to pass easily from one side of
the divide to the other. We also found trout in Atlantic Creek below
the pass, and in the upper Yellowstone they were abundant. Thus it is
certain that there is no obstruction, even in dry weather, to prevent
the passage of trout from the Snake River to Yellowstone Lake; it is
quite evident that trout do pass over in this way; and it is almost
certain that Yellowstone Lake was stocked with trout from the west via
Two-Ocean Pass."--EVERMANN.

=Mountain Chains.=--The Sierra Nevada constitutes also a very important
barrier to the diffusion of species. This is, however, broken by the
passage of the Columbia River, and many species thus find their way
across it. That the waters to the west of it are not unfavorable for
the growth of Eastern fishes is shown by the fact of the rapid spread
of the common Eastern catfish,[120] or horned pout, when transported
from the Schuylkill to the Sacramento. The catfish is now one of the
important food fishes of the San Francisco markets, and with the
Chinaman its patron, it has gone from California to Hawaii. The Chinese
catfish, described by Bleeker as _Ameiurus cantonensis_, was doubtless
carried home by some Chinaman returning from San Francisco. In like
fashion the small-mouthed black bass is now frequent in California
streams, as is also the blue-green sunfish, _Apomotis cyanellus_,
introduced as food for the bass.

The mountain mass of Mount Shasta is, as already stated, a considerable
barrier to the range of fishes, though a number of species find their
way around it through the sea. The lower and irregular ridges of the
Coast Range are of small importance in this regard, as the streams of
their east <DW72> reach the sea on the west through San Francisco Bay.
Yet the San Joaquin contains a few species not yet recorded from the
smaller rivers of southwestern California.

The main chain of the Alleghanies forms a barrier of importance
separating the rich fish fauna of the Tennessee and Ohio basins from
the scantier faunae of the Atlantic streams. Yet this barrier is crossed
by many more species than is the case with either the Rocky Mountains
or the Sierra Nevada. It is lower, narrower, and much more broken,--as
in New York, in Pennsylvania, and in Georgia there are several streams
which pass through it or around it. The much greater age of the
Alleghany chain, as compared with the Rocky Mountains, seems not to
be an element of any importance in this connection. Of the fish which
cross this chain, the most prominent is the brook trout,[121] which
is found in all suitable waters from Hudson's Bay to the head of the
Chattahoochee.

=Upland Fishes.=--A few other species are locally found in the head
waters of certain streams on opposite sides of the range. An example
of this is the little red "fallfish,"[122] found only in the mountain
tributaries of the Savannah and the Tennessee. We may suppose the same
agencies to have assisted these species that we have imagined in the
case of the Rocky Mountain trout, and such agencies were doubtless more
operative in the times immediately following the glacial epoch than
they are now. Prof. Cope calls attention also to the numerous caverns
existing in these mountains as a sufficient medium for the transfer of
many species. I doubt whether the main chains of the Blue Ridge or the
Great Smoky can be crossed in that way, though such channels are not
rare in the subcarboniferous limestones of the Cumberland range. In the
brooks at the head waters of the Roanoke River about Alleghany Springs
in Virginia, fishes of the Tennessee Basin are found, instead of those
characteristic of the lower Roanoke. In this case it is likely that we
have to consider the results of local erosion. Probably the divide has
been so shifted that some small stream with its fishes has been cut off
from the Holston and transferred to the Roanoke.

The passage of species from stream to stream along the Atlantic <DW72>
deserves a moment's notice. It is under present conditions impossible
for any mountain or upland fish, as the trout or the miller's
thumb,[123] to cross from the Potomac River to the James, or from
the Neuse to the Santee, by descending to the lower courses of the
rivers, and thence passing along either through the swamps or by way
of the sea. The lower courses of these streams, warm and muddy, are
uninhabitable by such fishes. Such transfers are, however, possible
farther north. From the rivers of Canada and from many rivers of New
England the trout does descend to the sea and into the sea, and farther
north the whitefish does this also. Thus these fishes readily pass
from one river basin to another. As this is the case now everywhere in
the North, it may have been the case farther south in the time of the
glacial cold. We may, I think, imagine a condition of things in which
the snow-fields of the Alleghany chain might have played some part in
aiding the diffusion of cold-loving fishes. A permanent snow-field on
the Blue Ridge in western North Carolina might render almost any stream
in the Carolinas suitable for trout, from its source to its mouth. An
increased volume of colder water might carry the trout of the head
streams of the Catawba and the Savannah as far down as the sea. We can
even imagine that the trout reached these streams in the first place
through such agencies, though of this there is no positive evidence.
For the presence of trout in the upper Chattahoochee we must account in
some other way.

It is noteworthy that the upland fishes are nearly the same in
all these streams until we reach the southern limit of possible
glacial influence. South of western North Carolina the faunae of the
different river basins appear to be more distinct from one another.
Certain ripple-loving types are represented by closely related but
unquestionably different species in each river basin, and it would
appear that a thorough mingling of the upland species in these rivers
has never taken place.

The best examples of this are the following: In the Santee basin
are found _Notropis pyrrhomelas_, _Notropis niveus_, and _Notropis
chloristius_; in the Altamaha, _Notropis xaenurus_ and _Notropis
callisemus_; in the Chattahoochee, _Notropis hypselopterus_ and
_Notropis eurystomus_; in the Alabama, _Notropis coeruleus_, _Notropis
trichroistius_, and _Notropis callistius_. In the Alabama, Escambia,
Pearl, and numerous other rivers is found _Notropis cercostigma_. This
species descends to the sea in the cool streams of the pine woods. Its
range is wider than that of the others, and in the rivers of Texas
it reappears in the form of a scarcely distinct variety, _Notropis
venustus_. In the Tennessee and Cumberland, and in the rivers of the
Ozark range, is _Notropis galacturus_; and in the upper Arkansas
_Notropis camurus_,--all distinct species of the same general type.
Northward, in all the streams from the Potomac to the Oswego, and
westward to the Des Moines and the Arkansas, occurs a single species
of this type, _Notropis whipplei_, varying eastward into _Notropis
analostanus_. But this species is not known from any of the streams
inhabited by any of the other species mentioned, although very likely
it is the parent stock of them all.

=Lowland Fishes.=--With the lowland species of the Southern rivers
it is different. Few of these are confined within narrow limits. The
streams of the whole South Atlantic and Gulf Coast flow into shallow
bays, mostly bounded by sand-spits or sand-bars which the rivers
themselves have brought down. In these bays the waters are often
neither fresh nor salt; or, rather, they are alternately fresh and
salt, the former condition being that of the winter and spring. Many
species descend into these bays, thus finding every facility for
transfer from river to river. There is a continuous inland passage in
fresh or brackish waters, traversable by such fishes, from Chesapeake
Bay nearly to Cape Fear; and similar conditions exist on the coasts
of Louisiana, Texas, and much of Florida. In Perdido Bay I have found
fresh-water minnows[124] and silversides[125] living together with
marine gobies[126] and salt-water eels.[127] Fresh-water alligator
gars[128] and marine sharks compete for the garbage thrown over from
the Pensacola wharves. In Lake Pontchartrain the fauna is a remarkable
mixture of fresh-water fishes from the Mississippi and marine fishes
from the Gulf. Channel-cats, sharks, sea-crabs, sunfishes, and mullets
can all be found there together. It is therefore to be expected that
the lowland fauna of all the rivers of the Gulf States would closely
resemble that of the lower Mississippi; and this, in fact, is the case.

The streams of southern Florida and those of southwestern Texas offer
some peculiarities connected with their warmer climate. The Florida
streams contain a few peculiar fishes;[129] while the rivers of Texas,
with the same general fauna as those farther north, have also a few
distinctly tropical types,[130] immigrants from the lowlands of Mexico.

=Cuban Fishes.=--The fresh waters of Cuba are inhabited by fishes
unlike those found in the United States. Some of these are evidently
indigenous, derived in the waters they now inhabit directly from marine
forms. Two of these are eyeless species,[131] inhabiting streams in
the caverns. They have no relatives in the fresh waters of any other
region, the blind fishes[132] of our caves being of a wholly different
type. Some of the Cuban fishes are common to the fresh waters of the
other West Indies. Of Northern types, only one, the alligator gar,[133]
is found in Cuba, and this is evidently a filibuster immigrant from the
coasts of Florida.

=Swampy Watersheds.=--The low and irregular watershed which separates
the tributaries of Lake Michigan and Lake Erie from those of the Ohio
is of little importance in determining the range of species. Many of
the distinctively Northern fishes are found in the headwaters of the
Wabash and the Scioto. The considerable difference in the general fauna
of the Ohio Valley as compared with that of the streams of Michigan is
due to the higher temperature of the former region, rather than to any
existing barriers between the river and the Great Lakes. In northern
Indiana the watershed is often swampy, and in many places large ponds
exist in the early spring.

At times of heavy rains many species will move through considerable
distances by means of temporary ponds and brooks. Fishes that have
thus emigrated often reach places ordinarily inaccessible, and people
finding them in such localities often imagine that they have "rained
down." Once, near Indianapolis, after a heavy shower, I found in a
furrow in a corn-field a small pike,[134] some half a mile from the
creek in which he should belong. The fish was swimming along in a
temporary brook, apparently wholly unconscious that he was not in his
native stream. Migratory fishes, which ascend small streams to spawn,
are especially likely to be transferred in this way. By some such means
any of the watersheds in Ohio, Indiana, or Illinois may be passed.

[Illustration: FIG. 193.--Creekfish or Chub-sucker, _Erimyzon sucetta_
(Lacepede). Nipisink Lake, Illinois. Family _Catostomidae_.]

It is certain that the limits of Lake Erie and Lake Michigan were once
more extended than now. It is reasonably probable that some of the
territory now drained by the Wabash and the Illinois was once covered
by the waters of Lake Michigan. The cisco[135] of Lake Tippecanoe,
Lake Geneva, and the lakes of the Oconomowoc chain is evidently a
modified descendant of the so-called lake herring.[136] Its origin most
likely dates from the time when these small deep lakes of Indiana and
Wisconsin were connected with Lake Michigan. The changes in habits
which the cisco has undergone are considerable. The changes in external
characters are but trifling. The presence of the cisco in these lakes
and its periodical disappearance--that is, retreat into deep water
when not in the breeding season--have given rise to much nonsensical
discussion as to whether any or all of these lakes are still joined to
Lake Michigan by subterranean channels. Several of the larger fishes,
properly characteristic of the Great Lake region,[137] are occasionally
taken in the Ohio River, where they are usually recognized as rare
stragglers. The difference in physical conditions is probably the sole
cause of their scarcity in the Ohio basin.

=The Great Basin of Utah.=--The similarity of the fishes in the
different streams and lakes of the Great Basin is doubtless to be
attributed to the general mingling of their waters which took place
during and after the Glacial Epoch. Since that period the climate in
that region has grown hotter and drier, until the overflow of the
various lakes into the Columbia basin through the Snake River has long
since ceased. These lakes have become isolated from each other, and
many of them have become salt or alkaline and therefore uninhabitable.
In some of these lakes certain species may now have become extinct
which still remain in others. In some cases, perhaps, the differences
in surroundings may have caused divergence into distinct species of
what was once one parent stock. The suckers in Lake Tahoe[138] and
those in Utah Lake are certainly now different from each other and
from those in the Columbia. The trout[139] in the same waters can be
regarded as more or less tangible species, while the whitefishes[140]
show no differences at all. The differences in the present faunas of
Lake Tahoe and Utah Lake must be chiefly due to influences which have
acted since the Glacial Epoch, when the whole Utah Basin was part of
the drainage of the Columbia.

=Arctic Species in Lakes.=--Connected perhaps with changes due to
glacial influences is the presence in the deep waters of the Great
Lakes of certain marine types,[141] as shown by the explorations of
Professor Sidney I. Smith and others. One of these is a genus of
fishes,[142] of which the nearest allies now inhabit the Arctic Seas.
In his review of the fish fauna of Finland,[143] Professor A. J.
Malmgren finds a number of Arctic species in the waters of Finland
which are not found either in the North Sea or in the southern portions
of the Baltic. These fishes are said to "agree with their 'forefathers'
in the Glacial Ocean in every point, but remain comparatively smaller,
leaner, almost starved." Professor Loven[144] also has shown that
numerous small animals of marine origin are found in the deep lakes of
Sweden and Finland as well as in the Gulf of Bothnia. These anomalies
of distribution are explained by Loven and Malmgren on the supposition
of the former continuity of the Baltic through the Gulf of Bothnia
with the Glacial Ocean. During the second half of the Glacial Period,
according to Loven, "the greater part of Finland and of the middle
of Sweden was submerged, and the Baltic was a great gulf of the
Glacial Ocean, and not connected with the German Ocean. By the gradual
elevation of the Scandinavian Continent, the Baltic became disconnected
from the Glacial Ocean and the Great Lakes separated from the Baltic.
In consequence of the gradual change of the salt water into fresh,
the marine fauna became gradually extinct, with the exception of the
glacial forms mentioned above."

It is possible that the presence of marine types in our Great Lakes
is to be regarded as due to some depression of the land which would
connect their waters with those of the Gulf of St. Lawrence. On this
point, however, our data are still incomplete.

To certain species of upland or mountain fishes the depression of the
Mississippi basin itself forms a barrier which cannot be passed. The
black-spotted trout,[145] very closely related species of which abound
in all waters of northern Asia, Europe, and western North America,
has nowhere crossed the basin of the Mississippi, although one of its
species finds no difficulty in passing Bering Strait. The trout and
whitefish of the Rocky Mountain region are all species different from
those of the Great Lakes or the streams of the Alleghany system. To
the grayling, the trout, the whitefish, the pike, and to arctic and
subarctic species generally, Bering Strait has evidently proved no
serious obstacle to diffusion; and it is not unlikely that much of the
close resemblance of the fresh-water faunae of northern Europe, Asia,
and North America is due to this fact. To attempt to decide from which
side the first migration came in regard to each group of fishes might
be interesting; but without a wider range of facts than is now in our
possession, most such attempts, based on guesswork, would have little
value. The interlocking of the fish faunas of Asia and North America
presents, however, a number of interesting problems, for migrations in
both directions have doubtless taken place.

=Causes of Dispersion Still in Operation.=--One might go on
indefinitely with the discussion of special cases, each more or less
interesting or suggestive in itself, but the general conclusion is in
all cases the same. The present distribution of fishes is the result
of the long-continued action of forces still in operation. The species
have entered our waters in many invasions from the Old World or from
the sea. Each species has been subjected to the various influences
implied in the term "natural selection," and under varying conditions
its representatives have undergone many different modifications. Each
of the six hundred fresh-water species we now know in the United States
may be conceived as making every year inroads on territory occupied
by other species. If these colonies are able to hold their own in the
struggle for possession, they will multiply in the new conditions,
and the range of the species becomes widened. If the surroundings are
different, new species or varieties may be formed with time; and these
new forms may again invade the territory of the parent species. Again,
colony after colony of species after species may be destroyed by other
species or by uncongenial surroundings.

The ultimate result of centuries on centuries of the restlessness of
individuals is seen in the facts of geographical distribution. Only
in the most general way can the history of any species be traced; but
could we know it all, it would be as long and as eventful a story as
the history of the colonization and settlement of North America by
immigrants from Europe. But by the fishes each river in America has
been a hundred times discovered, its colonization a hundred times
attempted. In these efforts there is no co-operation. Every individual
is for himself, every struggle a struggle of life and death; for each
fish is a cannibal, and to each species each member of every other
species is an alien and a savage.

FOOTNOTES:

[100] Except possibly the Sacramento.

[101] Unless the fauna of certain cave streams in the United States and
Cuba be regarded as forming an exception.

[102] For example, Elk River, Duck River, etc.

[103] There are three species of darters (_Cottogaster copelandi_
Jordan, _Hadropterus evides_ Jordan and Copeland, _Hadropterus
scierus_ Swain) which are now known only from the Ozark region or
beyond and from the uplands of Indiana, not yet having been found
at any point between Indiana and Missouri. These constitute perhaps
isolated colonies, now separated from the parent stock in Arkansas by
the prairie districts of Illinois, a region at present uninhabitable
for these fishes. But the non-occurrence of these species over the
intervening areas needs confirmation, as do most similar cases of
anomalous distribution.

[104] Thus, _Dorosoma cepedianum_ Le Sueur and _Pomolobus
chrysochloris_ Rafinesque have found their way into Lake Michigan
through canals.

[105] _Oncorhynchus tschawytscha_ Walbaum.

[106] _Oncorhynchus nerka_ Walbaum.

[107] "In this isolated region of North America, in this zoological
island of New England, as we may call it, we find neither Lepidosteus,
nor Amia, nor Polyodon, nor Amblodon (_Aplodinotus_), nor Grystes
(_Micropterus_), nor Centrarchus, nor Pomoxis, nor Ambloplites, nor
Calliurus (_Chaenobryttus_), nor Carpiodes, nor Hyodon, nor indeed
any of the characteristic forms of North American fishes so common
everywhere else, with the exception of two Pomotis (_Lepomis_), one
Boleosoma, and a few Catostomus."--AGASSIZ, _Amer. Journ. Sci. Arts_,
1854.

[108] _Catostomus_, _Pantosteus_, _Chasmistes_.

[109] _Gila_, _Ptychocheilus_, etc.

[110] _Salmo clarkii_ and its varieties.

[111] Genera _Notropis_, _Chrosomus_, etc.

[112] As the fresh-water surf-fish (_Hysterocarpus traski_) and the
species of salmon.

[113] _Salmo clarki_ Richardson.

[114] _Coregonus williamsoni_ Girard.

[115] _Salvelinus malma_ (Walbaum).

[116] _Thymallus tricolor_ Cope.

[117] _Salvelinus fontinalis_ Mitchill.

[118] _Notropis rubricroceus_ Cope, _Rhinichthys atronasus_ Mitchill,
etc.

[119] Evermann, A Reconnoissance of the Streams and Lakes of Western
Montana and Northwestern Wyoming, in Bull. U. S. Fish. Comm., XI, 1891,
24-28, pls. I and II; Jordan, The Story of a Strange Land, in Pop. Sci.
Monthly, Feb., 1892, 447-458; Evermann, Two-Ocean Pass, in Proc. Ind.
Ac. Sci., 1892, 29-34, pl. I; Evermann, Two-Ocean Pass, in Pop. Sci.
Monthly, June, 1895, with plate.

[120] _Ameiurus nebulosus_ Le Sueur: _Ameiurus catus_ Linnaeus.

[121] _Salvelinus fontinalis._

[122] _Notropis rubricroceus_ Cope.

[123] _Cottus ictalops_ Rafinesque.

[124] _Notropis cercostigma_, _Notropis xaenocephalus_.

[125] _Labidesthes sicculus._

[126] _Gobiosoma molestum._

[127] _Myrophis punctatus._

[128] _Lepisosteus tristoechus._

[129] _Jordanella_, _Rivulus_, _Heterandria_, etc.

[130] _Heros_, _Tetragonopterus_.

[131] _Lucifuga_ and _Stygicola_, fishes allied to the cusk, and
belonging to the family of _Brotulidae_.

[132] _Amblyopsis_, _Typhlichthys_.

[133] _Lepisosteus tristoechus._

[134] _Esox vermiculatus_ Le Sueur.

[135] _Argyrosomus sisco_ Jordan.

[136] _Argyrosomus artedi_ Le Sueur.

[137] As _Lota maculosa_; _Percopsis guttata_; _Esox masquinongy_.

[138] _Catostomus tahoensis_, in Lake Tahoe; _Catostomus macrocheilus_
and _discobolus_, in the Columbia; _Catostomus fecundus_; _Catostomus
ardens_; _Chasmistes liorus_ and _Pantosteus generosus_, in Utah Lake.

[139] _Salmo henshawi_ and _virginalis_.

[140] _Coregonus williamsoni._

[141] Species of _Mysis_ and other genera of Crustaceans, similar to
species described by Sars and others, in lakes of Sweden and Finland.

[142] _Triglopsis thompsoni_ Girard, a near ally of the marine species
_Oncocottus quadricornis_ L.

[143] Kritisk Oefversigt of Finlands Fisk-Fauna, Helsingfors, 1863.

[144] See Guenther, Zoological Record for 1864, p. 137.

[145] _Salmo fario_ L., in Europe; _Salmo labrax_ Pallas, etc., in
Asia; _Salmo gairdneri_ Richardson, in streams of the Pacific Coast;
_Salmo perryi_, in Japan; _Salmo clarki_ Richardson, throughout the
Rocky Mountain range to the Mexican boundary and the headwaters of the
Kansas, Platte, and Missouri.




CHAPTER XVIII

FISHES AS FOOD FOR MAN


=The Flesh of Fishes.=--Among all races of men, fishes are freely eaten
as food, either raw, as preferred by the Japanese and Hawaiians, or
else as cooked, salted, dried, or otherwise preserved.

The flesh of most fishes is white, flaky, readily digestible, and with
an agreeable flavor. Some, as the salmon, are charged with oil, which
aids to give an orange hue known as salmon color. Others have colorless
oil which may be of various consistencies. Some have dark-red flesh,
which usually contains a heavy oil which becomes acrid when stale.
Some fishes, as the sharks, have tough, coarse flesh. Some have flesh
which is watery and coarse. Some are watery and tasteless, some dry and
tasteless. Some, otherwise excellent, have the muscular area, which
constitutes the chief edible part of the fish, filled with small bones.

=Relative Rank of Food-fishes.=--The writer has tested most of the
noted food-fishes of the Northern Hemisphere. When properly cooked (for
he is no judge of raw fish) he would place first in the ranks as a
food-fish the eulachon, or candle-fish (_Thaleichthys pacificus_).

[Illustration: FIG. 194.--Eulachon, or Ulchen. _Thaleichthys pretiosus_
Girard. Columbia River. Family _Argentinidae_.]

This little smelt, about a foot long, ascends the Columbia River,
Frazer River, and streams of southern Alaska in the spring in great
numbers for the purpose of spawning. Its flesh is white, very delicate,
charged with a white and very agreeable oil, readily digested, and with
a sort of fragrance peculiar to the species.

[Illustration: FIG. 195.--Ayu, or Japanese Samlet, _Plecoglossus
altivelis_ Schlegel. Tanagawa, Tokyo, Japan.]

Next to this he is inclined to place the ayu (_Plecoglossus
altivelis_), a sort of dwarf salmon which runs in similar fashion in
the rivers of Japan and Formosa. The ayu is about as large as the
eulachon and has similar flesh, but with little oil and no fragrance.

[Illustration: FIG. 196.--Whitefish, _Coregonus clupeiformis_ Mitchill.
Ecorse, Mich.]

Very near the first among sea-fishes must come the pampano
(_Trachinotus carolinus_) of the Gulf of Mexico, with firm, white,
finely flavored flesh.

The red surmullet of Europe (_Mullus barbatus_) has been long famed for
its delicate flesh, and may perhaps be placed next. Two related species
in Polynesia, the munu and the kumu (_Pseudupeneus bifasciatus_ and
_Pseudupeneus porphyreus_), are scarcely inferior to it.

[Illustration: FIG. 197.--Golden Surmullet, _Mullus auratus_ Jordan &
Gilbert. Woods Hole, Mass.]

[Illustration: FIG. 198.--Spanish Mackerel, _Scomberomorus maculatus_
Mitchill. Family _Scombridae_. Key West.]

Side by side with these belongs the whitefish of the Great Lakes
(_Coregonus clupeiformis_). Its flesh, delicate, slightly gelatinous,
moderately oily, is extremely agreeable. Sir John Richardson records
the fact that one can eat the flesh of this fish longer than any other
without the feeling of cloying. The salmon cannot be placed in the
front ranks because, however excellent, the stomach soon becomes tired
of it. The Spanish mackerel (_Scomberomorus maculatus_), with flesh
at once rich and delicate, the great opah (_Lampris luna_), still
richer and still more delicate, the bluefish (_Pomatomus saltatrix_)
similar but a little coarser, the ulua (_Carangus sem_), the finest
large food-fish of the South Seas, the dainty California poppy-fish,
miscalled "Pampano" (_Palometa simillima_), and the kingfish firm and
well-flavored (_Scomberomorus cavalla_), represent the best of the
fishes allied to the mackerel.

[Illustration: FIG. 199.--Opah, or Moonfish, _Lampris luna_ (Gmelin).
Specimen in Honolulu market weighing 317-1/2 lbs. (Photograph by E. L.
Berndt.)--Page 323.]

[Illustration: FIG. 200.--Bluefish, _Pomatomus saltatrix_ (L.). New
York.]

[Illustration: FIG. 201.--Robalo, _Centropomus undecimalis_ (Bloch).
Florida.]

The shad (_Alosa sapidissima_), with its sweet, tender, finely oily
flesh, stands also near the front among food-fishes, but it sins above
all others in the matter of small bones. The weak-fish (_Cynoscion
nobilis_) and numerous relatives rank first among those with tender,
white, savorous flesh. Among the bass and perch-like fishes, common
consent places near the first the striped bass (_Roccus lineatus_),
the bass of Europe (_Dicentrarchus labrax_), the susuki of Japan
(_Lateolabrax japonicus_), the red tai of Japan (_Pagrus major_ and _P.
cardinalis_), the sheep's-head (_Archosargus probatocephalus_), the
mutton-fish or Pargo Criollo of Cuba (_Lutianus analis_), the European
porgy (_Pagrus pagrus_), the robalo (_Centropomus undecimalis_), the
uku (_Aprion virescens_) of Hawaii, the spadefish (_Chaetodipterus
faber_), and the black bass (_Micropterus dolomieu_).

[Illustration: FIG. 202.--Spadefish, _Chaetodipterus faber_ (L.).
Virginia.]

[Illustration: FIG. 203.--Small-mouthed Black Bass, _Micropterus
dolomieu_ (Lacepede). Potomac River.]

[Illustration: FIG. 204.--Speckled Trout (male), _Salvelinus
fontinalis_ (Mitchill). New York.]

[Illustration: FIG. 205.--Rainbow Trout, _Salmo irideus_ Gibbons.
Sacramento River, California.]

[Illustration: FIG. 206.--Rangeley Trout, _Salvelinus oquassa_
(Girard). Lake Oquassa, Maine.]

The various kinds of trout have been made famous the world over. All
are attractive in form and color; all are gamey; all have the most
charming of scenic surroundings, and, finally, all are excellent as
food, not in the first rank perhaps, but well above the second. Notable
among these are the European charr (_Salvelinus alpinus_), the American
speckled trout or charr (_Salvelinus fontinalis_), the Dolly Varden
or malma (_Salvelinus malma_), and the oquassa trout (_Salvelinus
oquassa_). Scarcely less attractive are the true trout, the brown
trout, or forelle (_Salmo fario_), in Europe, the rainbow-trout (_Salmo
irideus_), the steelhead (_Salmo gairdneri_), the cut-throat trout
(_Salmo clarkii_), and the Tahoe trout (_Salmo henshawi_), in America,
and the yamabe (_Salmo perryi_) of Japan. Not least of all these is the
flower of fishes, the grayling (_Thymallus_), of different species in
different parts of the world.

[Illustration: FIG. 207.--Steelhead Trout, _Salmo gairdneri_
Richardson. Columbia River.]

[Illustration: FIG. 208.--Tahoe Trout, _Salmo henshawi_ Gill & Jordan.
Lake Tahoe, California.]

[Illustration: FIG. 209.--The Dolly Varden Trout, _Salvelinus malma_
(Walbaum). Lake Pend d'Oreille, Idaho. (After Evermann.)]

[Illustration: FIG. 210.--Alaska Grayling, _Thymallus signifer_
Richardson. Nulato, Alaska.]

[Illustration: FIG. 211.--Pike, _Esox lucius_ L. Ecorse, Mich.]

[Illustration: FIG. 212.--Atka-fish, _Pleurogrammus monopterygius_
(Pallas). Atka Island.]

Other most excellent food-fishes are the eel (_Anguilla_ species),
the pike (_Esox lucius_), the muskallonge (_Esox Roccus_), the sole
of Europe (_Solea solea_), the sardine (_Sardinella pilchardus_), the
atka-fish (_Pleurogrammus monopterygius_) of Bering Sea, the pescado
blanco of Lake Chapala (_Chirostoma estor_ and other species), the
Hawaiian mullet (_Mugil cephalus_), the channel catfish (_Ictalurus
punctatus_), the turbot (_Scophthalmus maximus_), the barracuda
(_Sphyraena_), and the young of various sardines and herring, known
as whitebait. Of large fishes, probably the swordfish (_Xiphias
gladius_), the halibut (_Hippoglossus hippoglossus_), and the
king-salmon, or quinnat (_Oncorhynchus tschawytscha_), may be placed
first. Those people who feed on raw fish prefer in general the large
parrot-fishes (as _Pseudoscarus jordani_ in Hawaii), or else the young
of mullet and similar species.

[Illustration: FIG. 213.--Pescado blanco, _Chirostoma humboldtianum_
(Val.). Lake Chalco, City of Mexico.]

[Illustration: FIG. 214.--Red Goatfish, or Salmonete, _Pseudupeneus
maculatus_ Bloch. Family _Mullidae_ (Surmullets).]

=Abundance of Food-fishes.=--In general, the economical value of any
species depends not on its toothsomeness, but on its abundance and
the ease with which it may be caught and preserved. It is said that
more individuals of the herring (_Clupea harengus_ in the Atlantic,
_Clupea pallasi_ in the Pacific) exist than of any other species. The
herring is a good food-fish and whenever it runs it is freely sought.
According to Bjoernsoen, wherever the school of herring touches the coast
of Norway, there a village springs up, and this is true in Scotland,
Newfoundland, and from Killisnoo in Alaska to Otaru in Japan, and to
Strielok in Siberia. Goode estimates the herring product of the North
Atlantic at 1,500,000,000 pounds annually. In 1881 Professor Huxley
used these words:

[Illustration: FIG. 215.--Great Parrot-fish, or Guacamaia,
_Pseudoscarus guacamaia_ Bloch & Schneider. Florida.]

[Illustration: FIG. 216.--Striped Mullet, _Mugil cephalus_ (L.). Woods
Hole, Mass.]

"It is said that 2,500,000,000 or thereabout of herrings are every
year taken out of the North Sea and the Atlantic. Suppose we assume
the number to be 3,000,000,000 so as to be quite safe. It is a large
number undoubtedly, but what does it come to? Not more than that of the
herrings which may be contained in one shoal, if it covers half a dozen
square miles, and shoals of much larger size are on record. It is safe
to say that scattered through the North Sea and the Atlantic, at one
and the same time, there must be scores of shoals, any one of which
would go a long way toward supplying the whole of man's consumption of
herrings."

[Illustration: FIG. 217.--Mutton-snapper, or Pargo criollo, _Lutianus
analis_ (Cuv. & Val.). Key West.]

[Illustration: FIG. 218.--Herring, _Clupea harengus_ L. New York.]

[Illustration: FIG. 219.--Codfish, _Gadus callarias_ L. Eastport,
Maine.]

The codfish (_Gadus callarias_ in the Atlantic; _Gadus macrocephalus_
in the Pacific) likewise swarms in all the northern seas, takes the
hook readily, and is better food when salted and dried than it is when
fresh.

Next in economic importance probably stands the mackerel of the
Atlantic (_Scomber scombrus_), a rich, oily fish which bears salting
better than most.

[Illustration: FIG. 220.--Mackerel, _Scomber scombrus_ L. New York.]

Not less important is the great king-salmon, or quinnat (_Oncorhyanchus
tschawytscha_), and the still more valuable blue-back salmon, or
redfish (_Oncorhynchus nerka_).

[Illustration: FIG. 221.--Halibut, _Hippoglossus hippoglossus_
(Linnaeus). St. Paul Island, Bering Sea. (Photograph by U. S. Fur Seal
Commission.)]

The salmon of the Atlantic (_Salmo salar_), the various species of
sturgeon (_Acipenser_), the sardines (_Sardinella_), the halibut
(_Hippoglossus_), are also food-fishes of great importance.

=Variety of Tropical Fishes.=--In the tropics no one species is
represented by enormous numbers of individuals as is the case in
colder regions. On the other hand, the number of species regarded as
food-fishes is much greater in any given port. In Havana, about 350
different species are sold as food in the markets, and an equal number
are found in Honolulu. Upward of 600 different species appear in the
markets of Japan. In England, on the contrary, about 50 species make up
the list of fishes commonly used as food. Yet the number of individual
fishes is probably not greater about Japan or Hawaii than in a similar
stretch of British coast.

=Economic Fisheries.=--Volumes have been written on the economic value
of the different species of fishes, and it is not the purpose of the
present work to summarize their contents.

[Illustration: FIG. 222.--Fishing for Ayu with Cormorants in the
Tanagawa, near Tokyo. (After Photograph by J. O. Snyder by Sekko
Shimada.)]

Equally voluminous is the literature on the subject of catching fishes.
It ranges in quality from the quaint wisdom of the "Compleat Angler"
and the delicate wit of "Little Rivers" to elaborate discussions of the
most economic and effective forms and methods, of the beam-trawl, the
purse-seine, and the codfish hook. In general, fishes are caught in
four ways--by baited hooks, by spears, by traps, and by nets. Special
local methods, such as the use of the tamed cormorant[146] in the
catching of the ayu, by the Japanese fishermen at Gifu, may be set
aside for the moment, and all general methods of fishing come under
one of these four classes. Of these methods, the hook, the spear, the
seine, the beam-trawl, the gill-net, the purse-net, the sweep-net, the
trap and the weir are the most important. The use of the hook is again
extremely varied. In the deep sea long, sunken lines, are sometimes
used for codfish, each baited with many hooks. For pelagic fish, a
baited hook is drawn swiftly over the surface, with a "spoon" attached
which looks like a living fish. In the rivers a line is attached to a
pole, and when fish are caught for pleasure or for the joy of being
in the woods, recreation rises to the dignity of angling. Angling may
be accomplished with a hook baited with an earthworm, a grasshopper,
a living fish, or the larva of some insect. The angler of to-day,
however, prefers the artificial fly, as being more workmanlike and also
more effective than bait-fishing. The man who fishes, not for the good
company of the woods and brooks, but to get as many fish as possible
to eat or sell, is not an angler but a pot-fisher. The man who kills
all the trout he can, to boast of his skill or fortune, is technically
known as a trout-hog. Ethically, it is better to lie about your great
catches of fine fishes than to make them. For most anglers, also, it is
more easy.

=Fisheries.=--With the multiplicity of apparatus for fishing, there is
the greatest variety in the boats which may be used. The fishing-fleet
of any port of the world is a most interesting object, as are also the
fishermen with their quaint garb, plain speech, and their strange songs
and calls with the hauling in of the net.

[Illustration: FIG. 223.--Fishing for Ayu in the Tanagawa, Japan.
Emptying the pouch of the cormorant. (Photograph by J. O. Snyder.)]

For much information on the fishing apparatus in use in America the
reader is referred to the Reports of the Fisheries in the Tenth
Census, in 1880, under the editorship of Dr. George Brown Goode. In
these reports Goode, Stearns, Earle, Gilbert, Bean, and the present
writer have treated very fully of all economic relations of the
American fishes. In an admirable work entitled "American Fishes,"
Dr. Goode, with the fine literary touch of which he was master,
has fully discoursed of the game- and food-fishes of America with
especial reference to the habits and methods of capture of each. To
these sources, to Jordan and Evermann's "Food and Game Fishes of North
America," and to many other works of similar purport in other lands,
the reader is referred for an account of the economic and the human
side of fish and fisheries.

=Angling.=--It is no part of the purpose of this work to describe the
methods or materials of angling, still less to sing its praises as a
means of physical or moral regeneration. We may perhaps find room for
a first and a last word on the subject; the one the classic from the
pen of the angler of the brooks of Staffordshire, and the other the
fresh expression of a Stanford student setting out for streams such as
Walton never knew, the Purissima, the Stanislaus, or perchance his home
streams, the Provo or the Bear.

"And let me tell you, this kind of fishing with a dead rod, and laying
night-hooks, are like putting money to use; for they both work for the
owners when they do nothing but sleep, or eat, or rejoice, as you know
we have done this last hour, and sat as quietly and as free from cares
under this sycamore as Virgil's Tityrus and his Meliboeus did under
their broad beech-tree. No life, my honest scholar,--no life so happy
and so pleasant as the life of a well-governed angler; for when the
lawyer is swallowed up with business and the statesman is preventing
or contriving plots, then we sit on the cowslip-banks, hear the birds
sing, and possess ourselves in as much quietness as these silent silver
streams which we now see glide so quietly by us. Indeed, my good
scholar, we may say of angling, as Dr. Boteler said of strawberries,
'Doubtless God could have made a better berry, but doubtless God never
did'; and so, if I might be judge, 'God never made a more calm, quiet,
innocent recreation than angling.'

"I'll tell you, scholar, when I sat last on this primrose-bank, and
looked down these meadows, I thought of them as Charles the Emperor did
of Florence, 'That they were too pleasant to be looked on but only on
holidays.'

"Gentle Izaak! He has been dead these many years, but his disciples are
still faithful. When the cares of business lie heavy and the sound of
wheels jarring on cobbled streets grows painful, one's fingers itch for
the rod; one would away to the quiet brook among the pines, where one
has fished so often. Every man who has ever got the love of the stream
in his blood feels often this longing.

"It comes to me each year with the first breath of spring. There is
something in the sweetness of the air, the growing things, the 'robin
in the greening grass' that voices it. Duties that have before held in
their performance something of pleasure become irksome, and practical
thoughts of the day's work are replaced by dreamy pictures of a tent
by the side of a mountain stream--close enough to hear the water's
singing in the night. Two light bamboo rods rest against the tent-pole,
and a little column of smoke rising straight up through the branches
marks the supper fire. Jack is preparing the evening meal, and, as
I dream, there comes to me the odor of crisply browned trout and
sputtering bacon--was ever odor more delicious? I dare say that had
the good Charles Lamb smelled it as I have, his 'Dissertation on Roast
Pig' would never have been written. But then Charles Lamb never went
a-fishing as we do here in the west--we who have the mountains and the
fresh air so boundlessly.

"And neither did Izaak Walton for that matter. He who is sponsor for
all that is gentle in angling missed much that is best in the sport
by living too early. He did not experience the exquisite pleasure
of wading down mountain streams in supposedly water-proof boots and
feeling the water trickling in coolingly; nor did he know the joy of
casting a gaudy fly far ahead with a four-ounce rod, letting it drift,
insect-like, over that black hole by the tree stump, and then feeling
the seaweed line slip through his fingers to the _whirr_ of the reel.
And, at the end of the day, supper over, he did not squat around a big
camp-fire and light his pipe, the silent darkness of the mountains
gathering round, and a basketful of willow-packed trout hung in the
clump of pines by the tent. Izaak's idea of fishing did not comprehend
such joy. With a can of worms and a crude hook, he passed the day by
quiet streams, threading the worms on his hook and thinking kindly of
all things. The day's meditations over, he went back to the village,
and, mayhap, joined a few kindred souls over a tankard of ale at
the sign of the Red Lobster. But he missed the mountains, the water
rushing past his tent, the bacon and trout, the camp-fire--the physical
exaltation of it all. His kind of fishing was angling purely, while
modern Waltons, as a rule, eschew the worm.

[Illustration: FIG. 224.--Fishing for Tai, Tokyo Bay. (Photograph by J.
O. Snyder.)]

"To my mind, there is no real sport in any kind of fishing except
fly-fishing. This sitting on the bank of a muddy stream with your bait
sunk, waiting for a bite, may be conducive to gentleness and patience
of spirit, but it has not the joy of action in which a healthy man
revels. How much more sport is it to clamber over fallen logs that
stretch far out a-stream, to wade slipping over boulders and let
your fly drop caressingly on ripples and swirling eddies and still
holes! It is worth all the work to see the gleam of a silver side as
a half-pounder rises, and, with a flop, takes the fly excitedly to
the bottom. And then the nervous thrill as, with a deft turn of the
wrist, you hook him securely--whoever has felt that thrill cannot
forget it. It will come back to him in his law office when he should
be thinking of other things; and with it will come a longing for that
dear remembered stream and the old days. That is the hold trout-fishing
takes on a man.

"It is spring now and I feel the old longing myself, as I always do
when life comes into the air and the smell of new growth is sweet. I
got my rod out to-day, put it together, and have been looking over my
flies. If I cannot use them, I can at least muse over days of the past
and dream of those to come." (WALDEMAR YOUNG.)

FOOTNOTES:

[146] The cormorant is tamed for this purpose. A harness is placed
about its wings and a ring about the lower part of its neck. Two or
three birds may be driven by a boy in a shallow stream, a small net
behind him to drive the fish down the river. In a large river like that
of Gifu, where the cormorants are most used, the fishermen hold the
birds from the boats and fish after dark by torchlight. The bird takes
a great interest in the work, darts at the fishes with great eagerness,
and fills its throat and gular pouch as far down as the ring. Then
the boy takes him out of the water, holds him by the leg and shakes
the fishes out into a basket. When the fishing is over the ayu are
preserved, the ring is taken off from the bird's neck, and the zako or
minnows are thrown to him for his share. These he devours greedily.




CHAPTER XIX

DISEASES OF FISHES


=Contagious Diseases.=--As compared with other animals the fishes
of the sea are subject to but few specific diseases. Those in fresh
waters, being more isolated, are more frequently attacked by contagious
maladies. Often these diseases are very destructive. In an "epidemic"
in Lake Mendota, near Madison, Wis., Professor Stephen A. Forbes
reports a death of 300 tons of fishes in the lake. I have seen similar
conditions among the land-locked alewife in Cayuga and Seneca Lakes,
the dead fishes being piled on the beaches so as to fill the air with
the stench of their decay.

[Illustration: FIG. 225.--Menhaden, _Brevoortia tyrannus_ (Latrobe).
Woods Hole, Mass.]

=Crustacean Parasites.=--The external parasites of fishes are of little
injury. These are mainly lernaeans and other crustaceans (fish-lice)
in the sea, and in the rivers different species of leeches. These may
suck the blood of the fish, or in the case of certain crustaceans
which lie under the tongue, steal the food as it passes along, as is
done by _Cymothoa praegustator_, the "bug" of the mouth of the menhaden
(_Brevoortia tyrannus_).

[Illustration: FIG. 226.--Australian Flying-fish, _Exonautes unicolor_
(Valenciennes). Specimen from Tasman Sea, having parasitic lernaean
crustaceans, to which parasitic barnacles are attached. (After
Kellogg.)] The relation of this crustacean to its host suggested to
Latrobe, its discoverer, the relation of the "foretaster" in Roman
times to the tyrant whom he served. A similar commensation exists
in the mouth of a mullet (_Mugil hospes_) at Panama. The writer has
received, through the courtesy of Mr. A. P. Lundin, a specimen of a
flying-fish (_Exonautes unicolor_) taken off Sydney, Australia. To this
are attached three large copepod crustaceans of the genus _Penella_,
the largest over two inches long, and to the copepods in turn are
attached a number of barnacles (_Conchoderma virgatum_) so joined to
the copepods as to suggest strange flowers, like orchids, growing out
of the fish.

[Illustration: FIG. 227.--Black-nosed Dace, _Rhinichthys atronasus_
(Mitchill). East Coy Creek, W. N. Y. Showing black spots of parasitic
organisms. (From life by Mary Jordan Edwards.)]

=Myxosporidia, or Parasitic Protozoa.=--Internal parasites are
very numerous and varied. Some of them are bacteria, giving rise
to infectious diseases, especially in ponds and lakes. Others are
myxosporidia, or parasitic protozoans, which form warty appendages,
which burst, discharging the germs and leaving ulcers in their place.
In the report of the U. S. Fish Commissioner for 1892, Dr. R. R. Gurley
has brought together our knowledge of the protozoans of the subclass
_Myxosporidia_, to which these epidemics are chiefly due. These
creatures belong to the class of Sporozoa, and are regarded as animals,
their nearest relatives being the parasitic _Gregarinida_, from which
they differ in having the germinal portion of the spore consisting of a
single protoplasmic mass instead of falciform protoplasmic rods as in
the worm-like Gregarines. The _Myxosporidia_ are parasitic on fishes,
both fresh-water and marine, especially beneath the epidermis of the
gills and fins and in the gall-bladder and urinary bladder. In color
these protozoa are always cream-white. In size and form they vary
greatly. The cyst in which they lie is filled with creamy substance
made up of spores and granule matter.

Dr. Gurley enumerates as hosts of these parasites about sixty species
of fishes, marine and fresh-water, besides frogs, crustaceans,
sea-worms, and even the crocodile. In the sharks and rays the parasites
occur mainly in the gall-ducts, in the minnows within the gill cavity
and epidermis, and in the higher fishes mainly but not exclusively in
the same regions. Forty-seven species are regarded by Gurley as well
defined. The diseases produced by them are very obscurely known. These
parasites on American fishes have been extensively studied by Charles
Wardall Stiles, Edwin Linton, Henry B. Ward, and others.

According to Dr. Linton the parasitism which results from infection
with protozoan parasites will, of all kinds, be found to be the most
important. Epidemics among European fish have been repeatedly traced
to this source. The fatality which attends infection with psorosperms
appears to be due to a secondary cause, however, namely, to bacilli
which develop within the psorosperms (_Myxobolus_) tumors and give rise
to ulceration. The discharge of these ulcers then disseminates the
disease.

[Illustration: FIG. 228.--White Shiner, _Notropis hudsonius_ (Clinton),
with cysts of parasitic psorosperms. (After Gurley.)]

"Brief mention of the remedies there proposed may appropriately be
repeated here. Megnin sees no other method than to collect all the dead
and sick fishes and to destroy them by fire. Ludwig thinks that the
waters should be kept pure, and that the pollutions of the rivers by
communities or industrial establishments should be interdicted. Further
he says:

"That most dangerous contamination of the water by the _Myxosporidia_
from the ulcers cannot of course be stopped entirely, but it is evident
that it will be less if all fishermen are impressed with the importance
of destroying all diseased and dead fish instead of throwing them back
into the water. Such destruction must be so effected as to prevent the
re-entry of the germs into the water.

"Railliet says that it is expedient to collect the diseased fish and
to bury them at a certain depth and at a great distance from the
water-course. He further states that this was done on the Meuse with
success, so that at the end of some years the disease appeared to have
left no trace."

[Illustration: FIG. 229.--White Catfish, _Ameiurus catus_ (Linnaeus),
from Potomac River, infested by parasitic protozoa, _Ichthyophthirus
multifilis_ Fouquet. (After C. W. Stiles.)]

=Parasitic Worms: Trematodes.=--Parasitic worms in great variety exist
in the intestinal canal or in the liver or muscular substance of fishes.

Trematode worms are most common in fresh-water fishes. These usually
are sources of little injury, especially when found in the intestines,
but they may do considerable mischief when encysted within the body
cavity or in the heart or liver. Dr. Linton describes 31 species of
these worms from 25 different species of American fishes. In 20 species
of fishes from the Great Lakes, 102 specimens, Dr. H. B. Ward found
95 specimens infected with parasites, securing 4000 trematodes, 2000
acanchocephala, 200 cestodes, and 200 nematodes. In the bowfin (_Amia
calva_), trematodes existed in enormous numbers.

=Cestodes.=--Cestode worms exist largely in marine fishes, the adults,
according to Dr. Linton, being especially common in the spiral
valve of the shark. It is said that one species of human tape-worm
(_Bothriocephalus taenia_) has been got from eating the flesh of the
European tench (_Tinca tinca_).

=The Worm of the Yellowstone.=--The most remarkable case of parasitism
of worms of this type is that given by the trout of Yellowstone Lake
(_Salmo clarki_). This is thus described by Dr. Linton:

"One of the most interesting cases of parasitism in which direct injury
results to the host, which has come to my attention, is that afforded
by the trout of Yellowstone Lake (_Salmo clarki_). It was noticed by
successive parties who visited the lake in connection with government
surveys that the trout with which the lake abounded were, to a large
extent, infested with a parasitic worm, which is most commonly in the
abdominal cavity, in cysts, but which in time escapes from the cyst and
tunnels into the flesh of its host. Fish, when thus much afflicted, are
found to be lacking in vitality, weak, and often positively emaciated.

"It was my good fortune, in the summer of 1890, to visit this
interesting region for the purpose of investigating the parasitism
of the trout of Yellowstone Lake. The results of this special
investigation were published in the Bulletin of the U. S. Fish
Commission for 1889, vol. ix., pp. 337-358, under the title 'A
Contribution to the Life-history of _Dibothrium cordiceps_, a Parasite
Infesting the Trout of Yellowstone Lake.'

"I found the same parasite in the trout of Heart Lake, just across the
great continental divide from Yellowstone Lake, but did not find any
that had tunneled into the flesh of its host, while a considerable
proportion of the trout taken in Yellowstone Lake had these worms
in the flesh. Some of these worms were as much as 30 centimeters in
length when first removed; others which had lain in water a few hours
after removal before they were measured were much longer, as much as
54 centimeters. They are rather slender and of nearly uniform size
throughout, 2.5 to 3 millimeters being an average breadth of the
largest. I found the adult stage in the intestine of the large white
pelican (_Pelecanus erythrorhynchus_), which is abundant on the lake
and was found breeding on some small islands near the southern end of
the lake.

"In the paper alluded to above I attempted to account for two things
concerning this parasitism among the trout of Yellowstone Lake: First,
the abundance of parasitized trout in the lake; second, the migration
of the parasite into the muscular tissue of its host. The argument
cannot be well summarized in as short space as the requirements of this
paper demand. It is sufficient to say that what appear to me to be
satisfactory explanations are supplied by the peculiar conditions of
distribution of fish in the lakes of this national park. Until three
or four years ago, when the U. S. Fish Commission stocked some of the
lakes and streams of the park, the conditions with relation to fish
life in the three principal lakes were as follows: Shoshone Lake, no
fish of any kind; Heart Lake, at least three species, _Salmo clarki_,
_Leuciscus lineatus_, and _Catostomus ardens_; Yellowstone Lake, one
species, _Salmo clarki_. Shoshone and Yellowstone Lakes are separated
from the river systems which drain them by falls too high for fish to
scale. Heart Lake has no such barrier. The trout of Yellowstone Lake
are confined to the lake and to eighteen miles of river above the
falls. Whatever source of parasitism exists in the lake, therefore,
must continue to affect the fish all their lives. They cannot be going
and coming from the lake as the trout of Heart Lake may freely do. If
their food should contain eggs of parasites, or if the waters in which
they swim should contain eggs or embryos of parasites, they would be
continually exposed to infection, with no chance for a vacation trip
for recuperation. To quote from my report:

"'It follows, therefore, from the peculiar conditions surrounding the
trout of Yellowstone Lake, that if there is a cause of parasitism
present in successive years the trout are more liable to become
infested than they would be in waters where they had a more varied
range. Trout would become infested earlier and in greater relative
numbers, and the life of the parasites themselves--that is, their
residence as encysted worms--must be of longer duration than would
be the rule where the natural conditions are less exceptional....
There are probably not less than one thousand pelicans on the lake
the greater part of the time throughout the summer, of which at any
time not less than 50 per cent. are infested with the adult form of
the parasite, and, since they spend the greater part of their time on
or over the water, disseminate millions of tape-worm eggs each in the
waters of the lake. It is known that eggs of other dibothria hatch out
in the water, where they swim about for some time, looking much like
ciliated infusoria. Donnadieu found in his experiments on the adult
dibothria of ducks that the eggs hatched out readily in warm water
and very slowly in cold. If warm water, at least water that is warmer
than the prevailing temperature of the lake, is needed for the proper
development of these ova, the conditions are supplied in such places
as the shore system of geysers and hot springs on the west arm of the
lake, where for a distance of nearly three miles the shore is skirted
by a hot spring and geyser formation, with numerous streams of hot
water emptying into the lake, and large springs of hot water opening in
the floor of the lake near shore.

"'Trout abound in the vicinity of these warm springs, presumably on
account of the abundance of food there. They do not love the warm
water, but usually avoid it. Several persons with whom I talked on the
subject while in the park assert that diseased fish--that is to say,
those which are thin and affected with flesh worms--are more commonly
found near the warm water; that they take the bait readily but are
logy. I frequently saw pelicans swimming near the shore in the vicinity
of the warm springs on the west arm of the lake. It would appear that
the badly infested or diseased fish, being less active and gamy than
the healthy fish, would be more easily taken by their natural enemies,
who would learn to look for them in places where they most abound. But
any circumstances which cause the pelican and the trout to occupy the
same neighborhood will multiply the chances of the parasites developing
in both the intermediate and final host. The causes that make for the
abundance of the trout parasite conspire to increase the number of
adults. The two hosts react on each other and the parasite profits by
the reaction. About the only enemies the trout had before tourists,
ambitious to catch big strings of trout and photograph them with a
kodak, began to frequent this region, were the fish-eating birds,
and chief among these in numbers and voracity was the pelican. It is
no wonder, therefore, that the trout should have become seriously
parasitized. It may be inferred from the foregoing statements that the
reason why the parasite of the trout of Yellowstone Lake migrates into
the muscular tissue of its host must be found in the fact that the life
of the parasite within the fish is much more prolonged than is the case
where the conditions of life are less exceptional.

"The case just cited is probably the most signal one of direct injury
to the host from the presence of parasites that I have seen. I shall
enumerate more briefly a few additional cases out of a great number
that I have encountered in my special investigations on the entozoa of
fishes for the U. S. Fish Commission."

Many worms of this type abound in codfishes, bluefishes, striped bass,
and other marine fishes, rendering them lean and unfit for food.

=The Heart Lake Tape-worm.=--Another very interesting case of
parasitism is that of the large tape-worm (_Ligula catostomi_)
infecting the suckers, _Catostomus ardens_, in the warm waters of Witch
Creek, near Heart Lake, in the Yellowstone Park. Of this Dr. Linton
gives the following account:

[Illustration: FIG. 230.--Sucker, _Catostomus ardens_ (Jordan &
Gilbert), from Heart Lake, Yellowstone Park, infested by a flatworm,
_Ligula catostomi_ Linton, itself probably a larva of _Dibothrium_.
(After Linton.)]

"In the autumn of 1889 Dr. David Starr Jordan found an interesting
case of parasitism in some young suckers (_Catostomus ardens_) which
he had collected in Witch Creek, a small stream which flows into Heart
Lake, in the Yellowstone National Park. Specimens of these parasites
were sent to me for identification. They proved to be a species of
ligula, probably identical with the European _Ligula simplicissima_
Rud., which is found in the abdominal cavity of the tench. On
account of its larval condition in which it possesses few distinctive
characters, I described it under the name _Ligula catostomi_. These
parasites grow to a very large size when compared with the fish which
harbors them, often filling the abdominal cavity to such a degree as
to give the fish a deceptively plump appearance. The largest specimen
in Dr. Jordan's collection measured, in alcohol, 28.5 centimeters in
length, 8 millimeters in breadth at the anterior end, 11 millimeters
at a distance of 7 millimeters from the anterior end, and 1.5
millimeters near the posterior end. The thickness throughout was about
2 millimeters. The weight of one fish was 9.1 grams, that of its three
parasites 2.5 grams, or 27-1/2 per cent. the weight of the host. If a
man weighing 180 pounds were afflicted with tape-worms to a similar
degree, he would be carrying about with him 50 pounds of parasitic
impedimenta.

"In the summer of 1890 I collected specimens from the same locality. A
specimen obtained from a fish 19 centimeters in length measured while
living 39.5 centimeters in length and 15 millimeters in breadth at
the anterior end. Another fish 15 centimeters in length harbored four
parasites, 12, 13, 13, and 20 centimeters long, respectively, or 58
centimeters aggregate. Another fish 10 centimeters long was infested
with a single parasite which was 39 centimeters in length.

"These parasites were found invariably free in the body cavity. Dr.
Jordan's collections were made in October and mine in July of the
following year. Donnadieu has found that this parasite most frequently
attains its maximum development at the end of two years. It is
probable, therefore, that Dr. Jordan and I collected from the same
generation. Since these parasites, in this stage of their existence,
develop, not by levying a toll on the food of their host, after the
manner of intestinal parasites, but directly by the absorption of
the serous fluid of their host, it is quite evident that they work a
positive and direct injury. Since, however, they lie quietly in the
body cavity of the fish and possess no hard parts to cause irritation,
they work their mischief simply by the passive abstraction of the
nutritive juices of their host, and by crowding the viscera into
confined spaces and unnatural positions. The worms, in almost every
case, had attained such a size that they far exceeded in bulk the
entire viscera of their host.

"From the fact that the examples obtained were of comparatively the
same age, it may be justly inferred that the period of infection to
which the fish are subjected must be a short one. I did not discover
the final host, but it is almost certain to be one or more of the
fish-eating species of birds which visit that region, and presumably
one of which, in its migrations, pays but a brief visit to this
particular locality. This parasite was found only in the young suckers
which inhabit a warm tributary of Witch Creek. They were not found in
the large suckers of the lake. These young _Catostomi_ were found in
a single school, associated with the young of the chub (_Leuciscus
lineatus_), in a stream whose temperature was 95 deg. F. near where it
joined a cold mountain brook whose temperature was 46 deg. F. We seined
several hundred of these young suckers and chubs, ranging in length
from 6 to 19 centimeters. The larger suckers were nearly all infested
with these parasites, the smaller ones not so much, and the smallest
scarcely at all. Or, to give concrete examples: Of 30 fish ranging
in length from 14 to 19 centimeters, only one or two were without
parasites; of 45 specimens averaging about 10 centimeters in length,
15 were infested and 30 were not; of 65 specimens averaging about
9 centimeters in length, 10 were infested and 55 were not; of 62
specimens less than 9 centimeters in length, 2 were infested and 60
were not. None of the chubs were infested with this parasite.

"The conditions under which these fish were found are worthy of passing
notice. The stream which they occupied flowed with rather sluggish
current into a swift mountain stream, which it met almost at right
angles. The school of young chubs and suckers showed no inclination to
enter the cold water, even to escape the seine, but would dart around
the edge of the seine, in the narrow space between it and the bank,
in preference, apparently, to taking to the colder water. When not
disturbed by the seine they would swim up near to the line which marked
the division between the cold and the warm water, and seemed to be
gazing with open mouth and eyes at the trout which occasionally darted
past in the cold stream. The trout appeared to avoid the warm water,
while the chubs and suckers appeared to avoid the cold water. It may
be that what the latter really avoided was the special preserve of the
trout, since large chubs and suckers are found in abundance in the
lake, which is quite cold, a temperature of 40 deg. F. having been taken by
us at a depth of 124 feet.

"Since the eggs of this parasite, after the analogy of closely related
forms, in all probability are discharged into the water from the final
host and hatch out readily in warm water, where they may live for a
longer or shorter time as free-swimming planula-like forms, it will be
observed that the sluggish current and high temperature of the water in
which these parasitized fish occur give rise to conditions which are
highly favorable to infection.

"It may be of passing interest to state here what I have recorded
elsewhere, that ligulae, probably specifically identical with _L.
catostomi_, form an article of food in Italy, where they are sold in
the markets under the name _maccaroni piatti_; also in southern France,
where they are less euphemistically but more truthfully called the _ver
blanc_. So far as my information goes, this diet of worms is strictly
European.

"It is not necessary to prove cases of direct injury resulting from
the presence of parasites in order to make out a case against them. In
the sharp competition which nature forces on fishes in the ordinary
struggle for existence, any factor which imparts an increment either of
strength or of weakness may be a very potent one, and in a long term of
years may determine the relative abundance or rarity of the individuals
of a species. In most cases the interrelations between parasite and
host have become so adjusted that the evil wrought by the parasite
on its host is small. Parasitic forms, like free forms, are simply
developing along the lines of their being, but unlike most free forms
they do not contribute a fair share to the food of other creatures."

=Thorn-head Worms.=--The thorn-head worms called _Acanthocephala_ are
found occasionally in large numbers in different kinds of fishes. They
penetrate the coats of the intestines, producing much irritation and
finally waxy degeneration of the tissues.

According to Linton, there is probably no practical way of
counteracting the bad influences of worms of this order, since their
larval state is passed, in some cases certainly, and in most cases
probably, in small crustacea, which constitute a constant and necessary
source of food for the fish. The same remark which was made in another
connection with regard to the disposal of the viscera of fish applies
here. In no case should the viscera of fish be thrown back into the
water. In this order the sexes are distinct, and the females become at
last veritable sacs for the shelter and nourishment of enormous numbers
of embryos. The importance, therefore, of arresting the development of
as many embryos as possible is at once apparent.

=Nematodes.=--The round worms or nematodes are very especially abundant
in marine fishes, and particularly in the young. The study of these
forms has a large importance to man. Dr. Linton pertinently observes:

"Where there is exhaustive knowledge of the thing itself the
application of that knowledge toward getting good out of it or
averting evil that may come from it first becomes possible. For
example, a knowledge of the life-history of _Trichina spiralis_ and
its pathological effects on its host has taught people a simple way of
securing immunity from its often deadly effects. A knowledge of the
life-histories of the various species of taeniae which infest man and
the domestic animals, frequently to their serious hurt, has made it
possible to diminish their numbers, and may, in time, lead to their
practical extinction.

"So with the parasites of fishes. Whenever for any reason or reasons
parasitism of any sort becomes so prevalent with any species as to
amount to a disease, the remedy will be suggested, and in some cases
may be practically applied. If, for example, it were thought desirable
to counteract the influences which are at work to cause the parasitism
of the trout of Yellowstone Lake, it could be very largely accomplished
by breaking up the breeding-places of the pelican on the islands of
the lake. With regard to parasitism among the marine food-fishes,
the remedy while plainly suggested by the circumstances, might be
difficult of application. Yet something could be done even there,
if it were thought necessary to lessen the amount of parasitism. If
such precautions as the destruction of the parasites which abound
in the viscera of fish before throwing them back into the water,
and if no opportunity be lost of killing those sharks which feed on
the food-fishes, two sources of the prevalence of parasites would be
affected and the sum total of parasitism diminished. These remarks are
made not so much because such precautions are needed as to suggest
possible applications of knowledge which is already available."

=Parasitic Fungi.=--Fishes are often subject to wounds. If not too
serious these will heal in time, with or without scars. Some lost
portions may be restored, but not those including bone fin-rays or
scales. In the fresh waters, wounds are usually attacked by species of
fungus, notably _Saprolegnia ferox_, _Saprolegnia mixta_, and others,
which makes a whitish fringe over a sore and usually causes death.
This fungus is especially destructive in aquaria. This fungus is not
primarily parasitic, but it fixes itself in the slime of a fish or in
an injured place, and once established the animal is at its mercy.
Spent salmon are very often attacked by this fungus. In America the
spent salmon always dies, but in Scotland, where such is not the case,
much study has been given to this plant and the means by which it
may be exterminated. Dr. G. P. Clinton gives a useful account of the
development of _Saprolegnia_, from which we take the following:

"The minute structure and life-history of such fungous forms have been
so thoroughly made out by eminent specialists that no investigation
along this line was made, save to observe those phenomena which might
be easily seen with ordinary microscopic manipulations. The fungus
consists of branched, hyaline filaments, without septa, except as these
are found cutting off the reproductive parts of the threads. It is
made up of a root-like or rhizoid part that penetrates the fish and
a vegetative and reproductive part that radiates from the host. The
former consists of branched tapering threads which pierce the tissues
for a short distance, but are easily pulled out. The function of this
part is to obtain nourishment for the growth of the external parts.
Prostrate threads are found running through the natural slime covering
the fish, and from these are produced the erect radiating hyphae so
plainly seen when in the water. The development of these threads
appears to be very rapid when viewed under the microscope, although
the growth made under favorable conditions in two days is only about
a third of an inch. From actual measurements of filaments of the
fungus placed in water and watched under the microscope, it was found
that certain threads made a growth of about 3000 microns in an hour.
Two others, watched for twenty minutes, gave in that time a growth of
90 and 47 microns respectively; and yet another filament, observed
during two periods of five minutes each, made a growth of 28 microns
each time. In ordinary cultures the rate of growth depends upon the
condition of the medium, host, etc."

[Illustration: FIG. 231.--Quinnat Salmon, _Oncorhynchus tschawytscha_
(Walbaum). Monterey Bay. (Photograph by C. Rutter.)]

Professor H. A. Surface thus speaks of the attacks of _Saprolegnia_ on
the lamprey:

"The attack that attends the end of more lampreys than does any other
is that of the fungus (_Saprolegnia_ sp.). This looks like a gray
slime and eats into the exterior parts of the animal, finally causing
death. It covers the skin, the fins, the eyes, the gill-pouches,
and all parts, like leprosy. It starts where the lamprey has been
scratched or injured or where its mate has held it, and develops very
rapidly when the water is warm. It is found late in the season on all
lampreys that have spawned out, and it is almost sure to prove fatal,
as we have repeatedly seen with attacked fishes or lampreys kept in
tanks or aquaria. With choice aquarium fishes a remedy, or at least a
palliative, is to be found in immersion in salt water for a few minutes
or in bathing the affected parts with listerine. Since these creatures
complete the spawning process before the fungoid attack proves serious
to the individual, it can be seen that it affects no injury to the
race, as the fertilized eggs are left to come to maturity. Also, as it
is nature's plan that the adult lampreys die after spawning once, we
are convinced that death would ensue without the attack of the fungus;
and in fact this is to be regarded as a resultant of those causes that
produce death rather than the immediate cause of it. Its only natural
remedy is to be found in the depths of the lake (450 feet) where there
is a uniform or constant temperature of about 39 deg. Fahr., and where the
light of the noon-day sun penetrates with an intensity only about equal
to starlight on land on a clear but moonless night.

[Illustration: FIG. 232.--Young Male Quinnat Salmon, _Oncorhynchus
tschawytscha_, dying after spawning. Sacramento River. (Photograph by
Cloudsley Rutter.)]

"As light and heat are essential to the development of the fungus,
which is a plant growth and properly called a water mold, and as their
intensity is so greatly diminished in the depth of the lake, it is
probable that if creatures thus attacked should reach this depth they
might here find relief if their physical condition were otherwise
strong enough to recuperate. However, we have recently observed a
distinct tendency on the part of fungus-covered fishes to keep in the
shallower, and consequently warmer, parts of the water, and this of
course results in the more rapid growth of the sarcophytic plant, and
the death of the fishes is thus hastened.

"All kinds of fishes and fish-eggs are subject to the attacks of
such fungus, especially after having been even slightly scratched or
injured. As a consequence, the lamprey attacks on fishes cause wounds
that often become the seat of a slowly spreading but fatal fungus.
We have seen many nests of the bullhead, or horned pout (_Ameiurus
nebulosus_), with all the eggs thus destroyed, and we have found scores
of fishes of various kinds thus killed or dying. It is well known that
in many rivers this is the apparent cause of great mortality among
adult salmon. Yet we really doubt if it ever attacks uninjured fishes
that are in good strong physical condition which have not at least
had the slime rubbed from them when captured. It is contagious, not
only being conveyed from one infested fish to another, but from dead
flies to fishes." (For a further discussion of this subject see an
interesting and valuable Manual of Fish Culture, by the U. S. Fish
Commission, 1897.)

=Earthquakes.=--Occasionally an earthquake has been known to kill
sea-fishes in large numbers. The _Albatross_ obtained specimens
of _Sternoptyx diaphana_ in the Japanese Kuro Shiwo, killed by the
earthquakes of 1896, which destroyed fishing villages of the coast of
Rikuchu in northern Japan.

=Mortality of Tilefish.=--Some years ago in the Gulf Stream off
Newfoundland an immense mortality of the filefish (_Lopholatilus
chamaeleonticeps_) was reported by fishermen. This handsome and large
fish, inhabiting deep waters, died by thousands. For this mortality,
which almost exterminated the species, no adequate cause has been found.

As to the destruction of fresh-water fishes by larger enemies, we may
quote from Professor H. A. Surface. He says there is no doubt that
these three species, the lake lamprey (_Petromyzon marinus unicolor_),
the garpike (_Lepidosteus osseus_), and the mud-puppy (_Necturus
maculosus_), named "in order of destructiveness, are the three most
serious enemies of fishes in the interior of this State [New York],
each of which surely destroys more fishes annually than are caught by
all the fishermen combined. The next important enemies of fishes in
order of destructiveness, according to our observations and belief, are
spawn-eating fishes, water-snakes, carnivorous or predaceous aquatic
insects (especially larvae), and piscivorous fishes and birds." The
lamprey attaches itself to larger fishes, rasping away their flesh and
sucking their blood, as shown in the accompanying plate.

[Illustration: FIG. 233.--Catfishes, _Ameiurus nebulosus_ Le Sueur,
destroyed by lampreys (_Petromyzon marinus unicolor_ De Kay). Cayuga
Lake, N. Y. (Modified from photograph by Prof. H. A. Surface.)]




CHAPTER XX

THE MYTHOLOGY OF FISHES


=The Mermaid.=--A word may be said of the fishes which have no
existence in fact and yet appear in popular literature or in
superstition.

The mermaid, half woman and half fish, has been one of the most
tenacious among these, and the manufacture of their dried bodies from
the head, shoulders, and ribs of a monkey sealed to the body of a fish
has long been a profitable industry in the Orient. The sea-lion, the
dugong, and other marine mammals have been mistaken for mermaids, for
their faces seen at a distance and their movements at rest are not
inhuman, and their limbs and movements in the water are fish-like.

In China, small mermaids are very often made and sold to the curious.
The head and torso of a monkey are fastened ingeniously to the body
and tail of a fish. It is said that Linnaeus was once forced to leave
a town in Holland for questioning the genuineness of one of these
mermaids, the property of some high official. These monsters are still
manufactured for the "curio-trade."

=The Monkfish.=--Many strange fishes were described in the Middle Ages,
the interest usually centering in some supposed relation of their
appearance with the affairs of men. Some of these find their way into
Rondelet's excellent book, "Histoire Entiere des Poissons," in 1558.
Two of these with the accompanying plate of one we here reproduce.
Other myths less interesting grew out of careless, misprinted, or
confused accounts on the part of naturalists and travelers.

"In our times in Norway a sea-monster has been taken after a great
storm, to which all that saw it at once gave the name of monk; for it
had a man's face, rude and ungracious, the head shorn and smooth. On
the shoulders, like the cloak of a monk, were two long fins instead
of arms, and the end of the body was finished by a long tail. The
picture I present was given me by the very illustrious lady, Margaret
de Valois, Queen of Navarre, who received it from a gentleman who gave
a similar one to the emperor, Charles V., then in Spain. This gentleman
said that he had seen the monster as the portrait shows it in Norway,
thrown by the waves and tempests on the beach at a place called Dieze,
near the town called Denelopoch. I have seen a similar picture at Rome
not differing in mien. Among the sea-beasts, Pliny mentions a sea-mare
and a Triton as among the creatures not imaginary. Pausanias also
mentions a Triton."

[Illustration: FIG. 234.--"_Le monstre marin an habit de Moine._"
(After Rondelet.)]

Rondelet further says:

=The Bishop-fish.=--"I have seen a portrait of another sea-monster at
Rome, whither it had been sent with letters that affirmed for certain
that in 1531 one had seen this monster in a bishop's garb, as here
portrayed, in Poland. Carried to the king of that country, it made
certain signs that it had a great desire to return to the sea. Being
taken thither it threw itself instantly into the water."

[Illustration: FIG. 235.--"_Le monstre marin en habit d'Eveque._"
(After Rondelet.)]

=The Sea-serpent.=--A myth of especial persistency is that of the
sea-serpent. Most of the stories of this creature are seaman's yarns,
sometimes based on a fragment of wreck, a long strip of kelp, the power
of suggestion or the incitement of alcohol. But certain of these tales
relate to real fishes. The sea-serpent with an uprearing red mane like
that of a horse is the oarfish (_Regalecus_), a long, slender, fragile
fish compressed like a ribbon and reaching a length of 25 feet. We here
present a photograph of an oarfish (_Regalecus russelli_) stranded
on the California coast at Newport in Orange County, California. A
figure of a European species (_Regalecus glesne_) is also given showing
the fish in its uninjured condition. Another reputed sea-serpent
is the frilled shark (_Chlamydoselachus angineus_), which has been
occasionally noticed by seamen. The struggles of the great killer
(_Orca orca_) with the whales it attacks and destroys has also given
rise to stories of the whale struggling in the embrace of some huge
sea-monster. This description is correct, but the mammal is a monster
itself, a relative of the whale and not a reptile.

[Illustration: FIG. 236.--Oarfish, _Regalecus russelli_, on the beach
at Newport, Orange Co., Cal. (Photograph by C. P. Remsberg.)]

[Illustration: FIG. 237.--Glesnaes Oarfish, _Regalecus glesne_ Ascanius.
Newcastle, England. (After Day.)]

It is often hard to account for some of the stories of the sea-serpent.
A gentleman of unquestioned intelligence and sincerity lately described
to the writer a sea-serpent he had seen at short range, 100 feet long,
swimming at the surface, and with a head as large as a barrel. I do not
know what he saw, but I do know that memory sometimes plays strange
freaks.

Little venomous snakes with flattened tails (_Platyurus, Pelamis_) are
found in the salt bays in many tropical regions of the Pacific (Gulf
of California, Panama, East Indies, Japan), but these are not the
conventional sea-serpents.

Certain slender fishes, as the thread-eel (_Nemichthys_) and the
wolf-eel (_Anarrhichthys_), have been brought to naturalists as young
sea-serpents, but these of course are genuine fishes.

Whatever the nature of the sea-serpent may be, this much is certain,
that while many may be seen, none will ever be caught. The great
swimming reptiles of the sea vanished at the end of Mesozoic time, and
as living creatures will never be known of man.

As a record of the Mythology of Science, we may add the following
remarks of Rafinesque on the imaginary garpike (_Litholepis
adamantinus_), of which a specimen was painted for him by the wonderful
brush of Audubon:

"This fish may be reckoned the wonder of the Ohio. It is only found
as far up as the falls, and probably lives also in the Mississippi. I
have seen it, but only at a distance, and have been shown some of its
singular scales. Wonderful stories are related concerning this fish,
but I have principally relied upon the description and picture given
me by Mr. Audubon. Its length is from 4 to 10 feet. One was caught
which weighed 400 pounds. It lies sometimes asleep or motionless on
the surface of the water, and may be mistaken for a log or snag. It is
impossible to take it in any other way than with the seine or a very
strong hook; the prongs of the gig cannot pierce the scales, which are
as hard as flint, and even proof against lead balls! Its flesh is not
good to eat. It is a voracious fish. Its vulgar names are diamond-fish
(owing to its scales being cut like diamonds), devil-fish, jackfish,
garjack, etc. The snout is large, convex above, very obtuse, the eyes
small and black; nostrils small, round before the eyes; mouth beneath
the eyes, transversal with large angular teeth. Pectoral and abdominal
fins trapezoidal. Dorsal and anal fins equal, longitudinal, with many
rays. The whole body covered with large stone scales, lying in oblique
rows; they are conical, pentagonal pentaedral, with equal sides, from
half an inch to one inch in diameter, brown at first but becoming the
color of turtle-shell when dry. They strike fire with steel and are
ball-proof!"

[Illustration: FIG. 238.--Thread-eel, _Nemichthys avocetta_ Jordan &
Gilbert. Puget Sound.]




CHAPTER XXI

CLASSIFICATION OF FISHES


=Taxonomy.=--Classification, as Dr. Elliott Coues has well said,[147]
is a natural function of "the mind which always strives to make
orderly disposition of its knowledge and so to discover the reciprocal
relations and interdependencies of the things it knows. Classification
presupposes that there do exist such relations, according to which
we may arrange objects in the manner which facilitates their
comprehension, by bringing together what is like and separating what is
unlike, and that such relations are the result of fixed inevitable law.
It is therefore taxonomy (~taxis~, away; ~nomos~, law) or the rational,
lawful disposition of observed facts."

A perfect taxonomy is one which would perfectly express all the
facts in the evolution and development of the various forms. It
would recognize all the evidence from the three ancestral documents,
palaeontology, morphology, and ontogeny. It would consider structure
and form independently of adaptive or physiological or environmental
modifications. It would regard as most important those characters which
had existed longest unchanged in the history of the species or type.
It would regard as of first rank those characters which appear first
in the history of the embryo. It would regard as of minor importance
those which had arisen recently in response to natural selection or the
forced alteration through pressure of environment, while fundamental
alterations as they appear one after another in geologic time would
make the basal characters of corresponding groups in taxonomy. In a
perfect taxonomy or natural system of classification animals would not
be divided into groups nor ranged in linear series. We should imagine
series variously and divergently branched, with each group at its
earlier or lower end passing insensibly into the main or primitive
stock. A very little alteration now and then in some structure is
epoch-making, and paves the way through specialization to a new
class or order. But each class or order through its lowest types is
interlocked with some earlier and otherwise diverging group.

=Defects in Taxonomy.=--A sound system of taxonomy of fishes should
be an exact record of the history of their evolution. But in the
limitations of book-making, this transcript must be made on a flat
page, in linear series, while for centuries and perhaps forever whole
chapters must be left vacant and others dotted everywhere with marks of
doubt. For science demands that positive assertion should not go where
certainty cannot follow. A perfect taxonomy of fishes would be only
possible through the study, by some Artedi, Mueller, Cuvier, Agassiz,
Traquair, Gill, or Woodward, of all the structures of all the fishes
which have ever lived. There are many fishes living in the sea which
are not yet known to any naturalist, many others are known from one or
two specimens, but not yet accessible to students in other continents.
Many are known externally from specimens in bottles or drawings in
books, but have not been studied thoroughly by any one, and the vast
multitude of species have perished in Palaeozoic, Mesozoic, and Tertiary
seas without leaving a tooth or bone or fin behind them. With all this
goes human fallibility, the marring of our records, such as they are,
by carelessness, prejudice, dependence, and error. Chief among these
defects are the constant mistaking of analogy for homology, and the
inability of men to trust their own eyes as against the opinion of the
greater men who have had to form their opinions before all evidence was
in. Because of these defects, the current system of classification is
always changing with each accession of knowledge.

The result is, again to quote from Dr. Coues, "that the natural
classification, like the elixir of life or the philosopher's stone, is
a goal far distant."

=Analogy and Homology.=--_Analogy_, says Dr. Coues, "is the apparent
resemblance between things really unlike--as the wing of a bird and
the wing of a butterfly, as the lungs of a bird and the gills of a
fish. _Homology_ is the real resemblance, or true relation between
things, however different they may appear to be--as the wing of a bird
and the foreleg of a horse, the lungs of a bird and the swim-bladder
of a fish. The former commonly rests upon mere functional, i.e.
physiological, modifications; the latter is grounded upon structural,
i.e., morphological, identity or unity. Analogy is the correlative of
physiology, homology of morphology; but the two may be coincident,
as when structures identical in morphology are used for the same
purposes, and are therefore physiologically identical. Physiological
diversity of structure is incessant, and continually interferes with
morphological identity of structure, to obscure or obliterate the
indications of affinity the latter would otherwise express clearly....
We must be on our guard against those physiological appearances which
are proverbially deceptive!"

"It is possible and conceivable that every animal should have been
constructed upon a plan of its own, having no resemblance whatever
to the plan of any other animal. For any reason we can discover
to the contrary, that combination of natural forces which we term
life might have resulted from, or been manifested by, a series of
infinitely diverse structures; nor would anything in the nature of
the case lead us to suspect a community of organization between
animals so different in habit and in appearance as a porpoise and a
gazelle, an eagle and a crocodile, or a butterfly and a lobster. Had
animals been thus independently organized, each working out its life
by a mechanism peculiar to itself, such a classification as that now
under contemplation would be obviously impossible; a morphological or
structural classification plainly implying morphological or structural
resemblances in the things classified.

"As a matter of fact, however, no such mutual independence of animal
forms exists in nature. On the contrary, the members of the animal
kingdom, from the highest to the lowest, are marvelously connected.
Every animal has something in common with all its fellows--much with
many of them, more with a few, and usually so much with several that it
differs but little from them.

"Now, a morphological classification is a statement of these
gradations of likeness which are observable in animal structures, and
its objects and uses are manifold. In the first place, it strives to
throw our knowledge of the facts which underlie, and are the cause
of, the similarities discerned into the fewest possible general
propositions, subordinated to one another, according to their greater
or less degree of generality; and in this way it answers the purpose of
a _memoria technica_, without which the mind would be incompetent to
grasp and retain the multifarious details of anatomical science."

=Coues on Classification.=--It is obvious that fishes like other
animals may be classified in numberless ways, and as a matter of fact
by numberless men they have been classified in all sorts of fashions.
"Systems," again quoting from Dr. Coues, "have been based on this and
that set of characters and erected from this or that preconception in
the mind of the systematist.... The mental point of view was that every
species of bird (or of fish) was a separate creature, and as much of a
fixture in nature's museum as any specimen in a naturalist's cabinet.
Crops of classifications have been sown in the fruitful soil of such
blind error, but no lasting harvest has been reaped.... The genius
of modern taxonomy seems to be so certainly right, to be tending so
surely even if slowly in the direction of the desired consummation,
that all differences of opinion we hope will soon be settled, and
defect of knowledge, not perversity of mind, is the only obstacle in
the way of success. The taxonomic goal is not now to find the way in
which birds (or other animals) may be most conveniently arranged, but
to discover their pedigree, and so construct their family tree. Such
a genealogical table, or _phylum_ (~phylon~, tribe, race, stock),
as it is called, is rightly considered the only taxonomy worthy the
name--the only true or natural classification. In attempting this end,
we proceed upon the belief that, as explained above, all birds, like
all other animals and plants, are related to each other genetically, as
offspring are to parents, and that to discover their generic relations
is to bring out their true affinities--in other words, to reconstruct
the actual taxonomy of nature. In this view there can be but one
'natural' classification, to the perfecting of which all increase in
our knowledge of the structure of birds infallibly and inevitably
tends. The classification now in use or coming into use is the result
of our best endeavors to accomplish this purpose, and represents what
approach we have made to this end. It is one of the great corollaries
of that theorem of evolution which most naturalists are satisfied has
been demonstrated. It is necessarily a _morphological classification_;
that is, one based solely upon considerations of structure or form
(~morphe~, form, _morphe_), and for the following reasons: Every
offspring tends to take on precisely the form or structure of its
parents, as its natural physical heritage; and the principle involved,
or the _law of heredity_, would, if nothing interfered, keep the
descendants perfectly true to the physical characters of their
progenitors; they would 'breed true' and be exactly alike. But counter
influences are incessantly operative, in consequence of constantly
varying external conditions of environment; the plasticity of
organization of all creatures rendering them more or less susceptible
of modifications by such means, they become unlike their ancestors
in various ways and to different degrees. On a large scale is thus
accomplished, by natural selection and other natural agencies, just
what man does in a small way in producing and maintaining different
breeds of domestic animals. Obviously, amidst such ceaselessly shifting
scenes, degrees of likeness or unlikeness of physical structure
indicate with the greatest exactitude the nearness or remoteness
of organisms in kinship. Morphological characters derived from the
examination of structure are therefore the surest guides we can have to
the blood relationships we desire to establish; and such relationships
are the 'natural affinities' which all classification aims to discover
and formulate."

=Species as Twigs of a Genealogical Tree.=--In another essay Dr. Coues
has compared species of animals to "the twigs of a tree separated
from the parent stem. We name and arrange them arbitrarily in default
of a means of reconstructing the whole tree according to nature's
ramifications." If one had a tree, all in fragments, pieces of twig and
stem, some of them lost, some destroyed, and some not yet separated
from the mass not yet picked over, and wished to place each part where
he could find it, he would be forced to adopt some system of natural
classification. In such a scheme he would lay those parts together
which grew from the same branch. If he were compelled to arrange all
the fragments in a linear series, he would place together those of one
branch, and when these were finished he would begin with another. If
all this were a matter of great importance and extending over years or
over many lifetimes, with many errors to be made and corrected, a set
of names would be adopted--for the main trunk, for the chief branches,
the lesser branches, and on down to the twigs and buds.

A task of this sort on a world-wide scale is the problem of systematic
zoology. There is reason to believe that all animals and plants sprang
from a single stock. There is reasonable certainty that all vertebrate
animals are derived from a single origin. These vertebrate animals
stand related to each other, like the twigs of a gigantic tree of
which the lowermost branches are the aquatic forms to which we give
the name of fishes. The fishes are here regarded as composed of six
classes or larger lines of descent. Each of these, again, is composed
of minor divisions called orders. The different species or ultimate
kinds of animals are grouped in genera. A genus is an assemblage of
closely related species grouped around a central species as type. The
type of a genus is, in common usage, that species with which the name
of the genus was first associated. The name of the genus as a noun,
often with that of the species which is an adjective in signification
if not in form, constitutes the scientific name of the species. Thus
_Petromyzon_ is the genus of the common large lamprey, _marinus_ is
its species, and the scientific name of the species is _Petromyzon
marinus_. _Petromyzon_ means stone-sucker; _marinus_, of the sea, thus
distinguishing it from a species called _fluviatilis_, of the river. In
like fashion all animals and plants are named in scientific record or
taxonomy. Technical names are necessary because vernacular names fail.
Half a million kinds of animals are known, while not half a thousand
vernacular names exist in any language. And these are always loosely
used, half a dozen of them often for the same species, one name often
for a dozen species.

In the same way, whenever we undertake an exact description, we must
use names especially devised for that purpose. We cannot use the same
names for the bones of the head of a fish and those of the head of a
man, for a fish has a different series of bones, and this series is
different with different fishes.

=Nomenclature.=--A family in zoology is an assemblage of related
genera. The name of a family, for convenience, always ends in the
patronymic _idae_, and it is always derived from the leading genus,
that is, the one best known or earliest studied. Thus all lampreys
constitute the family _Petromyzonidae_. An order may contain one or
more families. An order is a division of a larger group; a family an
assemblage of related smaller groups. Intermediate groups are often
recognized by the prefixes sub or super. A subgenus is a division of a
genus. A subspecies is a geographic race or variation within a species;
a super-family a group of allied families. Binomial nomenclature, or
the use of the name of genus and species as a scientific name, was
introduced into science as a systematic method by Linnaeus. In the tenth
edition of his Systema Naturae, published in 1758, this method was first
consistently applied to animals. By common consent the scientific
naming of animals begins with this year, and no account is taken of
names given earlier, as these are, except by accident, never binomial.
Those authors who wrote before the adoption of the rule of binomials
and those who neglected it are alike "ruled out of court." The idea of
genus and species was well understood before Linnaeus, but the specific
name used was not one word but a descriptive phrase, and this phrase
was changed at the whim of the different authors.

[Illustration: FIG. 239.--Horned Trunkfish, Cowfish, or Cuckold,
_Lactophrys tricornis_ (Linnaeus). Charleston, S. C.]

=Nomenclature of Trunkfishes.=--Examples of such names are those of
the West Indian trunkfish, or cuckold (_Ostracion tricorne_, Linnaeus).
Lister refers to a specimen in 1686 as "_Piscis triangularis capiti
cornutu cui e media cauda cutanea aculeus longus erigitus_." This
Artedi alters in 1738 to _Ostracion triangulatus aculeis duobus in
capite et unico longiore superne ad caudam_. This is more accurately
descriptive and it recognizes the existence of a generic type,
_Ostracion_, or trunkfish, to cover all similar fishes. French writers
transformed this into various phrases beginning "Coffre triangulaire
a trois cornes," or some similar descriptive epithet, and in English
or German it was likely to wander still farther from the original. But
Linnaeus condenses it all in the word _tricornis_, which, although not
fully descriptive, is still a name which all future observers can use
and recognize.

It is true that common consent fixes the date of the beginning of
nomenclature at 1758. But to this there are many exceptions. Some
writers date genera from the first recognition of a collective idea
under a single name. Others follow even species back through the
occasional accidental binomials. Most British writers have chosen the
final and completed edition of the Systema Naturae, the last work of
Linnaeus, in 1766, in preference to the earlier volume. But all things
considered, justice and convenience alike seem best served by the use
of the edition of 1758.

=Synonymy and Priority.=--Synonymy is the record of the names applied
at different times to the same group or species. With characteristic
pungency Dr. Coues defines synonymy as "a burden and a disgrace
to science." It has been found that the only way to prevent utter
confusion is to use for each genus or species the first name applied
to it and no other. The first name, once properly given, is sacred
because it is the right name. All other later names whatever their
appropriateness are wrong names. In science, of necessity, a name is a
name without any necessary signification. For this reason and for the
further avoidance of confusion, it remains as it was originally spelled
by the author, obvious misprints aside, regardless of all possible
errors in classical form or meaning. The names in use are properly
written in Latin or in Latinized Greek, the Greek forms being usually
preferred as generic names, the Latin adjectives for names of species.
Many species are named in honor of individuals, these names being
usually given the termination of the Latin genitive, as _Sebastodes
gillii, Liparis agassizi_. In recent custom all specific names are
written with the small initial; all generic names with the capital.

One class of exceptions must be made to the law of priority. No generic
name can be used twice among animals, and no specific name twice in
the same genus. Thus the name _Diabasis_ has to be set aside in favor
of the next name _Haemulon_, because _Diabasis_ was earlier used for a
genus of beetles. The specific name _Pristipoma humile_ is abandoned,
because there was already a _humile_ in the genus _Pristipoma_.

=The Conception of Genus.=--In the system of Linnaeus, a genus
corresponds roughly to the modern conception of a family. Most of the
primitive genera contained a great variety of forms, as well as usually
some species belonging to other groups disassociated from their real
relationships.

As greater numbers of species have become known the earlier genera have
undergone subdivision until in the modern systems almost any structural
character not subject to intergradation and capable of exact definition
is held to distinguish a genus. As the views of these characters are
undergoing constant change, and as different writers look upon them
from different points of view, or with different ideas of convenience,
we have constant changes in the boundaries of genera. This brings
constant changes in the scientific names, although the same specific
name should be used whatever the generic name to which it may be
attached. We may illustrate these changes and the burden of synonymy as
well by a concrete example.

=The Trunkfishes.=--The horned trunkfish, or cuckold, of the West
Indies was first recorded by Lister in 1686, in the descriptive phrase
above quoted. Artedi, in 1738, recognized that it belonged with other
trunkfishes in a group he called _Ostracion_. This, to be strictly
classic, he should have written _Ostracium_, but he preferred a partly
Greek form to the Latin one. In the Nagg's Head Inn in London, Artedi
saw a trunkfish he thought different, having two spines under the tail,
while Lister's figure seemed to show one spine above. This Nagg's Head
specimen Artedi called "_Ostracion triangulatus duobus aculeis in
fronte et totidem in imo ventre subcaudalesque binis_."

Next came Linnaeus, 1758, who named Lister's figure and the species it
represented, _Ostracion tricornis_, which should in strictness have
been _Ostracion tricorne_, as ~ostrakion~, a little box, is a neuter
diminutive. The Nagg's Head fish he named _Ostracion quadricornis_. The
right name now is _Ostracion tricornis_, because the name _tricornis_
stands first on the page in Linnaeus' work, but _Ostracion quadricornis_
has been more often used by subsequent authors because it is more
truthful as a descriptive phrase. In 1798, Lacepede changed the name of
Lister's fish to _Ostracion listeri_, a needless alteration which could
only make confusion.

[Illustration: FIG. 240.--Horned Trunkfish, _Ostracion cornutum_
Linnaeus. East Indies. (After Bleeker.)]

In 1818, Dr. Samuel Latham Mitchill, receiving a specimen from below
New Orleans, thought it different from _tricornis_ and _quadricornis_
and called it _Ostracion sexcornutus_; Dr. Holard, of Paris, in
1857, named a specimen _Ostracion maculatus_, and at about the same
time Bleeker named two others from Africa which seem to be the same
thing, _Ostracion guineensis_ and _Ostracion gronovii_. Lastly, Poey
calls a specimen from Cuba _Acanthostracion polygonius_, thinking it
different from all the rest, which it may be, although my own judgment
is otherwise. This brings up the question of the generic name. Among
trunkfishes there are four-angled and three-angled kinds, and of each
form there are species with and without horns and spines. The original
_Ostracion_ of Linnaeus we may interpret as being _Ostracion cubicus_ of
the coasts of Asia, a species similar to the _Ostracion rhinorhynchus_.
This species, _cubicus_, we call the type species of the genus, as the
Nagg's Head specimen of Artedi was the type specimen of the species
_quadricornus_, and the one that was used for Lister's figure the type
specimen of _tricornis_.

_Ostracion cubicus_ is a four-angled species, and when the trunkfishes
were regarded as a family (_Ostraciidae_), the three-angled ones
were set off as a separate genus. For this two names were offered,
both by Swainson in 1839. For _trigonus_, a species without horns
before the eyes, he gave the name _Lactophrys_, and for _triqueter_,
a species without spines anywhere, the name of _Rhinesomus_. Most
recent American authors have placed the three-cornered species which
are mostly American in one genus, which must therefore be called
_Lactophrys_. Of this name _Rhinesomus_ is a synonym, and our species
should stand as _Lactophrys tricornis_. The fact that _Lactophrys_ as
a word (from Latin _laetus_, smooth; Greek ~ophrys~, eyebrow; or else
from _lactoria_, a milk cow, and ~ophrys~) is either meaningless or
incorrectly written makes no difference with the necessity for its use.

[Illustration: FIG. 241.--Spotted Trunkfish, _Lactophrys bicaudalis_
(Linnaeus). Cozumel Island, Yucatan.]

[Illustration: FIG. 242.--Spotted Trunkfish (face view), _Lactophrys
bicaudalis_ (Linnaeus).]

In 1862, Bleeker undertook to divide these fishes differently. Placing
all the hornless species, whether three-angled or four-angled, in
_Ostracion_, he proposed the name _Acanthostracion_ for the species
with horns, _tricornis_ being the type. But _Acanthostracion_ has
not been usually adopted except as the name of a section under
_Lactophrys_. The three-angled American species are usually set apart
from the four-angled species of Asia, and our cuckold is called
_Lactophrys tricornis_. But it may be with perfect correctness called
_Ostracion tricorne_, in the spirit called conservative. Or with
the "radical" systematists we may accept the finer definition and
again correctly call it _Acanthostracion tricorne_. But to call it
_quadricornis_ or _listeri_ or _maculatus_ with any generic name
whatever would be to violate the law of priority.

[Illustration: FIG. 243.--Spineless Trunkfish, _Lactophrys triqueter_
(Linnaeus). Tortugas.]

=Trinomial Nomenclature.=--By trinomial nomenclature we mean the
use of a second subordinate specific name to designate a geographic
subspecies, variety, or other intergrading race. Thus _Salmo clarki
virginalis_ indicates the variety of Clark's trout, or the cut-throat
trout, found in the lakes and streams of the Great Basin of Utah,
as distinguished from the genuine _Salmo clarkii_ of the Columbia.
Trinomials are not much used among fishes, as we are not yet able to
give many of the local forms correct and adequate definition such as
is awarded to similar variations among birds and mammals. Usually
varieties in ichthyology count as species or as nothing.

[Illustration: FIG. 244.--Hornless Trunkfish, _Lactophrys trigonus_
(Linnaeus). Tortugas, Florida.]

[Illustration: FIG. 245.--Hornless Trunkfish (face-view), _Lactophrys
trigonus_ (Linnaeus). Charleston, S. C.]

=Meaning of Species.=--Quoting once more from the admirable essay
of Dr. Coues on the taxonomy of birds: "The student cannot be too
well assured that no such things as species, in the old sense of
the word, exist in nature any more than have genera or families an
actual existence. Indeed they cannot be, if there is any truth in
the principles discussed in our earlier paragraphs. Species are
simply ulterior modifications, which once were, if they be not still,
inseparably linked together; and their nominal recognition is a
pure convention, like that of a genus. More practically hinges upon
the way we regard them than turns upon our establishment of higher
groups, simply because upon the way we decide in this case depends the
scientific labeling of specimens. If we are speaking of a robin, we do
not ordinarily concern ourselves with the family or order it belongs
to, but we do require a technical name for constant use. That name
is compounded of its genus, species, and variety. No infallible rule
can be laid down for determining what shall be held to be a species,
what a conspecies, subspecies, or variety. It is a matter of tact and
experience, like the appreciation of the value of any other group
in zoology. There is, however, a convention upon the subject, which
the present workers in ornithology in this country find available;
at any rate we have no better rule to go by. We treat as "specific"
any form, however little different from the next, that we do not
know or believe to intergrade with that next one, between which and
the next one no intermediate equivocal specimens are forthcoming,
and none, consequently, are supposed to exist. This is to imply that
differentiation is accomplished, the links are lost and the characters
actually become "specific." We treat as "varietal" of each other any
forms, however different in their extreme manifestation, which we know
to intergrade, having the intermediate specimens before us, or which we
believe with any good reason do intergrade. If the links still exist,
the differentiation is still incomplete, and the characters are not
specific, but only varietal, in the literal sense of these terms."

=Generalization and Specialization.=--A few terms in common use
may receive a moment's discussion. A type or group is said to be
specialized when it has a relatively large number of peculiarities
or when some one peculiarity is carried to an extreme. A sculpin is
a specialized fish having many unusual phases of development, as is
also a swordfish, which has a highly peculiar structure in the snout.
A generalized type is one with fewer peculiarities, as the herring in
comparison with the sculpin. In the process of evolution generalized
types usually give place to specialized ones. Generalized types are
therefore as a rule archaic types. The terms high and low are also
relative, a high type being one with varied structure and functions.
Low types may be primitively generalized, as the lancelet in comparison
with all other fishes, or the herring in comparison with the perch, or
they may be due to degradation, a loss of structures which have been
elaborately specialized in their ancestry. The sea-snail (_Liparis_),
an ally of the sculpin, with scales lost and fins deteriorated is an
example of a low type which is specialized as well as degraded.

=High and Low Forms.=--In the earlier history of ichthyology much
confusion resulted from the misconception of the terms "high" and
"low." Because sharks appeared earlier than bony fishes, it was assumed
that they should be lower than any of their subsequent descendants.
That the brain and muscular system in sharks was more highly developed
than in most bony fishes seemed also certain. Therefore it was thought
that the teleost series could not have had a common origin with the
series of sharks. It is now understood that evolution means chiefly
adaptation. The teleost is adapted to its mode of life, and to that end
it is specialized in fin and skeleton rather than in brain and nerves.
All degeneration is associated with specialization. The degeneration
of the blindfish is a specialization for better adaptation to life in
the darkness of caves; the degeneration of the deep-sea fish meets the
demands of the depths, the degeneration of the globefish means the
sinking of one line of functions in the extension of some other.

Referring to his own work on the fossil fishes in the early forties,
Professor Agassiz once said to the writer: "At that time I was on the
verge of anticipating the views of Darwin, but it seemed to me that the
facts were contrary to the theories of evolution. We had the highest
fishes first." This statement leads us to consider what is meant by
high and low. Undoubtedly the sharks are higher than the bony fishes in
the sense of being nearer to the higher vertebrates. In brain, muscle,
teeth, and reproductive structures they are also more highly developed.
In all skeletal and cranial characters the sharks stand distinctly
lower. But the essential fact, so far as evolution is concerned, is
not that the sharks are high or low. They are, in almost all respects,
distinctly generalized and primitive. The bony fishes are specialized
in various ways through adaptation to the various modes of life they
lead. Much of this specialization involves corresponding degeneration
of organs whose functions have ceased to be important. As a broad
proposition it is not true that "we had our highest fishes first," for
in a complete definition of high and low, the specialized perch or bass
stands higher. But whether true or not, it does not touch the question
of evolution which is throughout a process of adaptation to conditions
of life.

Referring to the position of Agassiz and his early friend and disciple,
Hugh Miller, Dr. Traquair (1900) uses these words in an address at
Bradford, England:

"It cannot but be acknowledged that the paleontology of fishes is not
less emphatic in the support of descent than that of any other division
of the animal kingdom. But in former days the evidence of fossil
ichthyology was by some read otherwise.

"It is now a little over forty years since Hugh Miller died: he who
was one of the first collectors of the fossil fishes of the Scottish
old red sandstone, and who knew these in some respects better than any
other man of his time, not excepting Agassiz himself. Yet his life was
spent in a fierce denunciation of the doctrine of evolution, then only
in its Lamarckian form, as Darwin had not yet electrified the world
with his 'Origin of Species.' Many a time I wonder greatly what Hugh
Miller would have thought had he lived a few years longer, so as to
have been able to see the remarkable revolution which was wrought by
the publication of that book.

"The main argument on which Miller rested was the 'high' state of
organization of the ancient fishes of the Paleozoic formations, and
this was apparently combined with a confident assumption of the
completeness of the geological record. As to the first idea, we know
of course that evolution means the passage from the more general to
the more special, and that as the general result an onward advance has
taken place; yet 'specialization' does not always or necessarily mean
'highness' of organization in the sense in which the term is usually
employed. As to the idea of the perfection of the geological record,
that of course is absurd.

"We do not and cannot know the oldest fishes, as they would not have
had hard parts for preservation, but we may hope to come to know
many more old ones, and older ones still than we do at present. My
experience on the subject of fossil ichthyology is that it is not
likely to become exhausted in our day.

"We are introduced at a period far back in geological history to
certain groups of fishes, some of which certainly are high in
organization as animals, but yet of generalized type, being fishes
and yet having the potentiality of higher forms. But because their
ancestors are unknown to us, that it is no evidence that they did
not exist, and cannot overthrow the morphological testimony in favor
of evolution with which the record actually does furnish us. We may
therefore feel very sure that fishes or 'fish-like vertebrates' lived
long ages before the oldest forms with which we are acquainted came
into existence.

"The modern type of bony fishes, though not so 'high' in many
anatomical points as that of the Selachii, Crossopterygii, Dipnoi,
Acipenseroidei, and Lepidosteoidei of the Palaeozoic and Mesozoic
eras, is more specialized in the direction of the fish proper, and,
as already indicated, specialization and 'highness' in the ordinary
sense of the word are not necessarily coincident. But ideas about these
things have undergone a wonderful change since those pre-Darwinian
days, and though we shall never be able fully to unravel the problems
concerning the descent of animals, we see many things a great deal more
clearly now than we did then."

Dr. Gill observes: "Perhaps there are no words in science that have
been productive of more mischief and more retarded the progress of
biological taxonomy than those words pregnant with confusion, High and
Low, and it were to be wished that they might be erased from scientific
terminology. They deceive the person to whom they are addressed. They
insensibly mislead the one who uses them. Psychological prejudices
and fancies are so inextricably associated with these words that the
use of them is provocative of such ideas. The words, generalized and
specialized, having become almost limited to the expression of the
ideas which the scientific biologist wishes to unfold by the others,
can with great gain be employed in their stead." ("Families of Fishes,"
1872.)

=The Problem of the Highest Fishes.=--As to which fishes should be
ranked highest and which lowest, Dr. Gill gives ("Families of Fishes,"
1872) the following useful discussion: "While among the mammals there
is almost universal concurrence as to the forms entitled to the first
as well as the last places, naturalists differ much as to the 'highest'
of the ichthyoid vertebrates, but are all of one accord respecting the
form to be designated as the 'lowest.' With that admitted lowest form
as a starting-point, inquiry may be made respecting the forms which are
successively _most nearly related_.

"No dissent has ever been expressed from the proposition that the
Leptocardians (_Branchiostoma_) are the lowest of the vertebrates;
while they have doubtless deviated much from the representatives of the
immediate line of descent of the higher vertebrates, and are probably
specialized considerably, in some respects, in comparison with those
vertebrates from which they (in common with the higher forms) have
descended, they undoubtedly have diverged far less, and furnish a
better hint as to the protovertebrates than any other form.

"Equally undisputed it is that most nearly related to the Leptocardians
are the Marsipobranchiates (_Lampreys_, etc.), and the tendency has
been rather to overlook the fundamental differences between the two,
and to approximate them too closely, than the reverse.

"But here unanimity ends, and much difference of opinion has prevailed
with respect to the succession in the system of the several
subclasses (by whatever name called) of true fishes: (1) Some (e.g.,
Cuvier, J. Mueller, Owen, Luetken, Cope) arranging next to the lowest
the Elasmobranchiates, and, as successive forms, the Ganoids and
Teleosteans; (2) while others (e.g., Agassiz, Dana, Dumeril, Guenther)
adopt the sequence Leptocardians, Marsipobranchiates, Teleosteans,
Ganoids, and Elasmobranchiates. The source of this difference
of opinion is evident and results partly from metaphysical or
psychological considerations, and partly from those based (in the case
of the Ganoids) on real similarities and affinities.

"The evidence in favor of the title of the Elasmobranchiates to the
'highest' rank is based upon (1) the superior development of the brain;
(2) the development of the egg, and the ovulation; (3) the possession
of a placenta; and (4) the complexity of the organs of generation.

"(1) It has not been definitely stated wherein the superior development
of the brain consists, and as it is not evident to the author, the
vague claim can only be met by this simple statement; it may be added,
however, that the brains comparable in essentials and most similar as
a whole to those of the Marsipobranchiates are those of the sharks. In
answer to the statement that the sharks exhibit superior intelligence,
and thus confirm the indications of cerebral structure, it may be
replied that the impression is a subjective one, and the author has
not been thus influenced by his own observations of their habits.
Psychological manifestations, at any rate, furnish too vague criteria
to be available in exact taxonomy.

"(2) If the development of the eggs, their small number, and their
investment in cases are arguments in favor of the high rank of the
Elasmobranchiates, they are also for the Marsipobranchiates, and
thus prove too much or too little for the advocates of the views
discussed. The variation in number of progeny among true fishes (e.g.,
Cyprinodonts, _Embiotocids_) also demonstrates the unreliability of
those modifications _per se_.

"(3) The so-called placenta of some Elasmobranchiates may be
_analogous_ to that of mammals, but that it is not _homologous_
(i.e., homogenetic) is demonstrable from the fact that all the forms
intervening between them and the specialized placental mammals are
devoid of a placenta, and by the variation (presence or want) among the
Elasmobranchiates themselves.

"(4) The organs of generation in the Elasmobranchiates are certainly
more complex than in most other fishes, but as the complexity results
from specialization of parts _sui generis_ and different from those of
the higher (quadruped) vertebrates, it is not evident what bearing the
argument has. If it is claimed simply on the ground of specialization,
irrespective of homological agreement with admitted higher forms,
then are we equally entitled to claim any specialization of parts
as evidence of high rank, or at least we have not been told within
what limits we should be confined. The Cetaceans, for example, are
excessively specialized mammals, and, on similar grounds, would rank
above the other mammals and man; the aye-aye exhibits in its dentition
excessive specialization and deviation from the primitive type (as
exhibited in its own milk teeth) of the Primates, and should thus also
rank above man. It is true that in other respects the higher primates
(even including man) may be more specialized, but the specialization is
not as obvious as in the cases referred to, and it is not evident how
we are to balance _irrelative_ specializations against each other, or
even how we shall subordinate such cases. We are thus compelled by the
_reductio ad absurdum_ to the confession that irrelative specialization
of single organs is untrustworthy, and are fain to return to that
better method of testing affinities by the equation of agreement in
whole and after the elimination of special teleological modifications.

"The question then recurs, What forms are the most _nearly allied_
to the Marsipobranchiates, and what show the closest approach in
_characteristic_ features? And in response thereto the evidence is
not undecisive. Wide as is the gap between Marsipobranchiates and
fishes, and comparatively limited as is the range of the latter among
themselves, the Elasmobranchiates are very appreciably more like, and
share more characters in common with them, than any other; so much is
this the case that some eminent naturalists (e.g., Pallas, Geoffroy,
St. Hilaire, Latreille, Agassiz, formerly Luetken) have combined the
two forms in a peculiar group, contradistinguished from the other
fishes. The most earnest and extended argument in English, in favor of
this combination has been published by Professor Agassiz in his 'Lake
Superior,' but that eminent naturalist subsequently arrived at the
opposite conclusions already indicated.

"The evidences of the closer affinity of the Elasmobranchiates (than
of any other fishes) with the Marsipobranchiates are furnished by (1)
the cartilaginous condition of the skeleton; (2) the post-cephalic
position of the branchiae; (3) the development of the branchiae and their
restriction to special chambers; (4) the larger number of the branchiae;
(5) the imperfect development of the skull; (6) the mode of attachment
of the teeth; (7) the slight degree of specialization of the rays of
the fins; and (8) the rudimentary condition of the shoulder-girdle."

FOOTNOTES:

[147] Key to North American Birds.




CHAPTER XXII

THE HISTORY OF ICHTHYOLOGY


Science consists of human experience, tested and placed in order. The
science of ichthyology represents our knowledge of fishes, derived
from varied experiences of man, tested by methods or instruments of
precision and arranged in orderly sequence. This science, in common
with every other, is the work of many persons, each in his own
field, and each contributing a series of facts, a series of tests of
the alleged facts of others, or some improvement in the method of
arrangement. As in other branches of science, this work has been done
by sincere, devoted men, impelled by a love for this kind of labor, and
having in view, as "the only reward they asked, a grateful remembrance
of their work." And in token of this reward it is well sometimes,
in grateful spirit, to go over the names of those who made even its
present stage of completeness possible.

We may begin the history of ichthyology with that of so many others of
the sciences, with the work of Aristotle (383-322 B.C.). This wonderful
observer recorded many facts concerning the structure and habits of the
fishes of Greece, and in almost every case his actual observation bears
the closest modern test. These observations were hardly "set in order."
The number of species he knew was small, about 118 in all, and it did
not occur to him that they needed classification. His ideas of species
were those of the fishermen, and the local vernacular supplied him with
the only names needed in his records.

As Dr. Guenther wisely observes, "It is less surprising that Aristotle
should have found so many truths as that none of his followers should
have added to them." For nearly 1800 years the scholars of the times
copied the words of Aristotle, confusing them by the addition of
fabulous stories and foolish superstitions, never going back to nature
herself, "who leads us to absolute truth whenever we wander." A few
observations were made by Caius Plinius, Claudius AElianus, Athenaeus and
others. Theophrastus (370-270 B.C.) wrote on the fishes which may live
out of water. About 400 A.D., Decius Magnus Ausonius wrote a pleasing
little poem on the Moselle, setting forth the merits of its various
fishes. It was not, however, until the middle of the seventeenth
century that any advance was made in the knowledge of fishes. At that
time the development of scholarship among the nations of Europe was
such that a few wise men were able to grasp the idea of species.

In 1553, Pierre Belon (1518-64) published his octavo volume of 448
pages, entitled "De Aquatilibus," in which numerous (110) species of
fishes of the Mediterranean were described, with tolerable figures,
and with these is a creditable attempt at classification. At about
this time Ulysses Aldrovandi, of Bologna, founded the first museum
of natural history and wrote on the fishes it contained. In 1554-58,
Ippolito Salviani (1513-72), a physician at Rome, published a work
entitled "Aquatilium Animalium Historia," with good figures of most of
the species, together with much general information as to the value and
habits of animals of the sea.

More important than these, but almost simultaneous with them, is the
great work of Guillaume Rondelet (1507-57), "De Piscibus Marinus"
(1554-55), at first written in Latin, later translated into French
and enlarged under other titles. In this work, 244 different species,
chiefly from the Mediterranean, are fairly described, and the
various fables previously current are subjected to severe scrutiny.
Recognizable woodcuts represent the different species. Classification,
Rondelet had none, except as simple categories for purposes of
convenience. More than usual care is given to the vernacular names,
French and Greek. He closes his book with these words:

"Or s'il en i a qui prennent les choses tant a la rigueur, qui ne
veulent rien apparouver qui ne soit du tout parfait, je les prie
de bien bon cueur de traiter telle, ou quelque autre histoire
parfaitement, sans qu'il i ait chose quelconque a redire et la
receverons e haut louerons bien vouluntiers. Cependant je scai bien,
et me console . . . avec grand travail . . . qu'on pourra trouver
plusieurs bones choses e dignes de louange ou proufit e contentement
des homes studieux e a l'honneur e grandissime admiration des tres
excellens e perfaits oeuvres de Dieu."

And with the many "bones choses" of the work of Rondelet, men were
too long satisfied, and it was not until the impulse of commerce had
brought them face to face with new series of animals not found in the
Mediterranean that the work of investigating fishes was again resumed.
About 1640, Prince Moritz (Maurice) of Nassau (1604-79) visited Brazil,
taking with him two physicians, Georg Marcgraf (1610-44) and Wilhelm
Piso. In the great work "Historia Naturalis Brasiliae," published at
Leyden (1648), Marcgraf described about one hundred species, all new
to science, under Portuguese names and with a good deal of spirit and
accuracy. This work was printed by Piso after Marcgraf's death, and his
 drawings--long afterward used by Bloch--are in the "History of
Brazil" reduced to small and crude woodcuts. This is the first study
of a local fish fauna outside the Mediterranean region and it reflects
great credit on Marcgraf and on the illustrious prince whose assistant
he was.

There were no other similar attempts of importance in ichthyology for
a hundred years, when Per Osbeck, an enthusiastic student of Linnaeus,
published (1757) the records of his cruise to China, under the name of
"Iter Chinensis." At about the same time another of Linnaeus' students,
Fredrik Hasselquist, published, in his "Iter Palestinum" the account
of his discoveries of fishes in Palestine and Egypt. More pretentious
than these and of much value as an early record is Mark Catesby's
(1679-1749) "Natural History of Carolina and the Bahamas," published
in 1749, with large  plates which are fairly correct except in
those cases in which the drawing was made from memory.

At about the same time, Hans Sloane (1660-1752) published his large
volume on the "Fishes of Jamaica," Patrick Browne (1720-90) wrote on
the fishes of the same region, while Father Charles Plumier (1646-1704)
made paintings of the fishes of Martinique, long after used by Bloch
and Lacepede. Dr. Alexander Garden (1730-91), of Charleston, S. C.,
collected fishes for Linnaeus, as did also Dr. Pehr Kalm in his travels
in the northern parts of the American colonies.

With the revival of interest in general anatomy several naturalists
took up the structure of fishes. Among these Guenther mentions Borelli,
Malpighi, Swammerdam, and Duverney. Other anatomists of later dates
were Albrecht von Heller (1708-77), Peter Camper (1722-89), Felix Vicq
d'Azyr (1748-94), and Alexander Monro (1783).

The basis of classification was first fairly recognized by John Ray
(1628-1705) and Francis Willughby (1635-72), who, with other and varied
scientific labors, undertook, in the "Historia Piscium," published
in Oxford in 1686, to bring order out of the confusion left by their
predecessors. This work, edited by Ray after Willughby's death, is
ostensibly the work of Willughby with additions by Ray. In this work
420 species were recorded, 180 of which were actually examined by the
authors, and the arrangement chosen by them pointed the way to a final
system of nomenclature.

Direct efforts in this direction, with a fairly clear recognition of
genera as well as species, were made by Lorenz Theodor Gronow, called
Gronovius, a German naturalist of much acumen, and by Jacob Theodor
Klein (1685-1757), whose work, "Historic Naturalis Piscium," published
about 1745, is of less importance, not being much of an advance over
the catalogue of Rondelet.

Far greater than any of these investigators, and earlier than either
Klein or Gronow, was he who has been justly called the Father of
Ichthyology, Petrus (Peter) Artedi (1705-35). Artedi was born in
Sweden. He was a fellow student of Linnaeus at Upsala, and he devoted
his short life wholly to the study of fishes. He went to Holland to
examine the collection of East and West Indian fishes of a rich Dutch
merchant in Amsterdam named Albert Seba, and there at the age of
twenty-nine he was, by accident, drowned in one of the Dutch canals.
"His manuscripts were fortunately rescued by an Englishman, Cliffort,"
and they were edited and published by Linnaeus in a series of five parts
or volumes.

Artedi divided the class of fishes into orders, and these orders
again into genera, the genera into species. The name of each species
consisted of that of the genus with a descriptive phrase attached. This
cumbersome system, called polynomial, used by Artedi, Gronow, Klein,
and others, was a great advance on the shifting vernacular, of which
it now took the place. But the polynomial method as a system was of
short duration. Linnaeus soon substituted for it the convenient, in fact
inevitable binomial system which has now endured for 150 years, and
which with certain modifications must form the permanent substructure
of the nomenclature in systematic zoology and botany.

The genera of Artedi are in almost all cases natural groups,
corresponding essentially equivalent to the families of to-day.
Families in ichthyology were first clearly recognized and defined by
Cuvier.

The following is a list of Artedi's genera and their arrangement:

ORDER MALACOPTERYGII.

  _Syngnathus_ (pipefishes) (4 species).
  _Cobitis_ (loaches) (3).
  _Cyprinus_ (carp and dace) (19).
  _Clupea_ (herrings) (4).
  _Argentina_ (argentines) (1).
  _Exocoetus_ (flying-fishes) (2).
  _Coregonus_ (whitefishes) (4).
  _Osmerus_ (smelts) (2).
  _Salmo_ (salmon and trout) (10).
  _Esox_ (pike) (3).
  _Echeneis_ (remoras) (1).
  _Coryphaena_ (dolphins) (3).
  _Ammodytes_ (sand-launces) (1).
  _Pleuronectes_ (flounders) (10).
  _Stromateus_ (butter-fishes) (1).
  _Gadus_ (codfishes) (11).
  _Anarhichas_ (wolf-fishes) (1).
  _Muraena_ (eels) (6).
  _Ophidion_ (cusk-eels) (2).
  _Anableps_ (four-eyed fish) (1).
  _Gymnotus_ (carapos) (1).
  _Silurus_ (catfishes) (1).

ORDER ACANTHOPTERYGII.

  _Blennius_ (blennies) (5).
  _Gobius_ (gobies) (4).
  _Xiphias_ (swordfishes) (1).
  _Scomber_ (mackerels) (5).
  _Mugil_ (mullets) (1).
  _Labrus_ (wrasses) (9).
  _Sparus_ (porgies) (15).
  _Sciaena_ (croakers) (2).
  _Perca_ (perch and bass) (7).
  _Trachinus_ (weavers) (2).
  _Trigla_ (gurnards) (10).
  _Scorpaena_ (scorpion-fishes) (2).
  _Cottus_ (sculpins) (5).
  _Zeus_ (john dories, etc.) (3).
  _Chaetodon_ (butterfly-fishes) (4).
  _Gasterosteus_ (sticklebacks) (3).
  _Lepturus_ (cutlass-fishes) (=_Trichiurus_) (1).

ORDER BRANCHIOSTEGI.

  _Balistes_ (trigger-fishes) (6).
  _Ostracion_ (trunkfishes) (22).
  _Cyclopterus_ (lumpfishes) (1).
  _Lophius_ (anglers) (1).

ORDER CHONDROPTERYGII.

  _Petromyzon_ (lampreys) (3).
  _Acipenser_ (sturgeons) (2).
  _Squalus_ (sharks) (14).
  _Raja_ (rays) (11).

In all 47 genera and 230 species of fishes were known from the whole
world in 1738.

The cetaceans, or whales, constitute a fifth order, Plagiuri, in
Artedi's scheme.

As examples of the nomenclature of species I may quote:

"_Zeus ventre aculeato, cauda in extremo circinata._" This polynomial
expression was shortened by Linnaeus to _Zeus faber_. The species was
called by Rondelet "_Faber sive Gallus Marinus_" and by other authors
"_Piscis Jovii_." "Jovii" suggested _Zeus_ to Artedi, and Rondelet's
name _faber_ became the specific name.

"_Anarhichas Lupus marinus nostras._" This became with Linnaeus
"_Anarhichas lupus_."

"_Clupea, maxilla inferiore longiore, maculis nigris carens: Harengus
vel Chalcis Auctorum, Herring vel Hering Anglis, Germanis Belgis._"
This became _Clupea harengus_ in the convenient binomial system of
Linnaeus.

The great naturalist of the eighteenth century, Carl von Linne,
known academically as Carolus Linnaeus, was the early associate and
close friend of Artedi, and from Artedi he obtained practically all
his knowledge of fishes. Linnaeus, professor in the University of
Upsala and for a time its rector, primarily a botanist, was a man of
wonderful erudition, and his great strength lay in his skill in the
orderly arrangement of things. In his lifetime, his greatest work, the
"Systema Naturae," passed through twelve editions. In the tenth edition,
in 1758, the binomial system of nomenclature was first consistently
applied to all animals. For this reason most naturalists use the
date of its publication as the beginning of zoological nomenclature,
although the English naturalists have generally preferred the more
complete twelfth edition, published in 1766. This difference in the
recognized starting-point has been often a source of confusion, as in
several cases the names of species were needlessly changed by Linnaeus
and given differently in the twelfth edition. In taxonomy it is not
nearly so important that a name be pertinent or even well chosen as
that it be stable. In changing his own established names, the father of
classification set a bad example to his successors, one which they did
not fail to follow.

In Linnaeus' system (tenth and twelfth editions) all of Artedi's genera
were retained save _Lepturus_, which name was changed to _Trichiurus_.
The following new genera were added: _Chimaera_, _Tetraodon_,
_Diodon_, _Centriscus_, _Pegasus_, _Callionymus_, _Uranoscopus_,
_Cepola_, _Mullus_, _Teuthis_, _Loricaria_, _Fistularia_, _Atherina_,
_Mormyrus_, _Polynemus_, _Amia_, _Elops_. The classification was
finally much altered: the Chondropterygia and Branchiostegi (with
_Syngnathus_) being called _Amphibia Nantes_, and divided into two
groups--_Spiraculis compositis_ and _Spiraculis solitariis_. The other
fishes were more naturally distributed according to the position of the
ventral fins into Pisces Apodes, Jugulares, Thoracici, and Abdominales.
The Apodes of Linnaeus do not form a homogeneous group, as members of
various distinct groups have lost their ventral fins in the process of
evolution. But the Jugulares, the Thoracici, and the Abdominales must
be kept as valid categories in any natural system.

Linnaeus' contributions to zoology consisted mainly of the introduction
of his most ingenious and helpful system of bookkeeping. By it
naturalists of all lands were able to speak of the same species by the
same name in whatever tongue. Unfortunately, ignorance, carelessness,
and perversity brought about a condition of confusion. For a long
period many species were confounded under one name. This source of
confusion began with Linnaeus himself. On the other hand, even with
Linnaeus, the same species often appeared under several different names;
in this matter it was not the system of naming which was at fault. It
was the lack of accurate knowledge, and sometimes the lack of just
and conscientious dealing with the work of other men. No system of
naming can go beyond the knowledge on which it rests. Ignorance of
fact produces confusion in naming. The earlier naturalists had no
conception of the laws of geographical distribution. The "Indies," East
or West, were alike to them, and "America" or "India" or "Africa" was a
sufficiently exact record of the origin of any specimen.

Moreover, no thought of the geological past of groups and species had
yet arisen, and without the conception of common origin, the facts of
homology had no significance. All classification was simply a matter of
arbitrary pigeon-holing the records of forms, rather than an expression
of actual blood relationship. To this confusion much was added
through love of novelty. Different authors changed names to suit their
personal tastes regardless of rights of priority. _Amia_ was altered
to _Amiatus_ by Rafinesque in 1815 because it was too short a name.
_Hiodon_ was changed to _Amphiodon_ because it sounded too much like
_Diodon_, _Batrachoides_ to _Batrictius_ because ~batrachos~ means a
frog, not a fish, and other changes even more wanton were introduced,
to be condemned and discarded by the more methodical workers of a later
period. With all its abuses, however, the binomial nomenclature made
possible systematic zoology and botany, and with the "Systema Naturae"
arose a new era in the science of living organisms.

In common with most naturalists of his day, the spirit of Linnaeus was
essentially a devout one. Admiration for the wonderful works of God was
breathed on almost every page. "O Jehovah! quam ampla sunt opera Tua"
is on the title-page of the "Systema Naturae," and the inscription over
the door of his home at Hammarby was to Linnaeus the wisdom of his life.
This inscription read: "Innocue vivito: Numen adest" (Live blameless:
God is here).

The followers of Linnaeus are divided into two classes, explorers and
compilers. To the first class belonged his own students and others
who ransacked all lands for species to be added to the lists of the
"Systema Naturae." Those men, mostly Scandinavian and Dutch, worked with
wonderful zeal, enduring every hardship and making great contributions
to knowledge, which they published in more or less satisfactory forms.
To these men we owe the beginnings of the science of geographical
distribution. Among the most notable of these are Pehr Osbeck and
Fredrik Hasselquist, already noted; Otto Fabricius (1744-1822), author
of an excellent "Fauna of Greenland"; Carl Peter Thunberg (1743-),
successor of Linnaeus as rector of the University of Upsala, who
collected fishes about Nagasaki, intrusting most of the descriptive
work to the less skillful hands of his students, Jonas Nicolas Ahl and
Martin Houttuyn; Martin Th. Bruennich, who collected at Marseilles the
materials for his "Pisces Massiliensis"; Petrus Forskal (1736-63),
whose work on the fishes of the Red Sea ("Descriptio Animalium,"
etc.), published posthumously in 1775, is one of the most accurate
of faunal lists, and one which shows a fine feeling for taxonomic
distinctions scarcely traceable in any previous author. Georg Wilhelm
Steller (1709-45), naturalist of Bering's expedition, gathered amid
incredible hardships the first knowledge of the fishes of Alaska and
Siberia, his notes being printed after his tragic death, by Pallas and
Krascheninnikov. Petrus Simon Pallas (1741-1811) gives the account
of his travels in the North Pacific in his most valuable volumes,
"Zoographia Russo-Asiatica"; Johann Georg Gmelin (1709-55) with Samuel
Theophilus Gmelin (1745-84), and Johann Anton Gueldenstaedt (1745-91),
like Steller, crossed Siberia, recording its animals. Johann David
Schoepf (1752-1800), a Hessian surgeon stationed at Long Island in the
Revolutionary War, gave an excellent account of the fishes about New
York.

Still other naturalists accompanied navigators around the globe,
collecting specimens and information as opportunity offered. John
Reinhold Forster (1729-98), with his son, John George Adam Forster
(1754-94), and Daniel Solander (1736-81), a student of Linnaeus,
and Sir Joseph Banks (1743-1820), sailed with Captain James Cook.
Philibert Commerson (1727-73) accompanied the explorer, Louis Antoine
de Bougainville, and furnished nearly all the original material used by
Lacepede. Other noted travelers of the early days were Pierre Sonnerat
and Mungo Park.

Still other naturalists, scarcely less useful, gave detailed accounts
of the fauna of their own native regions. Ablest of these was Anatole
Risso, an apothecary of Nice, who published in 1810 the "Ichthyologie
de Nice," an excellent work, afterward (1826) expanded by him into a
"Histoire Naturelle de l'Europe Meridionale."

Contemporary with Risso was a man of very different character,
Constantine Samuel Rafinesque (1784-1842), who wrote at Palermo in
1810 his "Caratteri di Alcuni Nuovi Generi" and his "Ittiologia
Siciliana." Later he went to America, where he was for a time professor
in the Transylvania University at Lexington, Ky. Brilliant, erudite,
irresponsible, fantastic, he wrote of the fishes of Sicily and later
("Ichthyologia Ohiensis," 1820) of the fishes of the Ohio River, with
wide knowledge, keen taxonomic insight, and a hopeless disregard of
the elementary principles of accuracy. Always eager for novelties,
restless and credulous, his writings have been among the most difficult
to interpret of any in ichthyology.

Earlier than Risso and Rafinesque, Thomas Pennant (1726-58) wrote of
the British fishes; Otto Fredrik Mueller of the fishes of Denmark; J. E.
Gunner, Bishop of Throendhjem, of fishes of Norway; Francis Valentijn
(1660-1730), Jan Nieuhof (1600-1671), Renard, and Castour of the fishes
of the Dutch East Indies; Duhamel du Monceau of the fisheries of
France; Francesco Cette of the fishes of Sicily; Jose Cornide of the
fishes of Spain; Ignacio Molina of the fishes of Chile; and Meidinger
of those of Austria. Some of these writers lived before Linnaeus. Others
knew little of the Linnaean system, and their records are generally in
the vernacular. Most important of this class is the work of Antonio
Parra, "Descripcion de Diferentes Piezas de Historia Natural de la Isla
de Cuba," published in Havana in 1787. In 1803, Patrick Russell gave
a valuable account, non-binomial, of "Two Hundred Fishes Collected at
Vizagapatam and on the Coast of Coromandel."

Papers on the fishes of Bering Sea and Japan by Wilhelm Theophilus
Tilesius (1775-1835), are published in the transactions of the early
societies of Russia. The collections of the traveler Krusenstern were
recorded by Tilesius. Stephen Krascheninnikov (1786) wrote a history of
Russia in Asia.

Other notable names among the early writers are those of Pierre Marie
Auguste Broussonet, of Montpelier, whose work (1780), too soon cut
short, showed marked promise; Fr. Faber, who wrote of the fishes of
Iceland; E. Blyth, who studied the fishes of the Andamans; A. G.
Desmarest, who made excellent studies of the fishes of Cuba; J. T.
Koelreuter and Everard Home in the East Indies; Geoffrey Saint-Hilaire,
who recorded the fishes of Egypt at the command of Napoleon. Others
equally notable were B. A. Euphrasen, Iwan Lepechin (1750-1802), John
Latham, W. E. Leach, George Montagu, C. Quensel, Jean-Antoine Scopoli,
Peter Ascanius, Francois Etienne de la Roche (1789-1812), Hans Stroem,
M. Vahl and Zuieuw.

The compilers who followed Linnaeus belonged to a wholly different
class. These were men of extensive learning, methodical ways,
sometimes brilliant, occasionally of deep insight, but more often, on
the whole, dull, plodding, and mechanical.

Earliest of those is Antoine Gouan, whose "Historia Piscium" was
published in Paris in 1770. In this work, which is of fair quality,
only genera were included, and the three new ones which he introduces
into the "System" (_Lepadogaster_, _Lepidopus_, and _Trachypterus_) are
still retained with his definition of them.

Johann Friedrich Gmelin (1748-1804), a relative of the explorers of
Siberia, published in 1788 a thirteenth edition of the "Systema Naturae"
of Linnaeus, adding to it the discoveries of Forskal, Forster, and
others who had written since Linnaeus' time. This work was useful as
bringing the compilation of Linnaeus to a later date, but it is not well
done, the compiler having little knowledge of the animals described and
little penetration in matters of taxonomy. Very similar in character,
although more lucid in expression, is the French compilation of the
same date (1788), "Tableau Encyclopedique et Methodique des Trois
Regnes de la Nature," by the Abbe J. P. Bonnaterre. Another volume of
the "Encyclopedie Methodique," of still less merit, was published as a
dictionary in Paris in 1787 by Rene Just Hauey. Another dictionary in
1817 even poorer was the work of Hippolyte Cloquet.

In 1792, Johann Julius Walbaum (1721-1800), a German compiler of a
little higher rank, gathered together the records of all known species,
using the work of Artedi as a basis and giving binominal names in
place of the vernacular terms used by Schoepf, Steller, Pennant, and
Krascheninnikov.

Far more pretentious and more generally useful, as well as containing
a large amount of original material, is the "Ichthyologia" of
Mark Eliezer Bloch, published in Berlin in various parts from
1782 to 1785. It was originally in German and divided into two
portions--"Oeconomische Naturgeschichte der Fische Deutschlands"
and "Naturgeschichte der auslandischen Fische." Bloch was a Jewish
physician, born at Anspach in 1723, and at the age of fifty-six began
to devote himself to ichthyology. In his great work is contained every
species which he had himself seen, every one which he could purchase
from collections, and every one of which he could find drawings made by
others.

That part which relates to the fishes of Germany is admirably done.
In the treatment of East Indian and American fishes there is much
guesswork and many errors of description and of fact, for which the
author was not directly responsible. To learn to interpret the personal
equation in the systematic work of other men is one of the most
delicate of taxonomic arts.

After the publication of these great folio volumes of plates, Dr.
Bloch began a systematic catalogue to include all known species. This
was published after his death by his collaborator, the philologist,
Dr. Johann Gottlob Schneider. This work, "M. E. Blochii Systema
Ichthyologia," contains 1519 species of fishes, and is the most
creditable compilation subsequent to the death of Linnaeus.

Even more important than the work of Bloch is that of the Comte de La
Cepede, who became with the progress of the French Revolution, "Citoyen
Lacepede," his original full name being Bernard Germain Etienne de
la Ville-sur-Illon, Comte de La Cepede. His great work, "Histoire
Naturelle des Poissons," was published originally in five volumes, in
Paris, from 1798 to 1803. It was brought out under great difficulties,
his materials being scattered, his country in a constant tumult. For
original material he depended largely on the collections and sagacious
notes of the traveler Commerson. Dr. Gill sums up the strength and
weakness of Lacepede's work in these terms:

"A work by an able man and eloquent writer even prone to aid rhetoric
by the aid of the imagination in absence of desirable facts, but
which because of undue confidence in others, default of comparison of
material from want thereof and otherwise, and carelessness generally is
entirely unreliable."

The work of Lacepede had a great influence upon subsequent
investigators, especially in France. A considerable number of the
numerous new genera of Rafinesque were founded on divisions made in the
analytical keys of Lacepede.

[Illustration: BERNARD GERMAIN DE LACEPEDE.

GEORGES DAGOBERT CUVIER.

LOUIS AGASSIZ.

JOHANNES MUeLLER.]

In 1803 and 1804, Dr. George Shaw published in London his "General
Zoology," the fishes forming part of volumes IV and V. This is a poor
compilation, the part concerning the fishes being mostly extracted from
Bloch and Lacepede. Another weak compilation for the supposed use of
students was the "Ichthyologie Analytique" of A. M. Constant Dumeril.
About 1815, Henri Ducrotay de Blainville wrote the "Faune Francaise"
and contributed important studies to the taxonomy of sharks.

With Georges Leopold Chretien Frederic Dagobert Cuvier (1769-1832)
and the "Regne Animal arrange apres son Organization" (1817; 1829-30)
we have the beginning of a new era in ichthyology. This period
is characterized by a recognition of the existence of a natural
classification inevitable in proportion to the exactness of our
knowledge, because based on the principles of morphology. The "Regne
Animal" is, in the history of ichthyology, not less important than the
"Systema Naturae" itself, and from it dates practically our knowledge
of families of fishes and the interrelations of the different groups.
The great facts of homology were clearly understood by Cuvier. Their
significance as indications of lines of descent were never grasped by
him, and this notwithstanding the fact that Cuvier was almost the first
to bring extinct forms into proper relations with those now living.

Dr. Guenther well says that the investigation of anatomy of fishes was
continued by Cuvier until he had succeeded in completing so perfect
a framework of the system of the whole class that his immediate
successors could content themselves with filling up those details for
which their master had no leisure. Indefatigable in examining all the
external and internal characters of the fishes of a rich collection, he
ascertained the natural affinities of the infinite variety of fishes,
and accurately defined the divisions, orders, families, and genera of
the class as they appear in the two original editions of the "Regne
Animal." His industry equaled his genius; he opened connections with
almost every accessible part of the globe; not only French travelers
and naturalists, but also Germans, Englishmen, Americans rivaled one
another to assist him with collections; and for many years the Museum
of the Jardin des Plantes was the Center where all ichthyological
treasures were deposited. Thus Cuvier brought together a collection the
like of which had never been seen before, and which, as it contains all
the materials on which his labors were based, must still be considered
to be one of the most important in existence.

"Those little low rooms, five in number" (in the museum of the Jardin
des Plantes), "they should be the Mecca of scientific devotees. Perhaps
every great zoologist of the past hundred years has sat in them and
discussed those problems of life which are always inviting solution and
are never solved. The spirits of great naturalists still haunt these
corridors and speak from the specimens their hands have set in order."
(THEODORE LYMAN.)

Cuvier's studies of the different species of fishes are contained in
the great "Histoire Naturelle des Poissons," the joint work of Cuvier
and his pupil and successor, Achille Valenciennes (1794-1865). Of this
work 22 volumes were published, from 1828 to 1849, containing 4514
nominal species, the greater portion being written after the death of
Cuvier (1832). The work was finally left unfinished on account of a
disagreement with the publisher. Dr. Gill tells me that at this time
Valenciennes made an unsuccessful appeal to the Smithsonian Institution
for assistance in the publication of the remaining chapters.

This is a most masterly work, indispensable to the student of fishes.
Its descriptions are generally fairly correct, its plates accurate,
and its judgments trustworthy. But with all this it is very unequal.
Too often nominal species are based on variations due to age or sex or
to the conditions of preservation of specimens. Many of the species
are treated very lightly by Cuvier; many of the descriptions of
Valenciennes are very mechanical, as though the author had grown weary
of the endless process, "a failing commonly observed among zoologists
when attention to descriptive details becomes to them a tedious task."

After the death of Valenciennes (1865) Dr. Auguste Dumeril began
another Natural History of the Fishes. Of this two volumes (1865-70)
were published covering sharks, ganoids, and other fishes not treated
by Cuvier and Valenciennes, his category beginning at the opposite
end of the fish series. The death of Dumeril left this catalogue also
unfinished. Dumeril's work is useful and carefully done, but his
excessive trust in slight differences has filled his book with nominal
species. Thus among the living ganoid fishes he recognizes 135 species,
the actual number being not far from 40.

We may anticipate the sequence of time by here referring to the
remaining attempts at a record of all the fishes in the world, Dr.
Albert C. L. G. Guenther, a naturalist of German birth, but resident in
London for many years, long the honored keeper of the British Museum,
published in eight volumes the "Catalogue of the Fishes of the British
Museum," from 1859 to 1870. In this monumental work, the one work most
essential to all systematic study of fishes, 6843 species are described
and 1682 doubtful species are mentioned. The book is a remarkable
example of patient industry. Its great merits are at once apparent, and
those of us engaged in the same line of study may pass by its faults
with the leniency which we may hope that posterity may bestow on ours.

The publication of this work gave an immediate impetus to the study of
fishes. The number of known species has been raised from 9000 to about
12,000 in the last thirty years, although meanwhile some hundreds of
species even accepted by the conservatism of Guenther have been erased
from the system.

A new edition of this work has been long in contemplation, and
in 1898 the first volume of it, covering the percoid fishes, was
published by Dr. George Albert Boulenger. This volume is one of the
most satisfactory in the history of ichthyology. It is based on
ample material. Its accepted species have been subject to thorough
criticism and in its classification every use has been made of the
teachings of morphology and especially of osteology. Its classification
is distinctly modern, and with the writings of the contemporary
ichthyologists of Europe and America, it is fully representative of
the scientific era ushered in by the researches of Darwin. The chief
criticism which one may apply to this work concerns most of the
publications of the British Museum. It is the frequent assumption that
those species not found in the greatest museum of the world do not
really exist at all. There are still many forms of life, very many,
outside the series gathered in any or all collections.

[Illustration: ALBERT GUeNTHER.

FRANZ STEINDACHNER.

GEORGE A. BOULENGER.

ROBERT COLLETT.]

We may now turn from the universal catalogues to the work on special
groups, on local faunas, or on particular branches of the subject of
ichthyology. These lines of study were made possible by the work of
Cuvier and Valenciennes and especially by that of Dr. Guenther.

Before taking up the students of faunal groups, we may, out of
chronological order, consider the researches of three great
taxonomists, who have greatly contributed to the modern system of the
classification of fishes.

Louis Agassiz (born at Motiers in western Switzerland in 1807; died at
Cambridge, Mass., in 1873) was a man of wonderful insight in zoological
matters and possessed of a varied range of scientific information,
scarcely excelled in any age--intellectually a lineal descendant of
Aristotle. His first work on fishes was the large folio on the fishes
collected by Jean Baptiste Spix (1781-1826) in Brazil, published at
Munich in 1827. After his establishment in America in 1846, soon after
which date, he became a professor in Harvard University, Agassiz
published a number of illuminating papers on the fresh-water fishes
of North America. He was the first to recognize the necessity of the
modern idea of genera among fishes, and most of the groups designated
by him as distinct genera are retained by later writers. He was also
the first to investigate the structure of the singular viviparous
surf-fishes of California, the names _Embiotoca_ and _Holconotus_
applied to these fishes being chosen by him.

His earlier work, "Recherches sur les Poissons des Eaux Douces,"
published in Europe, gave a great impetus to our knowledge of the
anatomy and especially of the embryology of the fresh-water fishes.
Most important of all his zoological publications was the "Recherches
sur les Poissons Fossiles," published at Neufchatel from 1833 to 1843.
This work laid the foundation of the systematic study of the extinct
groups of fishes. The relations of sharks were first appreciated by
Agassiz, and the first segregation of the ganoids was due to him.
Although he included in this group many forms not truly related either
to anything now called ganoids, nor even to the extinct mailed forms
which preceded them, yet the definition of this order marked a distinct
step in advance.

The great, genial, hopeful personality of Agassiz and his remarkable
skill as a teacher made him the "best friend that ever student had"
and gave him a large following as a teacher. Among his pupils in
ichthyology were Charles Girard (1822-1895), Frederick Ward Putnam,
Alexander Agassiz, Samuel Garman, Samuel H. Scudder, and the present
writer.

Johannes Mueller (1808-1858), of Berlin, was one of the greatest
of comparative anatomists. In his revision of Cuvier's "System of
Classification" he corrected many errors in grouping, and laid
foundations which later writers have not altered or removed. Especially
important is his classical work, "Ueber den Bau and die Grenzen der
Ganoiden." In this he showed some of the real fundamental characters of
that group of archaic fishes, and took from it the most heterogeneous
of the elements left in it by Agassiz. To Mueller we also owe the first
proper definition of the Leptocardii and the Cyclostomata, and, in
association with Dr. J. Henle, Mueller has given us one of the best
general accounts of the sharks ("Systematische Beschriebungen der
Plagiostomen"). To Mueller we owe an accession of knowledge in regard
to the duct of the air-bladder, and the groups called Physostomi,
Physoclysti, Dipneusti (Dipnoi), Pharyngognathi, and Anacanthini were
first defined by him.

In his work on Devonian fishes, the great British comparative
anatomist, Thomas Henry Huxley, first distinguished the group of
Crossopterygians, and separated it from the ganoids and dipnoans.

Theodore Nicholas Gill is the keenest interpreter of taxonomic facts
yet known in the history of ichthyology. He is the author of a vast
number of papers, the first bearing date of 1858, touching almost every
group and almost every phase of relation among fishes. His numerous
suggestions as to classification have been usually accepted in time
by other authors, and no one has had a clearer perception than he of
the necessity of orderly methods in nomenclature. Among the orders
first defined by Gill are the Eventognathi, Nematognathi, Pediculati,
Iniomi, Heteromi, Haplomi, Xenomi, and the group called Teleocephali,
originally framed to include all the bony fishes except those which
showed peculiar eccentricities or modifications. Dr. Gill's greatest
excellence has been shown as a scientific critic. Incisive, candid, and
friendly, there is scarcely an investigator in biology, in America,
who is not directly indebted to him for critical aid of the highest
importance. The present writer cannot too strongly express his own
obligations to this great teacher, his master in fish taxonomy. Dr.
Gill's work is not centered in any single great treatise, but is
diffused through a very large number of brief papers and catalogues,
those from 1861 to 1865 mostly published by the Academy of Natural
Sciences in Philadelphia, those of recent date by the United States
National Museum. For many years Dr. Gill has been identified with the
work of the Smithsonian Institution at Washington.

Closely associated with Dr. Gill was Dr. Edward Drinker Cope, of
Philadelphia, a tireless worker in almost every field of zoology, and
a large contributor to the broader fields of ichthyological taxonomy
as well as to various branches of descriptive zoology. Cope was one
of the first to insist on the close relation of the true ganoids with
the teleost fishes, the nearest related group of which he defined
as Isospondyli. At the same time he recognized the wide range of
difference even among the forms which Johannes Mueller had assembled
under that name. In breadth of vision and keenness of insight, Cope
ranked with the first of taxonomic writers. Always bold and original,
he was not at all times accurate in details, and to the final result in
classification his contribution has been less than that of Dr. Gill.
Professor Cope also wrote largely on American fresh-water fishes, a
large percentage of the Cyprinidae and Percidae of the eastern United
States having been discovered by him, as well as much of the Rocky
Mountain fauna. In later years his attention was absorbed by the fossil
forms, and most of the species of Cretaceous rocks and the Eocene
shales of Wyoming were made known through his ceaseless activity.

[Illustration: SPENCER FULLERTON BAIRD.

EDWARD DRINKER COPE.

THEODORE NICHOLAS GILL.

GEORGE BROWN GOODE.]

The enumeration of other workers in the great field of ichthyology
must assume something of the form of a catalogue. Part of the impulse
received from the great works of Cuvier and Valenciennes and of Guenther
was spent in connection with voyages of travel. In 1824 Quoy and
Gaimard published in Paris the great folio work on the fishes collected
by the corvette _l'Uranie_ and _la Physicienne_ in Freycinet's voyages
around the world, and in 1834 the same authors published the fishes
collected in Duperrey's voyage of the _Astrolabe_. In 1826 Lesson
published the fishes of Dumont D'Urville's voyage of the _Coquille_.
These three great works lie at the foundation of our knowledge of
the fishes of Polynesia. In 1839 Eydoux and Gervais published an
account of the fishes of the voyage of _La Favorite_. In 1853, also in
Paris, Hombron and Jacquinot gave an account of the fishes taken in
Dumont D'Urville's expedition to the South Pole. In England, Sir John
Richardson (1787-1865), a wise and careful naturalist, wrote of the
fishes collected by the _Sulphur_ (1845), the _Erebus_ and _Terror_
(1846), the _Samarang_, and the _Herald_. Lay and Bennett recorded
the species taken by Beechey's voyage on the _Blossom_. A most useful
work is the account of the species taken by Charles Darwin on the
voyage of the _Beagle_, prepared by the conscientious hand of Rev.
Leonard Jenyns. Still more important and far ranging is the voyage
of the _Challenger_, including the first important work in the deep
seas, one stately volume and parts of other volumes on fishes being
the work of Dr. Guenther. Other deep-sea work of equal importance has
been accomplished in the Atlantic and the Pacific by the U. S. Fish
Commission steamer _Albatross_. Its results in Central America, Alaska,
Japan, Hawaii, as well as off both coasts of the United States, have
been made known in different memoirs by Goode and Bean, Gilbert,
Garman, Gill, Jordan, Cramer, Ryder, and others. The deep-sea fish
collections of the _Fish Hawk_ and the _Blake_ have been studied by
Goode and Bean and Garman.

The deep-sea work of other countries may be briefly noticed. The French
vessels _Travailleur_ and _Talisman_ have made collections chiefly in
the Mediterranean and along the coast of Africa, the results having
been made known by Leon Valliant. The _Hirondelle_ about the Azores and
elsewhere has furnished material for Professor Robert Collett, of the
University of Christiania. Dr. Decio Vinciguerra, of Rome, has reported
on the collections of the _Violante_, a vessel belonging to the Prince
of Monaco. Dr. A. Alcock, of Calcutta, has had charge of the most
valuable deep-sea work of the _Investigator_ in the Indian Seas. Edgar
R. Waite and James Douglas Ogilby, of the Australian Museum at Sydney,
have described the collections of the _Thetis_, on the shores of the
New South Wales.

[Illustration: JOHANN REINHARDT.

EDWARD WALLER CLAYPOLE.

CARLOS BERG.

EDGAR R. WAITE.]

From Austria the voyage of the frigate _Novara_ has yielded large
material which has been described by Dr. Rudolph Kner. The cream of
many voyages of many Danish merchant vessels has been gathered in
the "Spolia Atlantica" and other truly classical papers of Christian
Frederik Luetken, of the University of Copenhagen, one of the most
accomplished naturalists of recent times.

F. H. von Kittlitz has written on the fishes seen by him in the
northern Pacific, and earlier and more important we may mention the
many ichthyological notes found in the records of travel in Mexico and
South America by Alexander von Humboldt (1796-1859).

The local faunal work in various nations has been very extensive. In
Great Britain we may note Parnell's "Natural History of the Fishes of
the Firth of Forth," published in Edinburgh in 1838, William Yarrell's
"History of British Fishes" (1859), the earlier histories of British
fishes by Edward Donovan and by William Turton, and the works of J.
Couch (1862) and Dr. Francis Day (1888), possessing similar titles.
The work of Day, with its excellent plates, will long be the standard
account of the relatively scant fish fauna of the British islands.
H. G. Seeley has prepared (1886) also a useful synopsis of "The
Fresh-water Fishes of Europe."

We may here notice without praise the pretentious work of William
Swainson (1838-39). W. Thompson has written of the fishes of Ireland,
and Rev. Richard T. Lowe and J. Y. Johnson have done most excellent
work on the fishes of Madeira. F. McCoy, better known for work on
fossil fishes, may be mentioned here.

The fish fauna of Scandinavia has been described more or less fully
by S. Kroeyer (1840), Robert Nilsson (1855), Fries and Ekstroem (1836),
Robert Collett, Robert Lilljeborg, and F. A. Smitt, besides special
papers by other writers, notably Reinhardt, L. Esmarck, Japetus
Steenstrup, Luetken, and A. W. Malm. Reinhardt, Kroeyer, Luetken, and
A. J. Malmgren have written of the Arctic fishes of Greenland and
Spitzbergen.

In Russia, Nordmann has described the fishes of the Black Sea
("Ichthyologic Pontique," Paris, 1840) and Eichwald those of the
Caspian. More recently, S. Herzenstein, Warpachowsky, K. Kessler, B.
N. Dybowsky, and others have written of the rich fauna of Siberia, the
Caucasus, and the scarcely known sea of Ochotsk. Stephan Basilevsky has
written of the fishes of northern China. A. Kowalevsky has contributed
very much to our knowledge of anatomy. Peter Schmidt has studied the
fishes of the Japan Sea.

In Germany and Austria the chief local works have been those of Heckel
and Kner on the fresh-water fishes of Austria (1858) and C. Th. von
Siebold on the fresh-water fishes of Central Europe (1863). German
ichthyologists have, however, often extended their view to foreign
regions where their characteristic thoroughness and accuracy has made
their work illuminating. The two memoirs of Eduard Rueppell on the
fishes of the Red Sea and the neighboring parts of Africa, "Atlas
zu der Reise im Noerdlichen Afrika," 1828, and "Neue Wirbelthiere,"
1837, rank with the very best of descriptive literature. Guenther's
illustrated "Fische der Suedsee," published in Hamburg, may be regarded
as German work. The excellent  plates are mostly from the hand
of Andrew Garrett. Other papers are those of Dr. Wilhelm Peters on
Asiatic fishes, the most important being on the fishes of Mozambique.
J. J. Heckel, Rudolph Kner, and Franz Steindachner, successively
directors of the Museum at Vienna, have written largely on fishes. The
papers of Steindachner cover almost every part of the earth and are
absolutely essential to any systematic study of fishes. No naturalist
of any land has surpassed Steindachner in industry or accuracy, and his
work has the advantage of the best illustrations of fishes made by any
artist, the noted Eduard Konopicky. In association with Dr. Doederlein,
formerly of Tokyo, Dr. Steindachner has given an excellent account
of the fishes of Japan. Other German writers are J. J. Kaup, who has
worked in numerous fields, but as a whole with little skill, Dr. S.
B. Klunzinger, who has given excellent accounts of the fishes of the
Red Sea, and Dr. Franz Hilgendorf, of the University of Berlin, whose
papers on the fishes of Japan and other regions have shown a high grade
of taxonomic insight. A writer of earlier date is W. L. von Rapp, who
wrote on the "Fische den Bodensees." J. F. Brandt has written of the
sturgeons of Russia, and Johann Marcusen, to whom we owe much of our
knowledge, of the Mormyri of Africa.

In Italy, Charles Lucien Bonaparte, Prince of Canino, has published an
elaborate "Fauna Italica" (1838) and in numerous minor papers has taken
a large part in the development of ichthyology. Many of the accepted
names of the large groups (as Elasmobranchii, Heterosomata, etc.) were
first suggested by Bonaparte. The work of Rafinesque has been already
noticed. O. G. Costa published (about 1850) a "Fauna of Naples." In
recent times Camillo Ranzani, of Bologna, wrote on the fishes of Brazil
and of the Mediterranean. Giovanni Canestrini, Decio Vinciguerra,
Enrico Hillyer Giglioli, Luigi Doederlein, and others have contributed
largely to our knowledge of Italian fishes, while Carlo F. Emery, F.
de Filippi, Luigi Facciola, and others have studied the larval growth
of different species. Camillo Ranzani, G. G. Bianconi, Domenico Nardo,
Cristoforo Bellotti, Alberto Perugia, and others have contributed to
different fields of ichthyology.

Nicholas Apostolides and, still later, Horace A. Hoffman and the
present writer, have written of the fishes of Greece.

In France, the fresh-water fishes are the subject of an important work
by Emile Blanchard (1866), and Emile Moreau has given us a convenient
account of the fish fauna of France. Leon Vaillant has written on
various groups of fishes, his monograph of the American darters
(Etheostominae) being a masterpiece so far as the results of the study
of relatively scanty material would permit. The "Mission Scientifique
au Mexique," by Valliant and F. Bocourt, is one of the most valuable
contributions to our knowledge of the fishes of that region. Dr. H. E.
Sauvage, of Boulogne-sur-Mer, has also written largely on the fishes
of Asia, Africa, and other regions. Among the most important of these
are the "Poissons de Madagascar," and a monograph of the sticklebacks.
Alexander Thominot and Jacques Pellegrin have also written, in the
Museum of the Jardin des Plantes, on different groups of fishes.
Earlier writers were Constant Dumeril, Alphonse Guichenot, L. Brissot
de Barneville, H. Hollard, an able anatomist, and Bibron, an associate
of Auguste Dumeril.

[Illustration: FELIPE POEY Y ALOY.

LEON VAILLANT.

LOUIS DOLLO.

DECIO VINCIGUERRA.]

In Spain and Portugal the chief work of local authors is that of J.
V. B. Bocage and F. de Brito Capello on the fishes of Portugal. So
far as the fishes of Spain are concerned, the most valuable memoir
is Steindachner's account of his travels in Spain and Portugal.
The principal studies of the Balkan region have also been made by
Steindachner. Jose Gogorza y Gonzalez, of the Museum of Madrid, has
given a list of the fishes of the Philippines. A still more elaborate
list, praiseworthy as a beginning, is the work of the Reverend Padre
Casto de Elera, professor of Natural History in the Dominican College
of Santo Tomas in Manila.

In Holland, the chief great works have been those of Schlegel and
Pieter van Bleeker. Professor H. Schlegel, of the University of Leyden,
described the fishes collected about Nagasaki by Ph. Fr. de Siebold and
Buerger. His work on fishes forms a large folio illustrated by 
plates, a volume of the "Fauna Japonica," published in Leyden from 1843
to 1847. Schlegel's work in every field is characterized by scrupulous
care and healthful conservatism, and the "Fauna Japonica" is a most
useful monument to his rare powers of discrimination.

Pieter von Bleeker (1819-78), a surgeon in the Dutch East Indies,
is the most voluminous writer in ichthyology. He began his work in
Java without previous training and in a very rich field where almost
everything was new. With many mistakes at first he rose to the front by
sheer force of industry and patience, and his later work, while showing
much of the "personal equation," is still thoroughly admirable. At his
death he was engaged in the publication of a magnificent folio work,
"Atlas Ichthyologique des Indes Orientales Neerlandaises," illustrated
by  plates. This work remains about two-thirds completed. The
writings of Dr. Bleeker constitute the chief source of our knowledge of
the fauna of the East Indies.

Dr. Van Lidth de Jeude, of the University of Leyden, is the author of a
few descriptive papers on fishes.

To Belgium we may assign part at least of the work of the eminent
Belgian naturalist, George Albert Boulenger, now long connected with
the British Museum. His various valuable papers on the fishes of the
Congo are published under the auspices of the "Congo Free State." To
Belgium also we may ascribe the work of Louis Dollo on the morphology
of fishes and on the deep-sea fishes obtained by the "Expedition
Antarctique Belge."

The fish fauna of Cuba has been the lifelong study of Dr. Felipe Poey
y Aloy (1799-1891), a pupil of Cuvier, for a half century or more
the honored professor of zoology in the University of Havana. Of
his many useful papers, the most extensive are his "Memorias sobre
la Historia Natural de la Isla de Cuba," followed by a "Repertorio"
and an "Enumeratio" in which the fishes are elaborately catalogued.
Poey devoted himself solely to the rich fish fauna of his native
island, in which region he was justly recognized as a ripe scholar
and a broad-minded gentleman. A favorite expression of his was "Comme
naturaliste, je ne suis pas espagnol: je suis cosmopolite." Before
Poey, Guichenot, of Paris, had written on the fishes collected in Cuba
by Ramon de la Sagra (1810-60). His account was published in Sagra's
"Historia de Cuba," and later Philip H. Gosse (1810-1888) wrote on the
fishes of Jamaica. Much earlier, Robert Hermann Schomburgk (1804-65)
wrote on the fishes of British Guiana. Other papers on the Caribbean
fishes were contributed by Johannes Mueller and F. H. Troschel, and by
Richard Hill and J. Hancock.

Besides the work in South America of Marcgraf, Agassiz, Reinhardt,
Luetken, Steindachner, Jenyns, Boulenger, and others already named,
we may note the local studies of Dr. Carlos Berg in Argentina, Dr.
R. A. Philippi, and Frederico T. Delfin in Chile, Miranda-Ribeiro in
Brazil, with Garman, J. F. Abbott, and others in recent times. Carl
H. Eigenmann and earlier Jordan and Eigenmann have studied the great
collections made in Brazil by Agassiz. Steindachner has described
the collections of Johann Natterer and Gilbert those made by Dr.
John Casper Branner. The most recent examinations of the myriads of
Brazilian river fishes have been made by Dr. Eigenmann. Earlier than
any of these (1855), Francis de Castelnau (1800-65) described many
Brazilian fishes and afterwards numerous fishes of Australia and
southern Africa, Alphonse Guichenot, of Paris, contributed a chapter
on fishes to Claude Gay's (1800-63) "History of Chile," and J. J. von
Tschudi, of St. Gallen, published an elaborate but uncritical "Fauna
Peruana" with  plates of Peruvian fishes.

In New Zealand, F. W. Hutton and J. Hector have published a valuable
work on the fishes of New Zealand, to which Dr. Gill added useful
critical notes in a study of "Antipodal Faunas." Later writers have
given us a good knowledge of the fishes of Australia. Notable among
them are Charles DeVis, William Macleay, H. de Miklouho-Maclay, James
Douglas Ogilby, and Edgar R. Waite. Clarke has also written on "Fishes
of New Zealand."

The most valuable work on the fishes of Hindustan is the elaborate
treatise on the "Fishes of India" by Surgeon Francis Day. In this all
the species are figured, the groups being arranged as in Guenther's
catalogue, a sequence which few non-British naturalists seem inclined
to follow. Cantor's "Malayan Fishes" is a memoir of high merit, as is
also McClelland's work on Indian fishes and the still earlier work of
Francis Buchanan Hamilton on the fishes of the Ganges. We may here
refer to Andrew Smith's papers on the fishes of the Cape of Good Hope
and to R. I. Playfair and A. Guenther's "Fishes of Zanzibar." T. C.
Jerdon, John Edward Gray, E. Tyrwhitt Bennett, and others have also
written on the fishes of India; J. C. Bennett has published several
excellent papers on the fishes of Polynesia and the East Indies.

In Japan, following the scattering papers of Thunberg, Tilesius, and
Houttuyn, and the monumental work of Schlegel, numerous species have
been recorded by James Carson Brevoort, Guenther, Gill, Eduard Nystroem,
Hilgendorf, and others. About 1884 Steindachner and Doederlein published
the valuable "Fische Japans," based on the collections made about Tokyo
by Dr. Doederlein. In 1881, Motokichi Namiye, then assistant curator in
the Imperial University, published the first list of Japanese fishes
by a native author. In 1900, Dr. Chiyomatsu Ishikawa, on the "Fishes
of Lake Biwa," was the first Japanese author to venture to name a
new species of fish (_Pseudogobio zezera_). This reticence was due
not wholly to lack of self-confidence, but rather to the scattered
condition of the literature of Japanese ichthyology. For this reason no
Japanese author has ever felt that any given undetermined species was
really new. Other Japanese ichthyologists of promise are Dr. Kamakichi
Kishinouye, in charge of the Imperial fisheries Bureau, Dr. Shinnosuke
Matsubara, director of the Imperial Fisheries Institute, Keinosuke
Otaki, S. Hatta, S. Nozawa, T. Kitahara, and Michitaro Sindo, and we
may look for others among the pupils of Dr. Kakichi Mitsukuri, the
distinguished professor of zoology in the Imperial University.

[Illustration: BASHFORD DEAN.

KAKICHI MITSUKURI.

CARL H. EIGENMANN.

FRANZ HILGENDORF.]

The most recent, as well as the most extensive, studies of the fishes
of Japan were made in 1999 by the present writer and his associate,
John Otterbein Snyder.

The scanty pre-Cuvieran work on the fishes of North America has
been already noticed. Contemporary with the early work of Cuvier is
the worthy attempt of Professor Samuel Latham Mitchill (1764-1831)
to record in systematic fashion the fishes of New York. Soon after
followed the admirable work of Charles Alexandre Le Sueur (1789-1840),
artist and naturalist, who was the first to study the fishes of the
Great Lakes and the basin of the Ohio. Le Sueur's engravings of
fishes, in the early publications of the Academy of Natural Sciences
in Philadelphia, are still among the most satisfactory representations
of the species to which they refer. Constantine Samuel Rafinesque
(1784-1842), the third of this remarkable but very dissimilar trio,
published numerous papers descriptive of the species he had seen or
heard of in his various botanical rambles. This culminated in his
elaborate but untrustworthy "Ichthyologia Ohiensis." The fishes of
Ohio received later a far more conscientious though less brilliant
treatment at the hands of Dr. Jared Potter Kirtland (1793-1877), an
eminent physician of Cleveland, Ohio. In 1842 the amiable and scholarly
James Ellsworth Dekay (1799-1851) published his detailed report on
the fishes of the "New York Fauna," and a little earlier (1836) in
the "Fauna Boreali-Americana" Sir John Richardson (1787-1865) gave a
most valuable and accurate account of the fishes of the Great Lakes
and Canada. Almost simultaneously, Rev. Zadock Thompson (1796-1856)
gave a catalogue of the fishes of Vermont, and David Humphreys Storer
(1804-91) began his work on the fishes of Massachusetts, finally
expanded into a "Synopsis of the Fishes of North America" (1846) and a
"History of the Fishes of Massachusetts" (1853-67). Dr. John Edwards
Holbrook (1794-1871), of Charleston, published (1855-60) his invaluable
record of the fishes of South Carolina, the promise of still more
important work, which was prevented by the outbreak of the Civil War
in the United States. The monograph on Lake Superior (1850) and other
publications of Louis Agassiz (1807-73) have been already noticed.
One of the first of Agassiz's students was Charles Girard (1822-95),
who came with him from Switzerland, and, in association with Spencer
Fullerton Baird (1823-87), described the fishes from the United States
Pacific Railway Surveys (1858) and the United States and Mexican
Boundary Surveys (1859). Professor Baird, primarily an ornithologist,
became occupied with executive matters, leaving Girard to finish these
studies of the fishes. A large part of the work on fishes published by
the United States National Museum and the United States Fish Commission
has been made possible through the direct help and inspiration of
Professor Baird. Among those engaged in this work, James William Milner
(1841-80), Marshall Macdonald (1836-95), and Hugh M. Smith may be noted.

Most eminent, however, among the students and assistants of Professor
Baird was his successor, George Brown Goode (1851-96), one of the most
accomplished of American naturalists, whose greatest work, "Oceanic
Ichthyology," published in collaboration with his long associate, Dr.
Tarleton Hoffman Bean, was barely finished at the time of his death.
The work of Theodore Nicholas Gill and Edward Drinker Cope has been
already noticed.

Other faunal writers of more or less prominence were William Dandridge
Peck (1763-1822) in New Hampshire, George Suckley (1830-69) in Oregon,
James William Milner (1841-80) in the Great Lake Region, Samuel Stehman
Haldeman (1812-80) in Pennsylvania, William O. Ayres (1817-91) in
Connecticut and California; Dr. John G. Cooper (died 1902), Dr. William
P. Gibbons and Dr. William N. Lockington (died 1902) in California;
Philo Romayne Hoy (1816-93) studied the fishes of Wisconsin, Charles
Conrad Abbott those of New Jersey, Silas Stearns (1859-88) those of
Florida, Stephen Alfred Forbes and Edward W. Nelson those of Illinois,
Oliver Perry Hay, later known for his work on fossil forms, those of
Mississippi, Alfredo Duges, of Guanajuato, those of Central Mexico.

Samuel Garman, at Harvard University, a student of Agassiz, is the
author of numerous valuable papers, the most notable being on the
sharks and on the deep-sea collections of the _Albatross_ in the
Galapagos region, the last illustrated by plates of most notable
excellence. Other important monographs of Garman treat of the
Cyprinodonts and the Discoboli.

The present writer began a "Systematic Catalogue of the Fishes of
North America" in 1875 in association with his gifted friend, Herbert
Edson Copeland (1849-76), whose sudden death, after a few promising
beginnings, cut short the undertaking. Later, Charles Henry Gilbert
(1860-), a student of Professor Copeland, took up the work and in 1883
a "Synopsis of the Fishes of North America" was completed by Jordan and
Gilbert. Later, Dr. Gilbert has been engaged in studies of the fishes
of Panama, Alaska, and other regions, and the second and enlarged
edition of the "Synopsis" was completed in 1898, as the "Fishes of
North and Middle America," in collaboration with another of the
writer's students, Dr. Barton Warren Evermann. A monographic review of
the Fishes of Puerto Rico was later (1900) completed by Dr. Evermann,
together with numerous minor works. Other naturalists whom the writer
may be proud to claim as students are Charles Leslie McKay (1854-83),
drowned in Bristol Bay, Alaska, while engaged in explorations, and
Charles Henry Bollman (1868-89), stricken with fever in the Okefinokee
Swamps in Georgia. Still others are Dr. Carl B. Eigenmann, the
indefatigable investigator of Brazilian fishes and of the blind fishes
of the caves; Dr. Oliver Peebles Jenkins, the first thorough explorer
of the fishes of Hawaii; Dr. Alembert Winthrop Brayton, explorer
of the streams of the Great Smoky Mountains; Dr. Seth Eugene Meek,
explorer of Mexico; John Otterbein Snyder, explorer of Mexico, Japan,
and Hawaii; Edwin Chapin Starks, explorer of Puget Sound and Panama
and investigator of fish osteology. Still other naturalists of the
coming generation, students of the present writer and of his lifelong
associate, Professor Gilbert, have contributed in various degrees to
the present fabric of American ichthyology. Among them are Mrs. Rosa
Smith Eigenmann, Dr. Joseph Swain, Wilbur Wilson Thoburn (1859-99),
Frank Cramer, Alvin Seale, Albert Jefferson Woolman, Philip H. Kirsch
(1860-1902), Cloudsley Rutter (died 1903), Robert Edward Snodgrass,
James Francis Abbott, Arthur White Greeley, Edmund Heller, Henry Weed
Fowler, Keinosuke Otaki, Michitaro Sindo, and Richard Crittenden
McGregor.

[Illustration: DAVID STARR JORDAN.

HERBERT EDSON COPELAND.

CHARLES HENRY GILBERT.

BARTON WARREN EVERMANN.]

Other facts and conclusions of importance have been contributed by
various persons with whom ichthyology has been an incident rather than
a matter of central importance.

=The Fossil Fishes.=[148]--The study of fossil fishes was begun
systematically during the first decades of the nineteenth century, for
it was then realized that of fossils of back-boned animals, fishes were
the only ones which could be determined from early Palaeozoic to recent
horizons, and that from the diversity of their forms they could serve
as reliable indications of the age of rocks. At a later time, when the
evolution of vertebrates began to be studied, fishes were examined
with especial care with a view of determining the ancestral line of
the Amphibians. The earliest work upon fossil fishes is, as one would
naturally expect, of a purely systematic value. Anatomical observations
were scanty and crude, but as the material for study increased, a more
satisfactory knowledge was gained of the structures of the various
major groups of fishes; and finally by a comparison of anatomical
results important light came to be thrown upon more fundamental
problems.

The study of fossil fishes can be divided for convenience into three
periods: (I) That which terminated in the _magnum opus_ of Louis
Agassiz; (II) that of the systematists whose major works appeared
between 1845 and the recent publication of the Catalogue of Fossil
Fishes of the British Museum (from this period date many important
anatomical observations); and (III) that of morphological work, roughly
from 1870 to the present. During this period detailed consideration
has been given to the phylogeny of special structures, to the
probable lines of descent of the groups of fossil fishes, and to the
relationships of terrestrial to aquatic vertebrates.

=First Period.=--=The Work of Louis Agassiz.=--The real beginning
of our knowledge of fossil fishes dates from the publication of the
classic volumes of Agassiz, "Recherches sur les Poissons Fossiles
(Neuchatel, 1833-44)." There had previously existed but a fragmentary
and widely scattered literature; the time was ripe for a great work
which should bring together a knowledge of this important vertebrate
fauna and the museums throughout Europe had been steadily growing in
their collections of fossils. Especially ripe, too, since the work
of Cuvier (1769-1832) had been completed and the classic anatomical
papers of J. Mueller (1802-56) were appearing. And Agassiz (1807-73) was
eminently the man for this mission. At the age of one and twenty he
had already mapped out the work, and from this time he devoted sixteen
active years to its accomplishment. One gets but a just idea of the
personality of Agassiz when he recalls that the young investigator
while in an almost penniless position contrived to travel over a large
part of Europe, mingle with the best people of his day, devote almost
his entire time to research, employ draughtsmen and lithographers,
support his own printing-house, and in the end publish his "Poissons
Fossiles" in a fashion which would have done credit to the wealthiest
amateur. With tireless energy he collected voluminous notes and
drawings numberless; he corresponded with collectors all over Europe
and prevailed upon them to loan him tons of specimens; in the meanwhile
he collated industriously the early but fragmental literature in such
works as those of de Blainville, Muenster, Murchison, Buckland, Egerton,
Redfield, W. C. Williamson, and others. Hitherto less than 300 species
of fossil fishes were known; at the end of Agassiz's work about 900
were described and many of them figured.

It is easy to see that such a work made a ready basis of future
studies. Doubtless, too, much is owing to the personal energy of
Agassiz that such keen interest was focused in the collection and
study of fossil fishes during the middle of the nineteenth century.
The actual value of Agassiz's work can hardly be overestimated; his
figures and descriptions are usually clear and accurate. And it is
remarkable, perhaps, that in view of the very wide field which he
covered that his errors are not more glaring and numerous. Upon the
purely scientific side, however, one must confess that the "Poissons
Fossiles" is of minor importance for the reason that as time has
gone by it has been found to yield no generalizations of fundamental
value. The classification of fishes advocated by Agassiz, based upon
the nature of the scales, has been shown to be convenient rather than
morphological. This indeed Agassiz himself appears to realize in a
letter written to Humboldt, but on the other hand he regards his
creation of the now discarded order of _Ganoids_, which was based upon
integumental characters, as his most important contribution to the
general study of ichthyology. And although there passed through his
hands a series of forms more complete than has perhaps been seen by
any later ichthyologist,[149] a series which demonstrates the steps
in the evolution of the various families and even orders of fishes,
he is nowhere led to such important philosophical conclusions as was,
for example, his contemporary, Johannes Mueller. And even to his last
day, in spite of the light which palaeontology must have given him, he
denied strenuously the truth of the doctrine of evolution, a result the
more remarkable since he has even given in graphic form the geological
occurrence of the various groups of fishes in a way which suggests
closely a modern phylogenetic table, and since at various times he has
emphasized the dictum that the history of the individual is but the
epitomized history of the race. The latter statement, which has been
commonly attributed to Agassiz, is clearly of much earlier origin; it
was definitely formulated by von Baer and Meckel, the former of whom
even as early as 1834 pronounced himself a distinct evolutionist.

[Illustration: RAMSAY HEATLEY TRAQUAIR.

ARTHUR SMITH WOODWARD.

KARL A. ZITTEL.

CHARLES R. EASTMAN.]

=Second Period.=--=Systematic Study of Fossil Fishes.=--On the ground
planted by Agassiz, many important works sprang up within the next
decades. In England a vigorous school of palaeichthyologists was
soon flourishing. Many papers of Egerton date from this time, and
the important work of Owen on the structure of fossil teeth and the
often-quoted papers of Huxley in the "British Fossil Remains." Among
other workers may be mentioned James Powrie, author of a number of
papers upon Scottish Devonian fossils; the enthusiastic Hugh Miller,
stone-mason and geologist; Montague Brown, Thomas Atthey, J. Young,
and W. J. Barkas, students upon Coal Measure fishes; E. Ray Lankester,
some of whose early papers deal with pteraspids; E. T. Newton, author
of important works on chimaeroids. The extensive works of J. W. Davis
deal with fishes of many groups and many horizons. Mr. Davis, like
Sir Philip Gray Egerton, was an amateur whose devotion did much to
advance the study of fossil fishes. The dean of British palaeichthyology
is at present Dr. R. H. Traquair, of the Edinburgh Museum of Science
and Arts. During four decades he has devoted himself to his studies
with rare energy and success, author of a host of shorter papers and
numerous memoirs and reports. Finally, and belonging to a younger
generation of palaeontologists, is to be named Arthur Smith Woodward,
curator of vertebrate palaeontology of the British Museum. Dr. Woodward
has already contributed many scores of papers to palaeichthyology,
besides publishing a four-volume Catalogue of the Fossil Fishes of the
British Museum, a compendial work whose value can only be appreciated
adequately by specialists.

In the United States the study of fossil fishes was taken up by J. H.
and W. C. Redfield, father and son, prior to the work of Agassiz, and
there has been since that time an active school of American workers.
Agassiz himself, however, is not to be included in this list, since
his interest in extinct fishes became almost entirely unproductive
during his life in America. Foremost among these workers was John
Strong Newberry (1822-92), of Columbia College, whose publications
deal with fishes of many horizons and whose work upon this continent
is not unlike that of Agassiz in Europe. He was the author of many
state reports, separate contributions, and two monographs, one upon the
palaeozoic fishes of North America, the other upon the Triassic fishes.
Among the earlier palaeontologists were Orestes H. St. John, a pupil
of Agassiz at Harvard, and A. H. Worthen (1813-88), director of the
Geological Survey of Illinois; also W. Gibbes and Joseph Leidy. The
late E. D. Cope (1840-97) devoted a considerable portion of his labors
to the study of extinct fishes. E. W. Claypole, of Buchtel College, is
next to be mentioned as having produced noteworthy contributions to
our knowledge of sharks, palaeaspids, and arthrodires, as has also A.
A. Wright, of Oberlin College. Among other workers may be mentioned
O. P. Hay, of the American Museum; C. R. Eastman, of Harvard, author
of important memoirs upon arthrodires and other forms; Alban Stewart,
a student of Dr. S. W. Williston at Kansas University, and Bashford
Dean. Among Canadian palaeontologists G. F. Matthew deserves mention for
his work on Cyathaspis, Principal Dawson for interesting references to
Mesozoic fishes, and J. F. Whiteaves for his studies upon the Devonian
fishes of Scaumenac Bay.

Belgian palaeontologists have also been active in their study of fishes.
Here we may refer to the work of Louis Dollo, of Brussels, of Max
Lohest, of P. J. van Beneden, of L. G. de Koninck, of T. C. Winckler,
and of R. Storms, the last of whom has done interesting work on
Tertiary fishes.

Foremost among Russian palaeichthyologists is to be named C. H. Pander,
long-time Academician in St. Petersburg, whose elaborate studies of
extinct lung-fishes, ostracophores, and crossopterygians published
between 1856 and 1860 will long stand as models of careful work. We
should also refer to the work of H. Asmuss and H. Trautschold, E.
Eichwald and of Victor Rohon, the last named having published many
important papers upon ostracophores during his residence in St.
Petersburg.

German palaeichthyologists include Otto Jaekel, of Berlin; O. M. Reis of
the Oberbergamt, in Munich; A. von Koenen, of Goettingen; A. Wagner, E.
Koken, and K. von Zittel. Among Austro-Hungarians are Anton Fritsch,
author of the _Fauna der Gaskohleformations Boemens_; Rudolf Kner, an
active student of living fishes as well, as is also Franz Steindachner.

French palaeichthyologists are represented by the veteran H. E. Sauvage,
of Boulogne-sur-Mer, V. Tholliere, M. Brongniart, and F. Priem. In
Italy Francesco Bassani, of Naples, is the author of many important
works dealing with Mesozoic and Tertiary forms; also was Baron Achille
di Zigno. Robert Collett, of Bergen, and G. Lindstroem are worthy
representatives of Scandinavia in kindred work.

=Third Period.=--=Morphological Work on Fossil Fishes.=--Among the
writers who have dealt with the problems of the relationships of the
Ostracophores as well as _Palaeospondylus_ and the Arthrodires may be
named Traquair, Huxley, Newberry, Smith Woodward, Rohon, Eastman, and
Dean; most recently William Patten. Upon the phylogeny of the sharks
Traquair, A. Fritsch, Hasse, Cope, Brongniart, Jaekel, Reis, Eastman,
and Dean. On Chimaeroid morphology mention may be made of the papers
of A. S. Woodward, Reis, Jaekel, Eastman, C. D. Walcott, and Dean. As
to Dipnoan relationships the paper of Louis Dollo is easily of the
first value; of especial interest, too, is the work of Eastman as
to the early derivation of the Dipnoan dentition. In this regard a
paper of Rohon is noteworthy, as is also that of Richard Semon on the
development of the dentition of recent Neoceratodus, since it contains
a number of references to extinct types. Interest notes on Dipnoan fin
characters have been given by Traquair. In the morphology of Ganoids,
the work of Traquair and A. S. Woodward takes easily the foremost rank.
Other important works are those of Huxley, Cope, A. Fritsch, and Oliver
P. Hay.

=Anatomists.=--Still more difficult of enumeration is the long list of
those who have studied the anatomy of fishes usually in connection with
the comparative anatomy or development of other animals. Pre-eminent
among these are Karl Ernst von Baer, Cuvier, Geoffroy St. Hilaire,
Louis Agassiz, Johannes Mueller, Carl Vogt, Carl Gegenbaur, William
Kitchen Parker, Francis M. Balfour, Thomas Henry Huxley, Meckel,
H. Rathke, Richard Owen, Kowalevsky, H. Stannius, Joseph Hyrtl,
Gill, Boulenger, and Bashford Dean. Other names of high authority
are those of Wilhelm His, Koelliker, Bakker, Rosenthal, Gottsche,
Miklucho-Macleay, Weber, Hasse, Retzius, Owsjannikow, H. Mueller,
Stieda, Marcusen, J. A. Ryder, E. A. Andrews, T. H. Morgan, G. B.
Grassi, R. Semon, Howard Ayers, R. R. Wright, J. P. McMurrich, C. O.
Whitman, A. C. Eyclesheimer, E. Pallis, Jacob Reighard, and J. B.
Johnston.

Besides all this, there has risen, especially in the United States,
Great Britain, Norway, and Canada and Australia, a vast literature of
commercial fisheries, fish culture, and angling, the chief workers in
which fields we may not here enumerate even by name.

FOOTNOTES:

[148] For these paragraphs on the history of the study of fossil fishes
the writer is indebted to the kind interest of Professor Bashford Dean.

[149] Dr. Arthur Smith Woodward excepted.




CHAPTER XXIII

THE COLLECTION OF FISHES


=How to Secure Fishes.=--In collecting fishes three things are vitally
necessary--a keen eye, some skill in adapting means to ends, and some
willingness to take pains in the preservation of material.

In coming into a new district the collector should try to preserve the
first specimen of every species he sees. It may not come up again.
He should watch carefully for specimens which look just a little
different from their fellows, especially for those which are duller,
less striking, or with lower fins. Many species have remained unnoticed
through generations of collectors who have chosen the handsomest or
most ornate specimens. In some groups with striking peculiarities, as
the trunkfishes, practically all the species were known to Linnaeus.
No collector could pass them by. On the other hand, new gobies or
blennies can be picked up almost every day in the lesser known parts of
the world. For these overlooked forms--herrings, anchovies, sculpins,
blennies, gobies, scorpion-fishes--the competent collector should be
always on the watch. If any specimen looks different from the rest,
take it at once and find out the reason why.

In most regions the chief dependence of the collector is on the markets
and these should be watched most critically. By paying a little more
for unusual, neglected, or useless fish, the supply of these will rise
to the demand. The word passed along among the people of Onomichi in
Japan, that "Ebisu the fish-god was in the village" and would pay more
for okose (poison scorpion-fishes) and umiuma (sea-horses) than real
fishes were worth soon brought (in 1900) all sorts of okose and umiuma
into the market when they were formerly left neglected on the beach.
Thus with a little ingenuity the markets in any country can be greatly
extended.

The collector can, if he thinks best, use all kinds of fishing tackle
for himself. In Japan he can use the "dabonawa" long lines, and secure
the fishes which were otherwise dredged by the _Challenger_ and
_Albatross_. If dredges or trawls are at his hand he can hire them and
use them for scientific purposes. He should neglect no kind of bottom,
no conditions of fish life which he can reach.

Especially important is the fauna of the tide-pools, neglected by
almost all collectors. As the tide goes down, especially on rocky
capes which project into the sea, myriads of little fishes will remain
in the rock-pools, the algae, and the clefts of rock. In regions like
California, where the rocks are buried with kelp, blennies will lie in
the kelp as quiescent as the branches of the algae themselves until the
flow of water returns.

A sharp three-tined fork will help in spearing them. The water in
pools can be poisoned on the coast of Mexico with the milky juice of
the "hava" tree, a tree which yields strychnine. In default of this,
pools can be poisoned by chloride of lime, sulphate of copper, or, if
small enough, by formaline. Of all poisons the commercial chloride of
lime seems to be most effective. By such means the contents of the pool
can be secured and the next tide carries away the poison. The water in
pools can be bailed out, or, better, emptied by a siphon made of small
garden-hose or rubber tubing. On rocky shores, dynamite can be used to
advantage if the collector or his assistant dare risk it and if the
laws of the country do not prevent.

Most effective in rock-pool work is the help of the small boy. In
all lands the collector will do well to take him into his pay and
confidence. Of the hundred or more new species of rock-pool fishes
lately secured by the writer in Japan, fully two-thirds were obtained
by the Japanese boys. Equally effective is the "muchacho" on the coasts
of Mexico.

Masses of coral, sponges, tunicates, and other porous or hollow
organisms often contain small fishes and should be carefully examined.
On the coral reefs the breaking up of large masses is often most
remunerative.

The importance of securing the young of pelagic fishes by tow-nets and
otherwise cannot be too strongly emphasized.

=How to Preserve Fishes.=--Fishes must be permanently preserved in
alcohol. Dried skins are far from satisfactory, except as a choice of
difficulties in the case of large species.

Dr. Guenther thus describes the process of skinning fishes:

"Scaly fishes are skinned thus: With a strong pair of scissors an
incision is made along the median line of the abdomen from the foremost
part of the throat, passing on one side of the base of the ventral
and anal fins to the root of the caudal fin, the cut, being continued
upward to the back of the tail close to the base of the caudal. The
skin of one side of the fish is then severed with the scalpel from
the underlying muscles to the median line of the back; the bones
which support the dorsal and caudal are cut through, so that these
fins remain attached to the skin. The removal of the skin of the
opposite side is easy. More difficult is the preparation of the head
and scapulary region. The two halves of the scapular arch which have
been severed from each other by the first incision are pressed toward
the right and left, and the spine is severed behind the head, so that
now only the head and shoulder bones remain attached to the skin.
These parts have to be cleaned from the inside, all soft parts, the
branchial and hyoid apparatus, and all smaller bones being cut away
with the scissors or scraped off with the scalpel. In many fishes which
are provided with a characteristic dental apparatus in the pharynx
(Labroids, Cyprinoids), the pharyngeal bones ought to be preserved and
tied with a thread to their specimen. The skin being now prepared so
far, its entire inner surface as well as the inner side of the head are
rubbed with arsenical soap; cotton-wool or some other soft material is
inserted into any cavities or hollows, and finally a thin layer of the
same material is placed between the two flaps of the skin. The specimen
is then dried under a slight weight to keep it from shrinking.

"The scales of some fishes, as for instance of many kinds of herrings,
are so delicate and deciduous that the mere handling causes them to rub
off easily. Such fishes may be covered with thin-paper (tissue paper
is the best) which is allowed to dry on them before skinning. There
is no need for removing the paper before the specimen has reached its
destination.

"Scaleless fishes, as siluroids and sturgeons, are skinned in the same
manner, but the skin can be rolled up over the head; such skins can
also be preserved in spirits, in which case the traveler may save to
himself the trouble of cleaning the head.

"Some sharks are known to attain to a length of thirty feet, and some
rays to a width of twenty feet. The preservation of such gigantic
specimens is much to be recommended, and although the difficulties
of preserving fishes increase with their size, the operation is
facilitated, because the skins of all sharks and rays can easily be
preserved in salt and strong brine. Sharks are skinned much in the
same way as ordinary fishes. In rays an incision is made not only from
the snout to the end of the fleshy part of the tail, but also a second
across the widest part of the body. When the skin is removed from the
fish, it is placed into a cask with strong brine mixed with alum, the
head occupying the upper part of the cask; this is necessary, because
this part is most likely to show signs of decomposition, and therefore
most requires supervision. When the preserving fluid has become
decidedly weaker from the extracted blood and water, it is thrown away
and replaced by fresh brine. After a week's or fortnight's soaking the
skin is taken out of the cask to allow the fluid to drain off; its
inner side is covered with a thin layer of salt, and after being rolled
up (the head being inside) it is packed in a cask the bottom of which
is covered with salt; all the interstices and the top are likewise
filled with salt. The cask must be perfectly water-tight."

=Value of Formalin.=--In the field it is much better to use formalin
(formaldehyde) in preference to alcohol. This is an antiseptic fluid
dissolved in water, and it at once arrests decay, leaving the specimen
as though preserved in water. If left too long in formalin fishes
swell, the bones are softened, and the specimens become brittle or even
worthless. But for ordinary purposes (except use as skeleton) no harm
arises from two or three months' saturation in formalin. The commercial
formalin can be mixed with about twenty parts of water. On the whole
it is better to have the solution too weak rather than too strong. Too
much formalin makes the specimens stiff, swollen, and intractable,
besides too soon destroying the color.

Formalin has the advantage, in collecting, of cheapness and of ease in
transportation, as a single small bottle will make a large amount of
the fluid. The specimens also require much less attention. An incision
should be made in the (right) side of the abdomen to let in the fluid.
The specimen can then be placed in formalin. When saturated, in the
course of the day, it can be wrapped in a cloth, packed in an empty
petroleum can, and at once shipped. The wide use of petroleum in all
parts of the world is a great boon to the naturalist.

Before preservation, the fishes should be washed, to remove slime and
dirt. They should have an incision to let the fluid into the body
cavity and an injection with a syringe is a useful help to saturation,
especially with large fishes. Even decaying fishes can be saved with
formalin.

=Records of Fishes.=--The collector should mark localities most
carefully with tin tags and note-book records if possible. He should,
so far as possible, keep records of life colors, and water-color
sketches are of great assistance in this matter. In spirits or formalin
the life colors soon fade, although the pattern of marking is usually
preserved or at least indicated. A mixture of formalin and alcohol is
favorable to the preservation of markings.

In the museum all specimens should be removed at once from formalin
to alcohol. No substitute for alcohol as a permanent preservative has
been found. The spirits derived from wine, grain, or sugar is much
preferable to the poisonous methyl or wood alcohol.

In placing specimens directly into alcohol, care should be taken not to
crowd them too much. The fish yields water which dilutes the spirit.
For the same reason, spirits too dilute are ineffective. On the other
hand, delicate fishes put into very strong alcohol are likely to
shrivel, a condition which may prevent an accurate study of their fins
or other structures. It is usually necessary to change a fish from
the first alcohol used as a bath into stronger alcohol in the course
of a few days, the time depending on the closeness with which fishes
are packed. In the tropics, fishes in alcohol often require attention
within a few hours. In formalin there is much less difficulty with
tropical fishes.

Fishes intended for skeletons should never be placed in formalin.
A softening of the bones which prevents future exact studies of
the bones is sure to take place. Generally alcohol or other spirits
(arrack, brandy, cognac, rum, sake "vino") can be tested with a match.
If sufficiently concentrated to be ignited, they can be safely used
for preservation of fishes. The best test is that of the hydrometer.
Spirits for permanent use should show on the hydrometer 40 to 60 above
proof. Decaying specimens show it by color and smell and the collector
should be alive to their condition. One rotting fish may endanger
many others. With alcohol it is necessary to take especial pains to
ensure immediate saturation. Deep cuts should be made into the muscles
of large fishes as well as into the body cavity. Sometimes a small
distilling apparatus is useful to redistil impure or dilute alcohol.
The use of formalin avoids this necessity.

Small fishes should not be packed with large ones; small bottles are
very desirable for their preservation. All spinous or scaly fishes
should be so wrapped in cotton muslin as to prevent all friction.

=Eternal Vigilance.=--The methods of treating individual groups
of fishes and of handling them under different climatic and other
conditions are matters to be learned by experience. Eternal vigilance
is the price of a good collection, as it is said to be of some other
good things. Mechanical collecting--picking up the thing got without
effort and putting it in alcohol without further thought--rarely serves
any useful end in science. The best collectors are usually the best
naturalists. The collections made by the men who are to study them and
who are competent to do so are the ones which most help the progress
of ichthyology. The student of a group of fishes misses half the
collection teaches if he has made no part of it himself.




CHAPTER XXIV

THE EVOLUTION OF FISHES


=The Geological Distribution of Fishes.=--The oldest unquestioned
remains of fishes have been very recently made known by Mr. Charles
D. Walcott, from rocks of the Trenton period in the Ordovician or
Lower Silurian. These are from Canon City in Colorado. Among these
is certainly a small Ostracophore (_Asteraspis desideratus_). With
it are fragments (_Dictyorhabdus_) thought to be the back-bone of a
Chimaera, but more likely, in Dean's view, the axis of a cephalopod,
besides bony, wrinkled scales, referred with doubt to a supposed
Crossopterygian genus called _Eriptychius_. This renders certain
the existence of _Ostracophores_ at this early period, but their
association with _Chimaeras_ and Crossopterygians is questionable.
Primitive sharks may have existed in Ordovician times, but thus far no
trace of them has been found.

[Illustration: FIG. 246.--Fragment of Sandstone from Ordovician
deposits, Canon City, Colo., showing fragments of scales, etc., the
earliest known traces of vertebrates. (From nature.)]

The fish-remains next in age in America are from the Bloomfield
sandstone in Pennsylvania of the Onondaga period in the upper
Silurian. The earliest in Europe are found in the Ludlow shales, both
of these localities being in or near the horizon of the Niagara rocks,
in the Upper Silurian Age.

It is, however, certain that these Lower Silurian remains do not
represent the beginning of fish-life. Probably _Ostracophores_, and
_Arthrodires_, with perhaps Crossopterygians and Dipnoans, existed
at an earlier period, together perhaps with unarmed, limbless forms
without jaws, of which no trace whatever has been left.

[Illustration: FIG. 247.--Fossil fish remains from Ordovician rocks,
Canon City, Colo. (After Walcott.) _a._ Scale of _Eriptychius
americanus_ Walcott. Family _Holoptychiidae?_ _b._ Dermal plate
of _Asteraspis desideratus_ Walcott. Family _Asterolepidae_. _c._
_Dictyorhabdus priscus_ Walcott, a fragment of uncertain nature,
thought to be a chordal sheath of a Chimaera, but probably part of a
Cephalopod (Dean). _Chimaeridae?_]

=The Earliest Sharks.=--The first actual trace of sharks is found
in the Upper Silurian in the form of fin-spines (_Onchus_), thought
to belong to primitive sharks, perhaps Acanthodeans possibly to
Ostracophores. With these are numerous bony shields of the mailed
Ostracophores, and somewhat later those of the more highly specialized
Arthrodires. Later appear the teeth of _Cochliodontidae_, with Chimaeras,
a few Dipnoans, and Crossopterygians.

=Devonian Fishes.=--In the Devonian Age the _Ostracophores_ increase
in size and abundance, disappearing with the beginning of the
Carboniferous. The Arthrodires also increase greatly in variety
and in size, reaching their culmination in the Devonian, but not
disappearing entirely until well in the Carboniferous. These two groups
(often united by geologists under the older name Placoderms) together
with sharks and a few Chimaeras made up almost exclusively the rich
fish-fauna of Devonian times. The sharks were chiefly Acanthodean and
Psammodont, as far as our records show. The supposed more primitive
type of _Cladoselache_ is not known to appear before the latter part
of the Devonian Age, while _Pleuracanthus_ and _Cladodus_, sometimes
regarded as still more primitive, are as yet found only in the
Carboniferous. It is clear that the records of early shark life are
still incomplete, whatever view we may adopt as to the relative rank of
the different forms. Chimaeroids occur in the Devonian, and with them a
considerable variety of Crossopterygians and Dipnoans. The true fishes
appear also in the Devonian in the guise of the Ganoid ancestors and
relatives of _Palaeoniscum_, all with diamond-shaped enameled scales. In
the Devonian, too, we find the minute creature _Palaeospondylus_, our
ignorance of which is concealed under the name _Cycliae_.

=Carboniferous Fishes.=--In the Carboniferous Age the sharks increase
in number and variety, the Ostracophores disappear, and the Arthrodires
follow them soon after, the last being recorded from the Permian.
Other forms of Dipnoans, Crossopterygians, and some Ganoids now appear
giving the fauna a somewhat more modern aspect. The _Acanthodei_ and
the _Ichthyotomi_ pass away with the Permian, the latest period of the
Carboniferous Age.

[Illustration: FIG. 248.--_Dipterus valenciennesi_ Agassiz, a Dipnoan.
(After Dean, from Woodward.)]

=Mesozoic Fishes.=--In the Triassic period which follows the Permian,
the earliest types of Ganoids give place to forms approaching the
garpike and sturgeon. The Crossopterygians rapidly decline. The
Dipnoans are less varied and fewer in number; the primitive sharks,
with the exception of certain Cestracionts, all disappear, only the
family of _Orodontidae_ remaining. Here are found the first true
bony fishes, doubtless derived from Ganoid stock, the allies and
predecessors of the great group of herrings. Herring-like forms become
more numerous in the Jurassic, and with them appear other forms which
give the fish-fauna of this period something of a modern appearance. In
the Jurassic the sharks become divided into several groups, _Notidani_,
Scyllioid sharks, Lamnoid sharks, angel-fishes, skates, and finally
Carcharioid sharks being now well differentiated. Chimaeras are still
numerous. The _Acanthodei_ have passed away, as well as the mailed
Ostrachopores and Arthrodires. The Dipnoans and Crossopterygians are
few. The early Ganoids have given place to more modern types, still in
great abundance and variety. This condition continues in the Cretaceous
period. Here the rays and modern sharks increase in number, the Ganoids
hold their own, and the other groups of soft-rayed fishes, as the
smelts, the lantern-fishes, the pikes, the flying-fishes, the berycoids
and the mackerels join the group of herring-like forms which represent
the modern bony fishes. In the Cretaceous appear the first spiny-rayed
fishes, derived probably from herring-like forms. These are allies or
ancestors of the living genus _Beryx_.

[Illustration: FIG. 249.--_Hoplopteryx lewesiensis_ (Mantell),
restored. English Cretaceous. Family _Berycidae_. (After Woodward.)]

Dr. Woodward observes:

"As soon as fishes with a completely osseous endoskeleton began to
predominate at the dawn of the Cretaceous period, specializations of
an entirely new kind were rapidly acquired. Until this time the skull
of the Actinopterygii had always been remarkably uniform in type. The
otic region of the cranium often remained incompletely ossified and was
never prominent or projecting beyond the roof bones; the supraoccipital
bone was always small and covered with the superficial plates; the
maxilla invariably formed the greater part of the upper jaw; the
cheek-plates were large and usually thick; while none of the head or
opercular bones were provided with spines or ridges. The pelvic fins
always retained their primitive remote situation, and the fin-rays
never became spines. During the Cretaceous period the majority of the
bony fishes began to exhibit modifications in all these characters, and
the changes occurred so rapidly that by the dawn of the Eocene period
the diversity observable in the dominant fish-fauna was much greater
than it had ever been before. At this remote period, indeed, nearly all
the great groups of bony fishes, as represented in the existing world,
were already differentiated, and their subsequent modifications have
been quite of a minor character."

[Illustration: FIG. 250.--A living Berycoid fish, _Paratrachichthys
prosthemius_ Jordan & Fowler. Misaki, Japan. Family _Berycidae_.]

[Illustration: FIG. 251.--Flying-fish, _Cypsilurus heterurus_
(Rafinesque). Family _Exocaetidae_ Woods Hole, Mass.]

[Illustration: FIG. 252.--The Schoolmaster Snapper, a Perch-like fish.
Family _Lutianidae_. Key West.]

=Tertiary Fishes.=--With the Eocene or first period of the Tertiary
great changes have taken place. The early families of bony fishes
nearly all disappear. The herring, pike, smelt, salmon, flying-fish,
and berycoids remain, and a multitude of other forms seem to
spring into sudden existence. Among these are the globefishes,
the trigger-fishes, the catfishes, the eels, the morays, the
butterfly-fishes, the porgies, the perch, the bass, the pipefishes,
the trumpet-fishes, the mackerels, and the John-dories, with the
sculpins, the anglers, the flounders, the blennies, and the cods.
That all these groups, generalized and specialized, arose at once
is impossible, although all seem to date from the Eocene times.
Doubtless each of them had its origin at an earlier period, and the
simultaneous appearance is related to the fact of the thorough study
of the Eocene shales, which have in numerous localities (London, Monte
Bolca, Licata, Mount Lebanon, Green River) been especially favorable
for the preservation of these forms. Practically fossil fishes have
been thoroughly studied as yet only in a very few parts of the earth.
The rocks of Scotland, England, Germany, Italy, Switzerland, Syria,
Ohio, and Wyoming have furnished the great bulk of all the fish remains
in existence. In some regions perhaps collections will be made which
will give us a more just conception of the origin of the different
groups of bony fishes. We can now only say with certainty that the
modern families were largely existent in the Eocene, that they sprang
from ganoid stock found in the Triassic and Jurassic, that several of
them were represented in the Cretaceous also, that the Berycoids were
earliest of the spiny-rayed fishes, and forms allied to herring the
earliest of the soft-rayed forms. Few modern families arose before the
Cretaceous. Few of the modern genera go back to the Eocene, many of
them arose in the Miocene, and few species have come down to us from
rocks older than the end of the Pliocene. The general modern type of
the fish-faunas being determined in the latter Eocene and the Miocene,
the changes which bring us to recent times have largely concerned
the abundance and variety of the individual species. From geological
distribution we have arising the varied problems of geographical
distribution and the still more complex conditions on which depend the
extinction of species and of types.

[Illustration: FIG. 253.--Decurrent Flounder, _Pleuronichthys
decurrens_ Jordan & Gilbert. San Francisco.]

=Factors of Extinction.=--These factors of extinction have been
recently formulated as follows by Professor Herbert Osborn. He
considers the process of extinction as of five different types:

"(1) That extinction which comes from modification or progressive
evolution, a relegation to the past as a result of the transmutation
into more advanced forms. (2) Extinction from changes of physical
environment which outrun the powers of adaptation. (3) The extinction
which results from competition. (4) The extinction which results from
extreme specialization and limitation to special conditions the loss of
which means extinction. (5) Extinction as a result of exhaustion."

=Fossilization of a Fish.=--When a fish dies he leaves no friends. His
body is at once attacked by hundreds of creatures ranging from the
one-celled protozoa and bacteria to individuals of his own species. His
flesh is devoured, his bones are scattered, the gelatinous substance
in them decays, and the phosphate of lime is in time dissolved in the
water. For this reason few fishes of the millions which die each year
leave any trace for future preservation. At the most a few teeth, a
fin-spine, or a bone buried in the clay might remain intact or in such
condition as to be recognized.

But now and then it happens that a dead fish may fall in more fortunate
conditions. On a sea bottom of fine clay the bones, or even the whole
body, may be buried in such a way as to be sealed up and protected
from total decomposition. The flesh will usually disappear and leave
no mark or at the most a mere cast of its surface. But the hard parts,
even the muscles may persist, and now and then they do persist, the
salts of lime unchanged or else silicified or subjected to some other
form of chemical substitution. Only the scales, the teeth, the bones,
the spines, and the fin-rays can be preserved in the rocks of sea or
lake bottom. In a few localities, as near Green River in Wyoming, Monte
Bolca, near Verona, and Mount Lebanon in Syria, the London clays, with
certain quarries in Scotland and lithographic stones in Germany, many
skeletons of fishes have been found pressed flat in layers of very fine
rock, their structures traced as delicately as if actually drawn on the
smooth stone. Fragments preserved in ruder fashion abound in the clays
and even the sandstones of the earliest geologic ages. In most cases,
however, fossil fishes are known from detached and scattered fragments,
many of them, especially of the sharks, by the teeth alone. Fishes have
occurred in all ages from the Silurian to the present time and probably
the very first lived long before the Silurian.

=The Earliest Fishes.=--No one can say what the earliest fishes were
like, nor do we know what was their real relation to the worm-like
forms among which men have sought their presumable ancestors, nor
to the Tunicates and other chordate forms, not fish-like, but still
degenerate relatives of the primeval fish.

From analogy we may suppose that the first fishes which ever were bore
some resemblance to the lancelet, for that is a fish-like creature with
every structure reduced to the lowest terms. But as the lancelet has no
hard parts, no bones, nor teeth, nor scales, nor fins, no traces of its
kind are found among the fossils. If the primitive fish was like it in
important respects, all record of this has probably vanished from the
earth.

=The Cyclostomes.=--The next group of living fishes, the Cyclostomes,
including the hagfishes and lampreys,--fishes with small skull and
brain but without limbs or jaws,--stands at a great distance above the
lancelet in complexity of structure, and equally far from the true
fishes in its primitive simplicity. In fact the lamprey is farther
from the true fish in structure than a perch is from an eagle. Yet
for all that it may be an offshoot from the primitive line of fish
descent. There is not much in the structure of the lamprey which may
be preserved in the rocks. But the cartilaginous skull, the back-bone,
fins, and teeth might leave their traces in soft clay or lithographic
stone. But it is certain that they have not done so in any rocks yet
explored, and it may be that the few existing lampreys owe their form
and structure to a process of degradation from a more complex and more
fish-like ancestry. The supposed lamprey fossil of the Devonian of
Scotland, _Palaeospondylus_, has little in common with the true lampreys.

=The Ostracophores.=--Besides the lampreys the Devonian seas swarmed
with mysterious creatures covered with an armor of plate, fish-like
in some regards, but limbless, without true jaws and very different
from the true fishes of to-day. These are called Ostracophori, and
some have regarded them as mailed lampreys, but they are more likely
to be a degenerate or eccentric offshoot from the sharks, as highly
modified or specialized lampreys, a side offshoot which has left no
descendants among recent forms. Recently Professor Patten has insisted
that the resemblance of their head-plates to those of the horseshoe
crab (_Limulus_) is indicative of real affinity.

Among these forms in mail-armor are some in which the jointed and
movable angles of the head suggest the pectoral spines of some
catfishes. But in spite of its resemblance to a fin, the spine in
_Pterichthyodes_ is an outgrowth of the ossified skin and has no more
homology with the spines of fishes than the mailed plates have with the
bones of a fish's cranium. In none of these fishes has any trace of
an internal skeleton been found. It must have retained its primitive
gelatinous character. There are, however, some traces of eyes, and
the mucous channels of the lateral line indicate that these creatures
possessed some other special senses.

[Illustration: FIG. 254.--An Ostracophore, _Cephalaspis lyelli_
Agassiz, restored. Devonian. (After Agassiz, per Dean.)]

Whatever the Ostracophores may be, they should not be included within
the much-abused term _Ganoidei_, a word which was once used in the
widest fashion for all sorts of mailed fishes, but little by little
restricted to the hard-scaled relatives and ancestors of the garpike of
to-day.

=The Arthrodires.=--Dimly seen in the vast darkness of Paleozoic time
are the huge creatures known as Arthrodires. These are mailed and
helmeted fishes, limbless so far as we know, but with sharp, notched,
turtle-like jaws quite different from those of the fish or those of
any animal alive to-day. These creatures appear in Silurian rocks and
are especially abundant in the fossil beds of Ohio, where Newberry,
Claypole, Eastman, Dean and others have patiently studied the broken
fragments of their armor. Most of them have a great casque on the head
with a shield at the neck and a movable joint connecting the two. Among
them was almost every variation in size and form.

[Illustration: FIG. 255.--An Arthrodire, _Dinichthys intermedius_
Newberry, restored. Devonian, Ohio. (Family after Dean.)]

These creatures have been often called ganoids, but with the true
ganoids like the garpike they have seemingly nothing in common. They
are also different from the Ostracophores. To regard them with Woodward
as derived from ancestral Dipnoans is to give a possible guess as to
their origin, and a very unsatisfactory guess at that. In any event
these have all passed away in competition with the scaly fishes and
sharks of later evolution, and it seems certain that they, like the
mailed Ostracophores, have left no descendants.

=The Sharks.=--Next after the lampreys, but a long way after them in
structure, come the sharks. With the sharks appear for the first time
true limbs and the lower jaw. The upper jaw is, however, formed from
the palate, and the shoulder-girdle is attached behind the skull.
"Little is known," says Professor Dean, "of the primitive stem of the
sharks, and even the lines of descent of the different members of the
group can only be generally suggested. The development of recent forms
has yielded few results of undoubted value to the phylogenist. It
would appear as if paleontology alone could solve the puzzles of their
descent."

Of the very earliest sharks in the Upper Silurian Age the remains are
too scanty to prove much save that there were sharks in abundance
and variety. Spines, teeth, fragments of shagreen, show that in some
regards these forms were highly specialized. In the Carboniferous Age
the sharks became highly varied and extensively specialized. Of the
Paleozoic types, however, all but a single family seems to have died
out, leaving Cestraciontes only in the Permian and Triassic. From these
the modern sharks one and all may very likely have descended.

=Origin of the Sharks.=--Perhaps the sharks are developed from the
still more primitive shark imagined as without limbs and with the teeth
slowly formed from modification of the ordinary shagreen prickles. In
determining the earliest among the several primitive types of shark
actually known we are stopped by an undetermined question of theory.
What is the origin of paired limbs? Are these formed, like the unpaired
fins, from the breaking up of a continuous fold of skin, in accordance
with the view of Balfour and others? Or is the primitive limb, as
supposed by Gegenbaur, a modification of the bony gill-arch? Or again,
as supposed by Kerr, is it a modification of the hard axis of an
external gill?

If we adopt the views of Gegenbaur or Kerr, the earliest type of limb
is the jointed _archipterygium_, a series of consecutive rounded
cartilaginous elements with a fringe of rays along its length.
Sharks possessing this form of limb (_Ichthyotomi_) appear in the
Carboniferous rocks, but are not known earlier. It may be that from
these the Dipnoans, on the one hand, may be descended and, on the
other, the true sharks and the Chimaeras; but there is no certainty that
the jointed arm or archipterygium of the Dipnoans is derived from the
similar pectoral fin of the _Ichthyotomi_.

On the other hand, if we regard the paired fins as parts of a lateral
fold of skin, we find primitive sharks to bear out our conclusions.
In _Cladoselache_ of the Upper Devonian, the pectoral and the ventral
fins are long and low, and arranged just as they might be if Balfour's
theory were true. _Acanthoessus_, with a spine in each paired fin and
no other rays, might be a specialization of this type or fin, and
_Climatius_, with rows of spines in place of pectorals and ventrals,
might be held to bear out the same idea. In all these the tail is
less primitive than in the _Ichthyotomi_. On the other hand, the vent
in _Cladoselache_ is thought by Dean to have been near the end of
the tail. If this is the case, it should indicate a very primitive
character. On the whole, though there is much to be said in favor of
the primitive nature of the _Ichthyotomi_ (_Pleuracanthus_) with the
tapering tail and jointed pectoral fin of a dipnoan, and other traits
of a shark, yet, on the whole, _Cladoselache_ is probably nearer the
origin of the shark-like forms.

The relatively primitive sharks called _Notidani_ have the weakly
ossified vertebrae joined together in pairs and there are six or seven
gill-openings. This group has persisted to our day, the frilled shark
(_Chlamydoselachus_) and the genera _Hexanchus_ and _Heptranchias_
still showing its archaic characters.

Here the sharks diverge into two groups, the one with the vertebrae
better developed and its calcareous matter arranged star-fashion.
This forms Hasse's group of _Asterospondyli_, the typical sharks. The
earliest forms (_Orodontidae_, _Heterodontidae_) approach the _Notidani_,
and so far as geological records go, precede all the other modern
sharks. One such ancient type, _Heterodontus_, including the bullhead
shark, and the Port Jackson shark, still persists. The others diverge
to form the three chief groups of the cat-sharks (_Scyliorhinus_,
etc.), the mackerel-sharks (_Lamna_, etc.), and the true sharks
(_Carcharhias_, etc.).

[Illustration: FIG. 256.--Mackerel-shark or Salmon-shark, _Lamna
cornubica_ (Gmelin). Santa Barbara, Cal.]

In the second group the vertebrae have their calcareous matter arranged
in rings, one or more about the notochordal center. In all these the
anal fin is absent, and in the process of specialization the shark
gradually gives place to the flattened body and broad fins of the ray.
This group is called _Tectospondyli_. Those sharks of this group with
one ring of calcareous matter in each vertebra constitute the most
primitive extreme of a group representing continuous evolution.

From _Cladoselache_ and _Chlamydoselachus_ through the sharks to the
rays we have an almost continuous series which reaches its highest
development in the devil rays or mantas of the tropical seas, _Manta_
and _Mobula_ being the most specialized genera and among the very
largest of the fishes. However different the rays and skates may appear
in form and habit, they are structurally similar to the sharks and have
sprung from the main shark stem.

[Illustration: FIG. 257.--Star-spined Ray, _Raja stellulata_ Jordan &
Gilbert. Monterey, Cal.]

=The Chimaeras.=--The most ancient offshoot from the shark stem, perhaps
dating from Silurian times and possibly separated at a period earlier
than the date of any known shark, is the group of _Holocephali_ or
Chimaeras, shark-like in essentials, but differing widely in details.
Of these there are but few living forms and the fossil types are known
only from dental plates and fin-spines. The living forms are found
in the deeper seas the world over, one of the simplest in structure
being the newly discovered _Rhinochimaera_ of Japan. The fusion of the
teeth into overlapping plates, the covering of the gills by a dermal
flap, the complete union of the palato-quadrate apparatus or upper jaw
with the skull and the development of a peculiar clasping spine on the
forehead of the male are characteristic of the Chimaeras. The group is
one of the most ancient, but it ends with itself, none of the modern
fishes being derived from Chimaeras.

[Illustration: FIG. 258.--A Deep-sea Chimaera, _Harriotta raleighiana_
Goode & Bean. Gulf Stream.]

[Illustration: FIG. 259.--An extinct Dipnoan, _Dipterus valenciennesi_
Agassiz. Devonian. (After Pander.)]

=The Dipnoans.=--The most important offshoot of the primitive sharks is
not the Chimaeras, nor even the shark series itself, but the groups of
Crossopterygians and Dipnoans, or lung-fishes, with the long chain of
their descendants. With the Dipnoan appears the lung or air-bladder,
at first an outgrowth from the ventral side of the oesophagus, as
it still is in all higher animals, but later turning over, among
fishes, and springing from the dorsal side. At first an arrangement
for breathing air, a sort of accessory gill, it becomes the sole
organs of respiration in the higher forms, while in the bony fishes
its respiratory function is lost altogether. The air-bladder is a
degenerate lung. In the Dipnoans the shoulder-girdle moves forward to
the skull, and the pectoral limb, a jointed and fringed archipterygium,
is its characteristic appendage. The shark-like structure of the mouth
remains.

The few living lung-fishes resemble the salamanders in many regards,
and some writers have ranged the class as midway between the primitive
sharks and the amphibians. These forms show their intermediate
characters in the development of lungs and in the primitive character
of the pectoral and ventral limbs. Those now extant give but little
idea of the great variety of extinct Dipnoans. The living genera are
three in number--_Neoceratodus_ in Australian rivers, _Lepidosiren_ in
the Amazon, and _Protopterus_ in the Nile. These are all mudfishes,
some of them living through most of the dry season encased in a cocoon
of dried mud. Of these forms _Neoceratodus_ is certainly the nearest to
the ancient forms, but its embryology, owing to the shortening of its
growth stages due to its environment, has thrown little light on the
question of its ancestry.

From some ally of the Dipnoans the ancestry of the amphibians, and
through them that of the reptiles, birds, and mammals may be traced,
although a good deal of evidence has been produced in favor of
regarding the primitive crossopterygian or fringe fin as the point of
divergence. It is not unlikely that the Crossopterygian gave rise to
Amphibian and Dipnoan alike.

In the process of development we next reach the characteristic fish
mouth in which the upper jaw is formed of maxillary and premaxillary
elements distinct from the skull. The upper jaw of the shark is part
of the palate, the palate being fused with the quadrate bone which
supports the lower jaw. That of the Dipnoan is much the same. The
development of a typical fish mouth is the next step in evolution, and
with its appearance we note the decline of the air-bladder in size and
function.

=The Crossopterygians.=--The fish-like mouth appears with the group
of Crossopterygians, fishes which still retain the old-fashioned
type of pectoral and ventral fin, the archipterygium. In the archaic
tail, enameled scales, and cartilaginous skeleton the Crossopterygian
shows its affinity with its Dipnoan ancestry. Thus these fishes unite
in themselves traits of the shark, lung-fish, and Ganoid. The few
living Crossopterygians, _Polypterus_ and _Erpetoichthys_, are not
very different from those which prevailed in Devonian times. The
larvae possess external gills with firm base and fringe-like rays,
suggesting a resemblance to the pectoral fin itself, which develops
from the shoulder-girdle just below it and would seem to give some
force to Kerr's contention that the archipterygium is only a modified
external gill. In _Polypterus_ the archipterygium has become short and
fan-shaped, its axis made of two diverging bones with flat cartilage
between. From this type it is thought that the arm of the higher
forms has been developed. The bony basis may be the humerus, from
which diverge radius and ulna, the carpal bones being formed of the
intervening cartilage.

[Illustration: FIG. 260.--An extinct Crossopterygian, _Holoptychius
giganteus_ Agassiz (1835). (After Agassiz, per Zittel.)]

=The Actinopteri.=--From the Crossopterygians springs the main branch
of the true fishes, known collectively as _Actinopteri_, or ray-fins,
those with ordinary rays on the paired fins instead of the jointed
archipterygium. The transitional series of primitive _Actinopteri_ are
usually known as Ganoids. The Ganoid differs from the Crossopterygian
in having the basal elements of the paired fins small and concealed
within the flesh. But other associated characters of the Crossopterygii
and Dipnoans are preserved in most of the species. Among these are the
mailed head and body, the heterocercal tail, the cellular air-bladder,
the presence of valves in the arterial bulb, the presence of a spiral
valve in the intestine and of a chiasma in the optic nerves. All these
characters are found in the earlier types so far as is known, and all
are more or less completely lost or altered in the teleosts or bony
fishes. Among these early types is every variety of form, some of
them being almost as long as deep, others arrow-shaped, and every
intermediate form being represented. An offshoot from this line is the
bowfin (_Amia calva_), among the Ganoids the closest living ally of the
bony fishes, showing distinct affinities with the great group to which
the herring and salmon belong. Near relatives of the bowfin flourished
in the Mesozoic, among them some with a forked tail, and some with a
very long one. From Ganoids of this type the vast majority of recent
fishes may be descended.

[Illustration: FIG. 261.--An ancient Ganoid fish, _Platysomus gibbosus_
Blainville. Family _Platysomidae_. (After Woodward.)]

[Illustration: FIG. 262.--A living Ganoid fish, the Short-nosed Gar,
_Lepisosteus platystomus_ Rafinesque. Lake Erie.]

Another branch of Ganoids, divergent from both garfish and bowfin and
not recently from the same primitive stock, included the sturgeons
(_Acipenser_, _Scaphirhynchus_, _Kessleria_) and the paddle-fishes
(_Polyodon_ and _Psephurus_). All these are regarded by Woodward as
degenerate descendants of the earliest Ganoids, _Palaeoniscidae_, of
Devonian and Carboniferous time.

[Illustration: FIG. 263.--A primitive Ganoid fish, _Palaeoniscum
macropomum_ (Agassiz), restored. Permian. Family _Potaconiscidae_.
(After Traquair.)]

[Illustration: FIG. 264.--A fossil Herring, _Diplomystus humilis_
Leidy. (From a specimen obtained at Green River, Wyo.) The scutes along
the back lost in the specimen. Family _Clupeidae_.]

=The Bony Fishes.=--All the remaining fishes have ossified instead
of cartilaginous skeletons. The dipnoan and ganoid traits one by one
are more or less completely lost. Through these the main line of fish
development continues and the various groups are known collectively as
bony fishes or teleosts.

[Illustration: FIG. 265.--A primitive Herring-like fish, _Holcolepis
lewesiensis_ Mantell, restored. Family _Elopidae_. English Chalk. (After
Woodward.)]

[Illustration: FIG. 266.--Ten-pounder, _Elops saurus_ L. An ally of the
earliest bony fishes. Virginia.]

The earliest of the true bony fishes or teleosts appear in Mesozoic
times, the most primitive forms being soft-rayed fishes with the
vertebrae all similar in form, allied more or less remotely to the
herring of to-day, but connected in an almost unbroken series with
the earliest ganoid forms. In these and other soft-rayed fishes the
pelvis still retains its posterior insertion, the ventral fins being
said to be abdominal. The next great stage in evolution brings the
pelvis forward, attaching it to the shoulder-girdle so that the ventral
fins are now thoracic as in the perch and bass. If brought to a point
in front of the pectoral fins, a feature of specialized degradation,
they become jugular as in the codfish. In the abdominal fishes the
air-bladder still retains its rudimentary duct joining it to the
oesophagus.

From the abdominal forms allied to the herring, the huge array
of modern fishes, typified by the perch, the bass, the mackerel,
the wrasse, the globefish, the sculpin, the sea-horse, and the cod
descended in many diverging lines. The earliest of the spine-rayed
fishes with thoracic fins belong to the type of _Berycidae_, a group
characterized by rough scales, the retention of a primitive bone
between the eyes, and the retention of the primitive larger number
of ventral rays. These appear in the Cretaceous or chalk deposits,
and show various attributes of transition from the abdominal to the
thoracic type of ventrals.

[Illustration: FIG. 267.--Cardinal-fish, a perch-like fish, _Apogon
semilineatus_ Schlegel. Misaki, Japan.]

[Illustration: FIG. 268.--Summer Herring, _Pomolobus aestivalis_
(Mitchill). Potomac River. Family _Clupeidae_.]

Another line of descent apparently distinct from that of the
herring and salmon extends through the characins to the loach,
carps, catfishes, and electric eel. The fishes of this series have
the anterior vertebrae coossified and modified in connection with the
hearing organ, a structure not appearing elsewhere among fishes. This
group includes the majority of fresh-water fishes. Still another great
group, the eels, have lost the ventral fins and the bones of the head
have suffered much degradation.

[Illustration: FIG. 269.--Fish with jugular ventral fins, _Bassozetus
catena_ Goode & Bean. Family _Brotulidae_. Gulf Stream.]

[Illustration: FIG. 270.--A specialized bony fish, _Trachicephalus
uranoscopus_. Family _Scorpaenidae_. From Swatow, China.]

The most highly developed fishes, all things considered, are doubtless
the allies of the perch, bass, and sculpin. These fishes have lost
the air-duct and on the whole they show the greatest development of
the greatest number of structures. In these groups their traits one
after another are carried to an extreme and these stages of extreme
specialization give way one after another to phases of degeneration.
The specialization of one organ usually involves degeneration of some
other. Extreme specialization of any organ tends to render it useless
under other conditions and may be one step toward its final degradation.

[Illustration: FIG. 271.--An African Catfish, _Chlarias breviceps_
Boulenger. Congo River. Family _Chlariidae_. (After Boulenger.)]

[Illustration: FIG. 272.--Silverfin, _Notropis whipplii_ (Girard).
White River, Indiana. Family _Cyprinidae_.]

We have thus seen, in hasty review, that the fish-like vertebrates
spring from an unknown and possibly worm-like stock, that from this
stock, before it became vertebrate, degenerate branches have fallen
off, represented to-day by the _Tunicates_ and _Enteropneustans_. We
have seen that the primitive vertebrate was headless and limbless
and without hard parts. The lancelet remains as a possible direct
offshoot from it; the cyclostome with brain and skull is a possible
derivative from archaic lancelets. The earliest fishes leaving
traces in the rocks were mailed ostracophores. From an unknown but
possibly lamprey-like stock sprang the sharks and chimaeras. The sharks
developed into rays in one right line and into the highest sharks along
another, while by a side branch through lost stages the primitive
sharks passed into Crossopterygians, into Dipnoans, or lung-fishes,
and perhaps into Ostracophores. All these types and others abound
in the Devonian Age and the early records were lost in the Silurian.
From the Crossopterygians or their ancestors or descendants by the
specialization of the lung and limbs, the land animals, at first
amphibians, after these reptiles, birds, and mammals, arose.

[Illustration: FIG. 273.--Moray, _Gymnothorax moringa_ Bloch. Family
_Muraenidae_ Tortugas.]

[Illustration: FIG. 274.--Amber-fish, _Seriola lalandi_ (Cuv. & Val.).
Family _Carangidae_. Woods Hole.]

In the sea, by a line still more direct, through the gradual emphasis
of fish-like characters, we find developed the Crossopterygians with
archaic limbs and after these the Ganoids with fish-like limbs but
otherwise archaic; then the soft-rayed and finally the spiny-rayed bony
fishes, herring, mackerel, perch, which culminate in specialized and
often degraded types, as the anglers, globefishes, parrot-fishes, and
flying gurnards; and from each of the ultimate lines of descent radiate
infinite branches till the sea and rivers are filled, and almost every
body of water has fishes fitted to its environment.

[Illustration: Geological Distribution of the Families of Elasmobranchs.

  A=Pliocene
  B=Miocene
  C=Eocene
  D=Cretaceous
  E=Jurassic
  F=Triassic
  G=Permian
  H=Coal Measures
  I=Sub-Carbonifero
  J=Devonian
  K=Silurian
  L=Ordovician

                 +-+-+-+-+-+-+-+-+-+-+-+-+
                 |A|B|C|D|E|F|G|H|I|J|K|L|
                 |-|-|-|-|-|-|-|-|-|-|-|-|
  Cladoselachidae | | | | | | | | |X|X| | |
  Acanthodii     | | | | | | |X|X|X|X| | |
  Pleuracanthidae | | | | | | |X|X|X|X| | |
  Cladodontidae   | | | | | | |X|X|X|X| | |
  Petalodontidae  | | | | | | |X|X|X| | | |
  Psammodontidae  | | | | | | | |X|X|X| | |
  Cochliodontidae | | | | | | | |X|X|X| | |
  Orodontidae     | | | | | | | |X|X| | | |
  Heterodontidae  |X|X|X|X|X|X|X| | | | | |
  Tamiobatidae    | | | | | | | | | |X| | |
  Hexanchidae     |X|X|X|X|X| | | | | | | |
  Lamnidae        |X|X|X|X|X| | | | | | | |
  Mitsukurinidae  |X|X|X|X| | | | | | | | |
  Odontaspidae    |X|X|X|X| | | | | | | | |
  Scyliorhinidae  |X|X|X|X|X| | | | | | | |
  Carchariidae    |X|X|X|X| | | | | | | | |
  Squalidae       |X|X|X|X| | | | | | | | |
  Dalatiidae      |X|X|X| | | | | | | | | |
  Squatinidae     |X|X|X|X|X| | | | | | | |
  Rhinobatidae    |X|X|X|X|X| | | | | | | |
  Pristididae     |X|X|X|X| | | | | | | | |
  Rajidae         |X|X|X|X| | | | | | | | |
  Narcobatidae    |X|X|X| | | | | | | | | |
  Dasyatidae      |X|X|X|X| | | | | | | | |
  Myliobatidae    |X|X|X|X|X| | | | | | | |
  Ptychodontidae  | | | |X| | | | | | | | |
  Chimaeridae      |X|X|X|X|X| | | | | | | |]




CHAPTER XXV

THE PROTOCHORDATA


=The Chordate Animals.=--Referring to our metaphor of the tree with
its twigs as used in the chapter on classification we find the fishes
with the higher vertebrates as parts of a great branch from which the
lower twigs have mostly perished. This great branch, phylum, or line of
descent is known in zoology as _Chordata_, and the organisms associated
with it or composing it are chordate animals.

The chordate animals are those which at some stage of life possess a
notochord or primitive dorsal cartilage which divides the interior of
the body into two cavities. The dorsal cavity contains the great nerve
centers or spinal cord; the ventral cavity contains the heart and
alimentary canal. In all other animals which possess a body cavity,
there is no division by a notochord, and the ganglia of the nervous
system if existing are placed on the ventral side or in a ring about
the mouth.

=The Protochordates.=--Modern researches have shown that besides the
ordinary back-boned animals certain other creatures easily to be
mistaken for mollusks or worms and being chordate in structure must be
regarded as offshoots from the vertebrate branch. These are degenerate
allies, as is shown by the fact that their vertebrate traits are shown
in their early or larval development and scarcely at all in their adult
condition. As Dr. John Sterling Kingsley has well said: "Many of the
species start in life with the promise of reaching a point high in the
scale, but after a while they turn around and, as one might say, pursue
a downward course, which results in an adult which displays but few
resemblances to the other vertebrates." In the Tunicates or Ascidians
(sea-squirts, sea-pears, and salpas), which constitute the class known
as _Tunicata_ or _Urochordata_, there is no brain, the notochord is
confined to the tail and is usually present only in the larval stage
of the animal when it has the form of a tadpole. In later life the
animal usually becomes quiescent, attached to some hard object, fixed
or floating. It loses its form and has the appearance of a hollow,
leathery sac, the body organs being developed in a tough tunic. There
are numerous families of Tunicates and the species are found in nearly
all seas. They suggest no resemblance to fishes and look like tough
clams without shells. The internal cavity being usually filled with
water it is squirted out through the two apertures when the animal is
handled. The class _Enteropneusta_ (_Adelochorda_, or _Hemichordata_),
includes the rather rare worm-like forms related to _Balanoglossus_.
Bateson has shown that these animals possess a notochord which is
developed in the anterior part of the body. They have no fins and
before the mouth is a long proboscis. Gill-slits are found in the
larval tunicate. In _Balanoglossus_ these persist through life as in
the fishes.

The remaining chordate forms constitute the vertebrates proper, not
worm-like nor mollusk-like, the notochord not disappearing with age,
except as it gives way, by specialized segmentation to the complex
structures of the vertebral column. These vertebrates, which are
permanently aquatic, are known in a popular sense as fishes. The fish,
in the broad sense, is a back-boned animal which retains the homologue
of the back-bone throughout life, which does not develop jointed
limbs, its locomotive members, if present, being developed as fins,
and which breathes through life the air contained in water by means of
gills. This definition excludes the Tunicates and Enteropneusta on the
one hand and the Amphibia or Batrachia with the reptiles, birds, and
mammals on the other. The Amphibia are much more closely related to
certain fishes than the classes of fishes are to each other. Still for
purposes of systematic study, the frogs and salamanders are left out
of the domain of ichthyology, while the Tunicata and the Enteropneusta
might well be included in it.

The known branchiferous or gill-bearing chordates living and extinct
may be first divided into eight classes--the _Enteropneusta_, the
_Tunicata_, the _Leptocardii_, or lancelets, the _Cyclostomi_, or
lampreys, the _Elasmobranchii_, or sharks, the _Ostracophori_ the
_Arthrodira_, and the _Teleostomi_, or true fishes. The first two
groups, being very primitive and in no respect fish-like in appearance,
are sometimes grouped together as _Protochordata_, the others with the
higher Chordates constituting the _Vertebrata_.

=Other Terms used in Classification.=--The Leptocardii are sometimes
called Acraniata (without skull), as distinguished from the higher
groups, Craniota, in which the skull is developed. The _Leptocardii_,
_Cyclostomi_, and _Ostracophori_ are sometimes called _Agnatha_
(without jaws) in contradistinction to the _Gnathostomi_ (jaw mouths),
which include the sharks and true fishes with the higher vertebrates.
The sharks and Teleostomes are sometimes brought together as _Pisces_,
or fishes, as distinguished from other groups not true fishes. To the
sharks and true fishes the collective name of _Lyrifera_ has been
given, these fishes having the harp-shaped shoulder-girdle, its parts
united below. The _Ostracophores_ and _Arthrodires_ agreeing in the
bony coat of mail, and both groups now extinct and both of uncertain
relationship, have been often united under the name of _Placoderms_,
and these and many other fishes have been again erroneously confounded
with the Ganoids. Again, the Teleostomi have been frequently divided
into three classes--_Crossopterygii_, _Dipneusti_ or _Dipnoi_, and
_Actinopterygii_. The latter may be again divided into _Ganoidei_
and _Teleostei_ and all sorts of ranks have been assigned to each of
these groups. For our purposes a division into eight classes is most
convenient, and lowest among these we may place the _Enteropneusta_.

=The Enteropneusta.=--Most simple, most worm-like, and perhaps most
primitive of all the Chordates is the group of worm-shaped forms,
forming the class of _Enteropneusta_. The class of _Enteropneusta_,
also called _Adelochorda_ or _Hemichordata_, as here recognized,
consists of a group of small marine animals allied to the genus
_Balanoglossus_, or acorn-tongues (~balanos~, acorn; ~glossa~, tongue).
These are worm-like creatures with fragile bodies buried in the sand or
mud, or living under rocks of the seashore and in shallow waters, where
they lie coiled in a spiral, with little or no motion. From the surface
of the body a mucous substance is secreted, holding together particles
by which are formed tubes of sand. The animal has a peculiar odor like
that of iodoform. At the front is a long muscular proboscis, very
sensitive, capable of great extension and contraction, largely used
in burrowing in the ground, and of a brilliant orange color in life.
Behind this is a collar which overlaps the small neck and conceals the
small mouth at the base of the proboscis. The gill-slits behind the
collar are also more or less concealed by it.

The body, which is worm-like, extends often to the length of two or
three feet. The gill-slits in the adult are arranged in regular pairs,
there being upwards of fifty in number much like the gill-slits of the
lancelet. As the animal grows older the slits become less conspicuous,
their openings being reduced to small slit-like pores.

In the interior of the proboscis is a rod-like structure which arises
as an outgrowth of the alimentary canal above the mouth. In development
and structure this rod so resembles the notochord of the lancelet
that it is regarded as a true notochord, though found in the anterior
region only. From the presence of gill-slits and notochord and from the
development and structure of the central nervous system _Balanoglossus_
was recognized by William Bateson, who studied an American species,
_Dolichoglossus kowalevskii_, at Hampton Roads in Virginia in 1885,
and at Beaufort in North Carolina, as a member of the Chordate series.
Unlike the Tunicates it represents a primitively simple, not a
degenerate, type. It seems to possess real affinities with the worms,
or possibly, as some have thought, with the sea-urchins.

[Illustration: FIG. 275.--"Tornaria" Larva of _Glossobalanus minutus_.
(After Minot.)]

A peculiar little creature, known as _Tornaria_, was once considered
to be the larva of a starfish. It is minute and transparent, floating
on the surface of the sea. It has no visible resemblance to the adult
_Balanoglossus_, but it has been reared in aquaria and shown to pass
into the latter or into the related genus _Glossobalanus_. No such
metamorphosis was found by Bateson in the more primitive genus
_Dolichoglossus_, studied by him. This adult animal may be, indeed,
a worm as it appears, but the presence of gill-slits, the existence
of a rudimentary notochord, and the character of the central nervous
system are distinctly fish-like and therefore vertebrate characters.
With the Chordates, and not with the worms, this class, _Enteropneusta_
(~enteron~, intestine; ~pnein~, to breathe), must be placed if its
characters have been rightly interpreted. It is possibly a descendant
of the primitive creatures which marked the transition from the archaic
worms, or possibly archaic Echinoderms, to the archaic Chordate type.

[Illustration: FIG. 276.--_Glossobalanus minutus_, one of the higher
Enteropneustans. (After Minot.)]

It is perhaps not absolutely certain that the notochord of
_Balanoglossus_ and its allies is a true homologue of the notochord
of the lancelet. There may be doubt even of the homologies of the
gill-slits themselves. But the balance of evidence seems to throw
_Balanoglossus_ on the fish side of the dividing line which separates
the lower Chordates from the worms.

It may be noticed that Hubrecht regards the proboscis of various marine
Nemertine worms as a real homologue of the notochord, and other writers
have traced with more or less success other apparent or possible
homologies between the Chordate and the Annelid series.

=Classification of Enteropneusta.=--Until recently the _Enteropneusta_
have been usually placed in a single family or even in a single genus.
The recent researches of Professor J. W. Spengel of Giessen and of
Professor William Emerson Ritter of the University of California, have
shown clearly that the group is much larger than had been generally
supposed, with numerous species in all the warm seas. In Spengel's
recent paper, "Die Benennung der Enteropneusten-Gattungen," three
families are recognized with nine genera and numerous species. At least
seven species are now known from the Pacific Coast of North America.

=Family Harrimaniidae.=--In _Harrimania maculosa_, lately described by
Dr. Ritter from Alaska, the eggs are large, with much food yolk, and
the process of development is probably, without _Tornaria_ stage. A
second species of _Harrimania_ (_H. kupferi_) is now recognized from
Norway and Greenland. This genus is the simplest in structure among
all the Enteropneustans and may be regarded as the lowest of known
Chordates, the most worm-like of back-boned animals.

[Illustration: FIG. 277.--_Harrimania maculosa_ (Ritter), the lowest of
chordate animals. An Enteropneustan from Alaska. (After Ritter.)]

In _Dolichoglossus kowalevskii_ the species studied by Bateson on the
Virginia coast, the same simplicity of development occurs. This genus,
with a third, _Stereobalanus_ (_canadensis_), constitutes in Spengel's
system the family of _Harrimaniidae_.

=Balanoglossidae.=--The family _Glandicepitidae_ contains the genera
_Glandiceps_, _Spengelia_, and _Schizocardium_. In the _Balanoglossidae_
(_Ptychoderidae_ of Spengel) the eggs are very small and numerous,
with little food yolk. The species in this family pass through the
Tornaria stage above described, a condition strikingly like that of the
larval starfish. This fact has given rise to the suggestion that the
Enteropneusta have a real affinity with the Echinoderms.

The _Balanoglossidae_ include the genera _Glossobalanus_,
_Balanoglossus_, and _Ptychodera_, the latter the oldest known member
of the group, its type, _Ptychodera flava_, having been described
by Eschscholtz from the Pacific Coast in 1825, while _Balanoglossus
clavigerus_ was found by Della Chiaje in 1829.

=Low Organization of Harrimaniidae.=--Apparently the _Harrimaniidae_,
with simpler structure, more extensive notochord, and direct
development, should be placed at the bottom as the most primitive
of the Enteropneustan series. Dr. Willey, however, regards its
characters as due to degeneration, and considers the more elaborate
_Balanoglossidae_ as nearest the primitive type. The case in this view
would have something in common with that of the _Larvacea_, which seems
to be the primitive Tunicates, but which may have been produced by the
degeneration of more complex forms.




CHAPTER XXVI

THE TUNICATES, OR ASCIDIANS


=Structure of Tunicates.=--One of the most singular groups of animals
is that known as Ascidians, or Tunicates. It is one of the most clearly
marked yet most heterogeneous of all the classes of animals, and in no
other are the phenomena of degeneration so clearly shown.

Among them is a great variety of form and habit. Some lie buried in
sand; some fasten themselves to rocks; some are imbedded in great
colonies in a gelatinous matrix produced from their own bodies, and
some float freely in long chains in the open sea. All agree in changing
very early in their development from a free-swimming or fish-like
condition to one of quiescence, remaining at rest or drifting with the
current.

Says Dr. John Sterling Kingsley: "Many of the species start in life
with the promise of reaching a point high in the scale, but after a
while they turn around and, as one might say, pursue a downward course
which results in an adult which displays but few resemblances to the
other vertebrates. Indeed, so different do they seem that the fact
that they belong here was not suspected until about thirty-five years
ago. Before that time, ever since the days of Cuvier, they were almost
universally regarded as mollusks, and many facts were adduced to show
that they belonged near the acephals (clams, oysters, etc.). In the
later years when the facts of development began to be known, this
association was looked on with suspicion, and by some they were placed
for a short time among the worms. Any one who has watched the phases
of their development cannot help believing that they belong here, the
lowest of the vertebrate series."

The following account of the structure and development of the Tunicate
is taken, with considerable modification and condensation, from
Professor Kingsley's chapter on the group in the Riverside Natural
History. For the changes suggested I am indebted to the kindness of
Professor William Emerson Ritter:

The Tunicates derive their name from the fact that the whole body is
invested with a tough envelope or "tunic." This tunic or test may be
either gelatinous, cartilaginous, or leathery. In some forms it is
perfectly transparent, in others it is translucent, allowing enough
light to pass to show the colors of the viscera, while in still others
it is opaque and variously . The tunic is everywhere only
loosely attached to the body proper, except in the region of the two
openings now to be mentioned. One of these openings occupies a more or
less central position, while the other is usually at one side, or it
may even be placed at the opposite end of the body. On placing one of
the Ascidians in a glass dish and sprinkling a little carmine or indigo
in the water, we can study some of the functions of the animal. As soon
as the disturbance is over, the animals will open the two apertures
referred to, when it will be seen that each is surrounded with blunt
lobes, the number of which varies with the species. As soon as they are
opened a stream of water will be seen to rush into the central opening,
carrying with it the carmine, and a moment later a reddish cloud will
be ejected from the other aperture. From this we learn that the water
passes through the body. Why it does so is to be our next inquiry. On
cutting the animal open we find that the water, after passing through
the first-mentioned opening (which may be called the mouth) enters a
spacious chamber, the walls of which are made up of fine meshes, the
whole appearing like lattice-work. Taking out a bit of this network and
examining it under the microscope, we find that the edges of the meshes
are armed with strong cilia, which are in constant motion, forcing the
water through the holes. Of course, the supply has to be made good, and
hence more water flows in through the mouth. This large cavity is known
as the branchial or pharyngeal chamber. It is, according to Professor
Ritter, "as we know from the embryology of the animal, the greatly
enlarged anterior end of the digestive tract; and as the holes, or
stigmata, as they are technically called, are perforations of the wall
for the passage of water for purposes of respiration, they are both
morphologically and physiologically comparable with the gill openings
of fishes." There can be no doubt, therefore, that the pharyngeal sac
of Ascidians is homologous with the pharynx of fishes.

Surrounding the mouth, or branchial orifice, just at its entrance into
the branchial chamber is a circle of tentacles. These are simple in
some genera, but elaborately branched in others.

In close connection with the cerebral ganglion, which is situated
between the two siphons, there is a large gland with a short
trumpet-shaped duct opening into the branchial sac a little distance
behind the mouth. The orifice of the duct is just within a ring
consisting of a ciliated groove that extends around the mouth outside
the circle of branchial tentacles. On the opposite side of the mouth
from the gland the ciliated groove joins another groove which is both
ciliated and glandular, and which runs backward along the upper floor
of the pharyngeal sac to its posterior extremity. This organ, called
the endostyle, is concerned in the transportation of the animal's
food through the pharyngeal sac to the opening of the oesophagus.
Comparative embryology makes it almost certain that the subneural gland
with its duct, described above, is homologous with the hypophesis
cerebri of true vertebrates, and that the endostyle is homologous with
the thyroid glands of vertebrates.

The water after passing through the branchial network is received
into narrow passages and conducted to a larger cavity--the cloacal or
atrial chamber. The general relations can he seen from our diagram,
illustrating a vertical and horizontal section. From the atrial chamber
the water flows out into the external world.

Now we can readily see how in the older works naturalists were misled
as to the affinities of the Tunicates. They regarded the tunic as
the equivalent of the mantle of the mollusks, while the incurrent
and excurrent openings corresponded to the siphons. In one genus,
_Rhodosoma_, the resemblance was even stronger, for there the tunic
is in two parts, united by a hinge line, and closed by an adductor
muscle. How and why these views were totally erroneous will be seen
when we come to consider the development of these animals.

At the bottom of the pharnygeal sac is the narrow oesophagus surrounded
with cilia, which force a current down into the digestive tract. The
branchial meshes serve as a strainer for the water, and the larger
particles which it contains fall down until they are within reach
of the current going down the oesophagus. After passing through the
throat, they come to the stomach, where digestion takes place, and then
the ejectamenta are carried out through the intestine and poured into
the bottom of the atrial cavity.

The heart lies on the ventral side of the stomach and is surrounded by
a well-developed pericardium. The most remarkable fact connected with
the circulation is that the heart, after beating a short time, forcing
the blood through the vessels, will suddenly stop for a moment and then
resume its beats; but, strange to say, after the stoppage the direction
of the circulation is reversed, the blood taking an exactly opposite
course from that formerly pursued. This most exceptional condition
was first seen in the transparent _Salpa_, but it may be witnessed in
the young of most genera. We have already referred to the branchial
chamber. The walls of this chamber, besides acting as a strainer, are
also respiratory organs. The meshes of which they are composed are in
reality tubes through which the blood circulates and thus is brought in
contact with a constantly renewed supply of fresh water.

The central nervous system in the adults of all except the _Larvacea_
is reduced to a single ganglion placed near the mouth thus indicating
the dorsal side. In forms like _Cynthia_ it holds the same relative
position with regard to the mouth, but by the doubling of the body (to
be explained further on) it is also brought near the atrial aperture,
where it is shown in our first diagram.

=Development of Tunicates.=--The sexes are combined in the same
individual, though usually the products ripen at different times. As a
rule, the earlier stages of the embryo are passed inside the cloacal
chamber, though in some the development occurs outside the body.
As a type of the development we will consider that of one of the
solitary forms, leaving the many curious modifications to be noticed
in connection with the species in which they occur. This will be best,
since these forms show the relationship to the other vertebrates in the
clearest manner.

[Illustration: FIG. 278.--Development of the larval Tunicate to the
fixed condition. (From Seeliger, per Parker & Haswell.) _a_, larva;
_b_, intermediate stage; _c_, adult.]

The egg undergoes a total segmentation and a regular gastrulation.
Soon a tail appears, and under the microscope the young embryo,
which now begins its free life, appears much like the tadpole of
the frog. It has a large oval body and a long tail which lashes
about, forcing the animal forward with a wriggling motion. Nor is the
resemblance superficial; it pervades every part of the structure, as
may be seen from the adjacent diagram. The mouth is nearly terminal
and communicates with a gill-chamber provided with gill-clefts. At
the posterior end of the gill-chamber begins the alimentary tract,
which pursues a convoluted course to the vent. In the tail, but not
extending to any distance into the body, is an axial cylinder, the
notochord, which here, as in all other vertebrates, arises from the
hypoblast; and above it is the spinal cord (epiblastic in origin),
which extends forward to the brain, above the gill-chamber. Besides,
the animal is provided with organs of sight and hearing, which,
however, are of peculiar construction and can hardly be homologized
with the corresponding organs in vertebrates. So far the correspondence
between the two types is very close, and if we knew nothing about the
later stages, one would without doubt predict that the adult tunicate
would reach a high point in the scale of vertebrates. These high
expectations are never fulfilled; the animal, on the contrary, pursues
a retrograde course, resulting in an adult whose relationship to the
true vertebrates never would have been suspected had its embryology
remained unknown.

[Illustration: FIG. 279.--Anatomy of Tunicate. (After Herdman, per
Parker & Haswell.)]

After the stage described this retrograde movement begins. From various
parts of the body lobes grow out, armed on their extremities with
sucking-disks. These soon come in contact with some subaquatic object
and adhere to it. Then the notochord breaks down, the spinal cord is
absorbed, the tail follows suit, the intestine twists around, and the
cloaca is formed, the result being much like the diagram near the head
of this section. In forms like _Appendicularia_, little degeneration
takes place, so far as is known, the tail, with its notochord and
neural chord, persisting through life.

=Reproduction of Tunicates.=--As to the reproduction of the Tunicates,
Dr. Ritter writes: "In addition to the sexual method of reproduction,
many tunicates reproduce asexually by budding. The capacity for bud
reproduction appears to have been acquired by certain simple Ascidians
in connection with, probably as a result of, their having given up the
free-swimming life and become attached and consequently degenerate.

"Instructive as the embryonic development of the creatures is from the
standpoint of evolution, the bud method of development is scarcely less
so from the same point of view. The development of the adult zooid
from the simple bud has been conclusively shown to be by a process in
many respects fundamentally unlike that by which the individual is
developed from the egg. We have then in these animals a case in which
practically the same results are reached by developmental processes
that are, according to prevailing conceptions of animal organizations,
fundamentally different. This fact has hardly a parallel in the animal
kingdom."

=Habits of Tunicates.=--The Tunicates are all marine, some floating or
swimming freely, some attached to rocks or wharves, others buried in
the sand. They feed on minute organisms, plants, or animals, occasional
rare forms being found in their stomachs. Some of them possess a single
median eye or eye-like structure which may not do more than recognize
the presence of light. No fossil Tunicates are known, as they possess
no hard parts, although certain Ostracoderms have been suspected,
though on very uncertain grounds, to be mailed Tunicates, rather than
mailed lampreys. It is not likely that this hypothesis has any sound
foundation. The group is divided by Herdman and most other recent
authorities into three orders, viz., the _Larvacea_, the _Ascidiacea_,
and the _Thaliacea_.

=Larvacea.=--In the most primitive order the animals are minute and
free-swimming, never passing beyond the tadpole stage. The notochord
and the nervous chord persist through life, the latter with ganglionic
segmentations at regular intervals. The species mostly float in
the open sea, and some of them form from their own secretions a
transparent gelatinous envelope called a "house." This has two
apertures and a long chamber "in which the tail has room to vibrate."

The order consists of a single small family, _Appendiculariidae_. The
lowest type is known as _Kowalevskia_, a minute creature without
heart or intestine found floating in the Mediterranean. It is in
many respects the simplest in structure among _Chordate_ animals.
_Oikopleura_ (Fig. 288) is another genus of this group.

=Ascidiacea.=--In the _Ascidiacea_ the adult is usually attached to
some object, and the two apertures are placed near each other by the
obliteration of the caudal area. The form has been compared to a
"leathern bottle with two spouts."

[Illustration: FIG. 280.--_Ascidia adhaerens_ Ritter. Glacier Bay,
Alaska. (After Ritter.)]

The suborder _Ascidiae simplices_ includes the solitary Ascidians or
"sea-squirts," common on our shores, as well as the social forms in
which an individual is surrounded by its buds. The common name arises
from the fact that when touched they contract, squirting water from
both apertures. The _Ascidiidae_ comprise the most familiar solitary
forms, some of them the largest of the Tunicates and represented on
most coasts. In the _Molgulidae_ and most _Ascidiae compositae_ the young
hatch out in the cloaca, from which "these tadpoles swim out as yellow
atoms," while in a new genus, _Euherdmania_, described by Ritter,
from the coast of California, the embryos are retained through their
whole larval stage in the oviduct of the parent. They form, according
to Kingsley, adhesive processes on the body, but those of _Molgula_
cannot use them in becoming attached to rocks, since they are entirely
inclosed in a peculiar envelope. This envelope is after a while very
adhesive, and if the little tadpole happens to touch any part of
himself to a stone or shell he is fastened for life. Thus "I have
frequently seen them adhere by the tail, while the anterior part was
making the most violent struggles to escape. Soon, however, they settle
down contentedly, absorb the tail, and in a few weeks assume the adult
structure."

In the family _Cynthiidae_ the brightly- red and yellow species
of _Cynthia_ are known as sea-peaches by the fishermen. The sea-pears,
_Boltenia_, are fastened to long stalks. These have a leathery and
wrinkled tunic, to which algae and hydroids freely attach themselves.
Into the gill-cavity of these forms small fishes, blennies, gobies, and
pearl-fishes often retreat for protection.

[Illustration: FIG. 281.--_Styela yacutatensis_ (Ritter), a simple
Ascidian. Family _Molgulidae_. Yakutat Bay, Alaska. (After Ritter.)]

The social Ascidians constitute the _Clavellinidae_. They are similar
to the _Ascidiidae_ in form, but each individual sends out a bud which
forms a stern bearing another individual at the end. By this means
large colonies may be formed.

The suborder, _Ascidiae compositae_, contains the compound Ascidians or
colonies enveloped in a common gelatinous "test." These colonies are
usually attached to rock or seaweed, and the individuals are frequently
regularly and symmetrically arranged. The bodies are sometimes complex
in form.

[Illustration: FIG. 282.--_Styela greeleyi_ Ritter. Family _Molgulidae_.
Lukanin, Pribilof Islands. (After Ritter.)]

[Illustration: FIG. 283.--_Cynthia superba_ Ritter. A Tunicate from
Puget Sound. Family _Cynthiidae_. (After Ritter.)]

In the _Botryllidae_ and _Polystyelidae_ the individuals are not
segmented and in the former family are arranged in star-shaped groups
about a common cloaca, into which the atrial siphons of the different
individuals open. The group springs by budding from the tadpole, or
larva, which has attached itself to some object. These forms are often
brightly . _Botryllus gouldi_ is a species very common along
our North Atlantic coast, forming gray star-shaped masses sometimes
an inch across on eel-grass (_Zostera_) and on flat-leaved seaweeds.
_Goodsiria dura_, a representative of the _Polystyelidae_, is one of
the most common Ascidians on the California coast southward, where the
brick-red masses incrusting on seaweeds of various kinds, and on other
Ascidians, are frequently thrown ashore in great quantities during
heavy storms.

[Illustration: FIG. 284.--_Botryllus magnus_ Ritter. A compound
Ascidian. Shumagin Islands, Alaska. (After Ritter.)]

In _Didemnidae_ the body is more complex, of two parts, called the
"thorax" and "abdomen." In _Amaroecium_, the "sea pork" of the
fishermen, the body is in three parts and the individuals are very
long. These sometimes form great masses a foot or more long, "
like boiled salt pork, but more translucent." Other families of this
type are the _Distomidae_ and the _Polyclinidae_.

In the suborder _Luciae_, including the family _Pyrosomidae_, the
colonies are thimble-shaped and hollow, the incurrent openings being on
the outer surface of the thimble, the outgoing stream opening within.
_Pyrosoma_ is highly phosphorescent. In the tropical seas some colonies
reach a length of two or three feet. It is said that a description of
a colony was once written by a naturalist on a page illumined by the
colony's own light. "Each of the individuals has a number of cells near
the mouth the function of which is to produce the light."

=Thaliacea.=--In the order _Thaliacea_ the Tunicates have the two
orifices at opposite ends of the body. All are free-swimming and
perfectly transparent. The principal family is that of _Salpidae_. The
gill-cavity in Salpa is much altered, the gills projecting into it
dividing it into two chambers.

In these forms we have the phenomena of alternation of generations.
A sexual female produces eggs, and from each hatches a tadpole larva
which is without sex. This gives rise to buds, some at least of the
individuals arising which in turn produce eggs.

[Illustration: FIG. 285.--_Botryllus magnus_ Ritter. Part of colony.
(After Ritter.)]

In the family _Salpidae_ two kinds of individuals occur, the solitary
salpa, or female, and the chain salpa, or bisexual males. The latter
are united together in long bands, each individual forming a link
in the chain held together by spurs extending from one to the next.
From each solitary individual a long process or cord grows out, this
dividing to form the chain. Each chain salpa produces male reproductive
organs and each develops as well a single egg. The egg is developed
within the body attached by a sort of placenta, while the spermatozoa
are cast into the sea to fertilize other eggs. From each egg develops
the solitary salpa and from her buds the chain of bisexual creatures.
Dr. W. K. Brooks regards these as nursing males, the real source of
the egg being perhaps the solitary female. Of this extraordinary
arrangement the naturalist-poet Chamisso, who first described it,
said: "A salpa mother is not like its daughter or its own mother, but
resembles its sister, its granddaughter, and its grandmother." But
it is misleading to apply such terms taken from the individualized
human relationship to the singular communal system developed by these
ultra-degenerate and strangely specialized Chordates.

[Illustration: FIG. 286.--_Botryllus magnus_ Ritter, a single Zooid.
Shumagin Islands, Alaska. (After Ritter.)]

[Illustration: FIG. 287.--_Aplidiopsis jordani_ Ritter, a compound
Ascidian. Lukanin Beach, Pribilof Islands. (After Ritter.)]

The Salpas abound in the warm seas, the chains often covering the water
for miles. They are perfectly transparent, and the chains are often
more than a foot in length. In Doliolum the body is barrel-shaped
and the gills are less modified than in Salpa. The alternation of
generations in this genus is still more complicated than in Salpa,
for here we have not only a sexual and a non-sexual generation, the
individuals of which differ from each other, but there is further a
differentiation among the asexually produced individuals themselves;
so that we have in all three instead of two sorts of animals in the
complete life cycle. Besides the proliferating stolon situated on the
ventral side, the bud-producing individual possesses a dorsal process
larger than the stolon proper. The buds become completely severed from
the true stolon at an early stage and actually crawl along the side of
the parent up to the dorsal process, upon which they arrange themselves
in three rows, two lateral and one median. The buds of the lateral
rows become nutritive and respiratory zooids, while those of the
median row, ultimately at least, give rise in turn to the egg-producing
individuals.

=Origin of Tunicates.=--There can be little doubt that the _Tunicata_
form an offshoot from the primitive Chordate stock, and the structure
of their larva in connection with that of the lancelet throws a large
light on the nature of their common parents. "We may conclude," says
Dr. Arthur Willey, "that the proximate ancestor of the Vertebrates was
a free-swimming animal intermediate in organization between an Ascidian
tadpole and Amphioxus, possessing the dorsal mouth, hypophysis,
and restricted notochord of the former and the myotomes, coelomic
epithelium, and straight alimentary canal of the latter. The ultimate
or primordial ancestor of the Vertebrates would, on the contrary, be a
worm-like animal whose organization was approximately on a level with
that of the bilateral ancestors of the Echinoderms."

[Illustration: FIG. 288.--Adult Tunicate of the group Larvacea,
Oikopleura. Family _Appendiculariidae_. (After Fol, per Parker &
Haswell.)]

=Degeneration of Tunicates.=--There is no question, furthermore,
Professor Ritter observes, "that most of the group has undergone great
degeneration in its evolutionary course. Just what the starting-point
was, however, is a matter on which there is considerable difference
of opinion among authorities. According to one view, particularly
championed by Professor W. K. Brooks, _Appendicularia_ is very near
the ancestral form. The ancestor was consequently a small, marine,
free-swimming creature. From this ancestor the Ascidiacea were evolved
largely through the influence of the attached habit of life, and the
tadpole stage in their development is a recapitulation of the ancestral
form, just as the tadpole stage in the frog's life is a repetition of
the fish ancestry of the frog.

"According to the most common view _Appendicularia_ is not an ancestral
form at all, but is the tadpole stage of the _Ascidiacea_ that has
failed to undergo metamorphosis and has become sexually mature in the
larval condition, as the larva of certain Amphibians and insects are
known to never pass into the adult state but reproduce their kind
sexually in the larval condition. By this view the tadpole of such
Ascidian as _Ciona_, for example, represents more closely the common
ancestor of the group than does any other form we know. This view is
especially defended by Professor K. Heider and Dr. Arthur Willey."




CHAPTER XXVII

THE LEPTOCARDII, OR LANCELETS


=The Lancelet.=--The lancelet is a vertebrate reduced to its very
lowest terms. The essential organs of vertebrate life are there,
but each one in its simplest form unspecialized and with structure
and function feebly differentiated. The skeleton consists of a
cartilaginous notochord inclosed in a membranous sheath. There is
no skull. No limbs, no conspicuous processes, and no vertebrae are
present. The heart is simply a long contractile tube, hence the name
_Leptocardii_ (from ~leptos~, slender; ~kardia~, heart). The blood is
colorless. There is a hepatic portal circulation. There is no brain,
the spinal cord tapering in front as behind. The water for respiration
passes through very many gill-slits from the pharynx into the atrium,
from which it is excluded through the atripore in front of the vent. A
large chamber, called the atrium, extends almost the length of the body
along the ventral and lateral regions. It communicates with the pharynx
through the gill-slits and with the exterior through a small opening in
front of the vent, the atripore. The atrium is not found in forms above
the lancelets.

The reproductive organs consist of a series of pairs of segmentally
arranged gonads. The excretory organs consist of a series of tubules in
the region of the pharynx, connecting the body-cavity with the atrium.
The mouth is a lengthwise slit without jaws, and on either side is a
row of fringes. From this feature comes the name _Cirrostomi_, from
cirrus, a fringe of hair, and ~stoma~, mouth. The body is lanceolate
in form, sharp at either end. From this fact arises a third name,
_Amphioxus_, from ~amphi~, both; ~oxys~, sharp. Dorsal and anal fins
are developed as folds of the skin supported by very slender rays.
There are no other fins. The alimentary canal is straight, and is
differentiated into pharynx and intestine; the liver is a blind sac
arising from the anterior end of the intestine. A pigment spot in the
wall of the spinal cord has been interpreted as an eye. Above the snout
is a supposed olfactory pit which some have thought to be connected
with the pineal structure. The muscular impressions along the sides are
very distinct and it is chiefly by means of the variation in numbers
of these that the species can be distinguished. Thus in the common
lancelet of Europe, _Branchiostoma lanceolatum_, the muscular bands are
35+14+12=61. In the common species of the Eastern coasts of America,
_Branchiostoma caribaeum_, these are 35+14+9=58, while in the California
lancelet, _Branchiostoma californiense_, these are 44+16+9=69.

=Habits of Lancelets.=--Lancelets are slender translucent worm-like
creatures, varying from half an inch (_Asymmetron lucayanum_) to
four inches (_Branchiostoma californiense_) in length. They live
buried in sand in shallow waters along the coasts of warm seas. One
species, _Amphioxides pelagicus_, has been taken at the depth of 1000
fathoms, but whether at the bottom or floating near the surface is
not known. The species are very tenacious of life and will endure
considerable mutilation. Some of them are found on almost every coast
in semi-tropical and tropical regions.

=Species of Lancelets.=--The Mediterranean species ranges northward
to the south of England. Others are found as far north as Chesapeake
Bay, San Diego, and Misaki in Japan, where is found a species called
_Branchiostoma belcheri_. The sands at the mouth of San Diego Bay
are noted as producing the largest of the species of lancelets,
_Branchiostoma californiense_. From the Bahamas comes the smallest,
the type of a distinct genus, _Asymmetron lucayanum_, distinguished
among other things by a projecting tail. Other supposed genera are
_Amphioxides_ (_pelagicus_), dredged in the deep sea off Hawaii and
supposed to be pelagic, the mouth without cirri; _Epigonichthys_
(_cultellus_), from the East Indies, and _Heteropleuron_ (_bassanum_),
from Bass Straits, Australia. These little animals are of great
interest to anatomists as giving the clue to the primitive structure
of vertebrates. While possibly these have diverged widely from their
actual common ancestry with the fishes, they must approach near to
these in many ways. Their simplicity is largely primitive, not, as in
the Tunicates, the result of subsequent degradation.

[Illustration: FIG. 289.--California Lancelet, _Branchiostoma
californiense_ Gill. (From San Diego.)]

The lancelets, less than a dozen species in all, constitute a single
family, _Branchiostomidae_. The principal genus, _Branchiostoma_,
is usually called _Amphioxus_ by anatomists. But while the name
_Amphioxus_, like lancelet, is convenient in vernacular use, it has no
standing in systematic nomenclature. The name _Branchiostoma_ was given
to lancelets from Naples in 1834, by Costa, while that of _Amphioxus_,
given to specimens from Cornwall, dates from Yarrell's work on the
British fishes in 1836. The name Amphioxus may be pleasanter or shorter
or more familiar or more correctly descriptive than _Branchiostoma_,
but if so the fact cannot be considered in science as affecting the
duty of priority.

The name _Acraniata_ (without skull) is often used for the lower
Chordates taken collectively, and it is sometimes applied to the
lancelets alone. It refers to those chordate forms which have no
skull nor brain, as distinguished from the _Craniota_, or forms with
a distinct brain having a bony or cartilaginous capsule for its
protection.

=Origin of Lancelets.=--It is doubtless true, as Dr. Willey suggests,
that the Vertebrates became separated from their worm-like ancestry
through "the concentration of the central nervous system along the
dorsal side of the body and its conversion into a hollow tube."
Besides this trait two others are common to all of them, the presence
of the gill-slits and that of the notochord. The gill-slits may have
served primarily to relieve the stomach of water, as in the lowest
forms they enter directly into the body-cavity. The primitive function
of the notochord is still far from clear, but its ultimate use of its
structures in affording protection and in furnishing a fulcrum for the
muscles and limbs is of the greatest importance in the processes of
life.

[Illustration: FIG. 289_a_.--Gill-basket of Lamprey.]




CHAPTER XXVIII

THE CYCLOSTOMES, OR LAMPREYS


=The Lampreys.=--Passing upward from the lancelets and setting aside
the descending series of Tunicates, we have a long step indeed to the
next class of fish-like vertebrates. During the period this great gap
represents in time we have the development of brain, skull, heart, and
other differentiated organs replacing the simple structures found in
the lancelet.

The presence of brain without limbs and without coat-of-mail
distinguishes the class of _Cyclostomes_, or lampreys (~kuklos~,
round; ~stoma~, mouth). This group is also known as _Marsipobranchi_
(~marsipion~, pouch; ~branchos~, gill); _Dermopteri_ (~derma~, skin;
~pteron~, fin); and _Myzontes_ (~myzao~, to suck). It includes the
forms known as lampreys, slime-eels, and hagfishes.

=Structure of the Lamprey.=--Comparing a Cyclostome with a lancelet we
may see many evidences of specialization in structure. The Cyclostome
has a distinct head with a cranium formed of a continuous body of
cartilage modified to contain a fish-like brain, a cartilaginous
skeleton of which the cranium is evidently a differentiated part.
The vertebrae are undeveloped, the notochord being surrounded by its
membranes, without bony or cartilaginous segments. The gills have the
form of fixed sacs, six to fourteen in number, on each side, arranged
in a cartilaginous structure known as "branchial basket" (fig. 289_a_),
the elements of which are not clearly homologous with the gill-arches
of the true fishes. Fish-like eyes are developed on the sides of the
head. There is a median nostril associated with a pituitary pouch,
which pierces the skull floor. An ear-capsule is developed. The brain
is composed of paired ganglia in general appearance resembling the
brain of the true fish, but the detailed homology of its different
parts offers considerable uncertainty. The heart is modified to form
two pulsating cavities, auricle and ventricle. The folds of the dorsal
and anal fins are distinct, supported by slender rays.

The mouth is a roundish disk, with rasping teeth over its surface
and with sharper and stronger teeth on the tongue. The intestine is
straight and simple. The kidney is represented by a highly primitive
pronephros and no trace exists of an air-bladder or lung. The skin is
smooth and naked, sometimes secreting an excessive quantity of slime.

From the true fishes the Cyclostomes differ in the total absence of
limbs and of shoulder and pelvic girdles, as well as of jaws. It has
been thought by some writers that the limbs were ancestrally present
and lost through degeneration, as in the eels. Dr. Ayers, following
Huxley, finds evidence of the ancestral existence of a lower jaw. The
majority of observers, however, regard the absence of limbs and jaws in
Cyclostomes as a primitive character, although numerous other features
of the modern hagfish and lamprey may have resulted from degeneration.
There is no clear evidence that the class of Cyclostomes, as now known
to us, has any great antiquity, and its members may be all degenerate
offshoots from types of greater complexity of structure.

=Supposed Extinct Cyclostomes.=--No species belonging to the class
of Cyclostomes has been found fossil. We may reason theoretically
that the earliest fish-like forms were acraniate or lancelet-like,
and that lamprey-like forms would naturally follow these, but this
view cannot be substantiated from the fossils. Lancelets have no hard
parts whatever, and could probably leave no trace in any sedimentary
deposit. The lampreys stand between lancelets and sharks. Their teeth
and fins at least might occasionally be preserved in the rocks, but
no structures certainly known to be such have yet been recognized. It
is however reasonably certain that the modern lamprey and hagfish are
descendants, doubtless degraded and otherwise modified from species
which filled the gap between the earliest chordate animals and the
jaw-bearing sharks.

=Conodontes.=--Certain structures found as fossils have been from time
to time regarded as Cyclostomes, but in all such cases there is doubt
as to the real nature of the fossil relic in question or as to the
proper interpretation of its relationship.

Thus the _Conodontes_ of the Cambrian, Silurian, and Devonian have
been regarded as lingual teeth of extinct Cyclostomes. The _Cycliae_
of the Devonian have been considered as minute lampreys, although the
vertebral segments are highly specialized, to a degree far beyond the
condition seen in the lampreys of to-day. The Ostracophores have been
regarded as monstrous lampreys in coat of mail, and the possibility of
a lamprey origin even for Arthrodires has been suggested. The _Cycliae_
and _Ostracophori_ were apparently without jaws or limbs, being in this
regard like the _Cyclostomes_, but their ancestry and relationships are
wholly problematical.

[Illustration: FIG. 290.--_Polygnathus dubium_ Hinde. A Conodont from
the New York Devonian. (After Hinde.)]

The nature of the Conodontes is still uncertain. In form they resemble
teeth, but their structure is different from that of the teeth of any
fishes, agreeing with that of the teeth of annelid worms. Some have
compared them to the armature of Trilobites. Some fifteen nominal
genera are described by Pander in Russia, and by Hinde about Lake
Erie and Lake Ontario. Some of these, as _Drepaniodus_, are simple,
straight or curved grooved teeth or tooth-like structures; others, as
_Prioniodus_, have numerous smaller teeth or denticles at the base of
the larger one.

=Orders of Cyclostomes.=--The known Cyclostomes are naturally
divided into two orders, the _Hyperotreta_, or hagfishes, and the
_Hyperoartia_, or lampreys. These two orders are very distinct from
each other. While the two groups agree in the general form of the body,
they differ in almost every detail, and there is much pertinence in
Lankester's suggestions that each should stand as a separate class. The
ancestral forms of each, as well as the intervening types if such ever
existed, are left unrecorded in the rocks.

=The Hyperotreta, or Hagfishes.=--The _Hyperotreta_ (~hyperoa~,
palate; ~tretos~, perforate), or hagfishes, have the nostril highly
developed, a tube-like cylinder with cartilaginous rings penetrating
the palate. In these the eyes are little developed and the species are
parasitic on other fishes. In _Polistotrema stouti_, the hagfish of
the coast of California, is parasitic on large fishes, rockfishes, or
flounders. It usually fastens itself at the throat or isthmus of its
host and sometimes at the eyes. Thence it works very rapidly to the
inside of the body. It there devours all the muscular part of the fish
without breaking the skin or the peritoneum, leaving the fish a living
hulk of head, skin, and bones. It is especially destructive to fishes
taken in gill-nets. The voracity of the Chilean species _Polistotrema
dombeyi_ is equally remarkable. Dr. Federico T. Delfin finds that
in seven hours a hagfish of this species will devour eighteen times
its own weight of fish-flesh. The intestinal canal is a simple tube,
through which most of the food passes undigested. The eggs are large,
each in a yellowish horny case, at one end of which are barbed threads
by which they cling together and to kelp or other objects. In the
California hagfish, _Polistotrema stouti_, great numbers of these eggs
have been found in the stomachs of the males.

[Illustration: FIG. 291.--California Hagfish, _Polistotrema stouti_
Lockington.]

Similar habits are possessed by all the species in the two families,
_Myxinidae_ and _Eptatretidae_. In the _Myxinidae_ the gill-openings
are apparently single on each side, the six gills being internal and
leading by six separate ducts to each of the six branchial sacs. The
skin is excessively slimy, the extensible tongue is armed with two
cone-like series of strong teeth. About the mouth are eight barbels.

Of _Myxine_, numerous species are known--_Myxine glutinosa_, in the
north of Europe; _Myxine limosa_, of the West Atlantic; _Myxine
australis_, and several others about Cape Horn, and _Myxine garmani_
in Japan. All live in deep waters and none have been fully studied. It
has been claimed that the hagfish is male when young, many individuals
gradually changing to female, but this conclusion lacks verification
and is doubtless without foundation.

In the _Eptatretidae_ the gill-openings, six to fourteen in number, are
externally separate, each with its own branchial sac as in the lampreys.

The species of the genus _Eptatretus_ (_Bdellostoma_, _Heptatrema_,
and _Homea_, all later names for the same group) are found only in the
Pacific, in California, Chile, Patagonia, South Africa, and Japan. In
general appearance and habits these agree with the species of _Myxine_.
The species with ten to fourteen gill-openings (_dombeyi_: _stouti_)
are sometimes set off as a distinct genus (_Polistotrema_), but in
other regards the species differ little, and frequent individual
variations occur. _Eptatretus burgeri_ is found in Japan and
_Eptatretus forsteri_ in Australia.

=The Hyperoartia, or Lampreys.=--In the order _Hyperoartia_, or
lampreys, the single nostril is a blind sac which does not penetrate
the palate. The seven gill-openings lead each to a separate sac, the
skin is not especially covered with mucus, the eyes are well developed
in the adult, and the mouth is a round disk armed with rasp-like
teeth, the comb-like teeth on the tongue being less developed than
in the hagfishes. The intestine in the lampreys has a spiral valve.
The eggs are small and are usually laid in brooks away from the sea,
and in most cases the adult lamprey dies after spawning. According to
Thoreau, "it is thought by fishermen that they never return, but waste
away and die, clinging to rocks and stumps of trees for an indefinite
period, a tragic feature in the scenery of the river-bottoms worthy to
be remembered with Shakespeare's description of the sea-floor." This
account is not far from the truth, as recent studies have shown.

The lampreys of the northern regions constitute the family of
_Petromyzonidae_. The larger species (_Petromyzon_, _Entosphenus_) live
in the sea, ascending rivers to spawn, and often becoming land-locked
and reduced in size by living in rivers only. Such land-locked marine
lampreys (_Petromyzon marinus unicolor_) breed in Cayuga Lake and
other lakes in New York. The marine forms reach a length of three
feet. Smaller lampreys of other genera six inches to eighteen inches
in length remain all their lives in the rivers, ascending the little
brooks in the spring, clinging to stones and clods of earth till
their eggs are deposited. These are found throughout northern Europe,
northern Asia, and the colder parts of North America, belonging to the
genera _Lampetra_ and _Ichthyomyzon_. Other and more aberrant genera
from Chile and Australia are _Geotria_ and _Mordacia_, the latter
forming a distinct family, _Mordaciidae_. In _Geotria_, a large and
peculiar gular pouch is developed at the throat. In _Macrophthalmia_
_chilensis_ from Chile the eyes are large and conspicuous.

=Food of Lampreys.=--The lampreys feed on the blood and flesh of
fishes. They attach themselves to the sides of the various species,
rasp off the flesh with their teeth, sucking the blood till the fish
weakens and dies. Preparations made by students of Professor Jacob
Reighard in the University of Michigan show clearly that the lamprey
stomach contains muscular tissue as well as the blood of fishes. The
river species do a great deal of mischief, a fact which has been the
subject of a valuable investigation by Professor H. A. Surface, who has
also considered the methods available for their destruction. The flesh
of the lamprey is wholesome, and the larger species, especially the
great sea lamprey of the Atlantic, _Petromyzon marinus_, are valued as
food. The small species, according to Prof. Gage, never feed on fishes.

[Illustration: FIG. 292.--Lamprey, _Petromyzon marinus_ L. Woods Hole,
Mass.]

=Metamorphosis of Lampreys.=--All lampreys, so far as known, pass
through a distinct metamorphosis. The young, known as the _Ammocoetes_
form, are slender, eyeless, and with the mouth narrow and toothless.
From Professor Surface's paper on "The Removal of Lampreys from the
Interior Waters of New York" we have the following extracts (slightly
condensed):

[Illustration: FIG. 293.--_Petromyzon marinus unicolor_ (De Kay). Mouth
of Lake Lamprey, Cayuga Lake. (After Gage.)]

[Illustration: FIG. 294.--_Lampetra wilderi_ Jordan & Evermann. Larval
Brook Lamprey in its burrow in a glass filled with sand. (After Gage.)]

[Illustration: FIG. 295.--_Lampetra wilderi_ Jordan & Evermann. Mouth
of Brook Lamprey. Cayuga Lake. (After Gage.)]

"In the latter part of the fall the young lampreys, _Petromyzon marinus
unicolor_, the variety land-locked in the lakes of Central New York,
metamorphose and assume the form of the adult. They are now about six
or eight inches long. The externally segmented condition of the body
disappears. The eyes appear to grow out through the skin and become
plainly visible and functional. The mouth is no longer filled with
vertical membranous sheets to act as a sieve, but it contains nearly
one hundred and fifty sharp and chitinous teeth, arranged in rows
that are more or less concentric and at the same time presenting the
appearance of circular radiation. These teeth are very strong, with
sharp points, and in structure each has the appearance of a hollow
cone of chitin placed over another cone or papilla. A little below the
center of the mouth is the oral opening, which is circular and contains
a flattened tongue which bears finer teeth of chitin set closely
together and arranged in two interrupted (appearing as four) curved
rows extending up and down from the ventral toward the dorsal side
of the mouth. Around the mouth is a circle of soft membrane finally
surrounded by a margin of fimbriae or small fringe. This completes the
apparatus with which the lamprey attaches itself to its victims, takes
its food, carries stones, builds and tears down its nest, seizes its
mate, holds itself in position in a strong current, and climbs over
falls."

=Mischief Done by Lampreys.=--"The most common economic feature in
the entire life history of these animals is their feeding habits in
this (spawning) stage, their food now consisting wholly of the blood
(and flesh) of fishes. A lamprey is able to strike its suctorial mouth
against a fish, and in an instant becomes so firmly attached that it
is very rarely indeed that the efforts of the fish will avail to rid
itself of its persecutor. When a lamprey attaches itself to a person's
hand in the aquarium, it can only be freed by lifting it from the
water. As a rule it will drop the instant it is exposed to the open
air, although often it will remain attached for some time even in the
open air, or may attach itself to an object while out of water.

"Nearly all lampreys that are attached to fish when they are caught
in nets will escape through the meshes of the nets, but some are
occasionally brought ashore and may hang on to their victim with
bulldog pertinacity.

"The fishes that are mostly attacked are of the soft-rayed species,
having cycloid scales, the spiny-rayed species with ctenoid scales
being most nearly immune from their attacks. We think there may be
three reasons for this: 1st, the fishes of the latter group are
generally more alert and more active than those of the former, and may
be able more readily to dart away from such enemies; 2d, their scales
are thicker and stronger and appear to be more firmly imbedded in the
skin, consequently it is more difficult for the lampreys to hold on and
cut through the heavier coat-of-mail to obtain the blood of the victim;
3d, since the fishes of the second group are wholly carnivorous and in
fact almost exclusively fish-eating when adult, in every body of water
they are more rare than those of the first group, which are more nearly
omnivorous. According to the laws and requirements of nature the fishes
of the first group must be more abundant, as they become the food for
those of the second, and it is on account of their greater abundance
that the lampreys' attacks on them are more observed.

"There is no doubt that the bullhead, or horned pout (_Ameiurus
nebulosus_), is by far the greatest sufferer from lamprey attacks in
Cayuga Lake. This may be due in part to the sluggish habits of the
fish, which render it an easy victim, but it is more likely due to
the fact that this fish has no scales and the lamprey has nothing to
do but to pierce the thick skin and find its feast of blood ready for
it. There is no doubt of the excellency of the bullhead as a food-fish
and of its increasing favor with mankind. It is at present the most
important food- and market-fish of the State (New York), being caught
by bushels in the early part of June when preparing to spawn. As we
have observed at times more than ninety per cent. of the catch attacked
by lampreys, it can readily be seen how very serious are the attacks
of this terrible parasite which is surely devastating our lakes and
streams."

=Migration or "Running" of Lampreys.=--"After thus feeding to an
unusual extent, their reproductive elements (gonads) become mature
and their alimentary canals commence to atrophy. This duct finally
becomes so occluded that from formerly being large enough to admit a
lead-pencil of average size when forced through it, later not even
liquids can pass through, and it becomes nearly a thread closely
surrounded by the crowding reproductive organs. When these changes
commence to ensue, the lampreys turn their heads against the current
and set out on their long journeys to the sites that are favorable for
spawning, which here may be from two to eight miles from the lake. In
this migration they are true to their instincts and habits of laziness
in being carried about, as they make use of any available object, such
as a fish, boat, etc., that is going in their direction, fastening to
it with their suctorial mouths and being borne along at their ease.
During this season it is not infrequent that as the Cornell crews
come in from practice and lift their shells from the water, they find
lampreys clinging to the bottoms of the boats, sometimes as many as
fifty at one time. They are likely to crowd up all streams flowing
into the lake, inspecting the bed of the stream as they go. They do
not stop until they reach favorable spawning sites, and if they find
unsurmountable obstacles in their way, such as vertical falls or dams,
they turn around and go down-stream until they find another, up which
they go. This is proved every spring by the number of adult lampreys
which are seen temporarily in Pall Creek and Cascadilla Creek. In each
of these streams, about a mile from its mouth, there is a vertical fall
over thirty feet in height which the lampreys cannot surmount, and in
fact they have never been seen attempting to do so. After clinging with
their mouths to the stones at the foot of the falls for a few days,
they work their way down-stream, carefully inspecting all the bottom
for suitable spawning sites. They do not spawn in these streams because
there are too many rocks and no sand, but finally enter the only stream
(the Cayuga Lake inlet) in which they find suitable and accessible
spawning sites.

[Illustration: FIG. 296.--Kamchatka Lamprey, _Lampetra camtschatica_
(Tilesius). Kamchatka.]

"The three-toothed lampreys (_Entosphenus tridentatus_) of the West
Coast climb low falls or rapids by a series of leaps, holding with
their mouths to rest, then jumping and striking again and holding, thus
leap by leap gaining the entire distance.

"The lampreys here have never been known to show any tendency or
ability to climb, probably because there are no rapids or mere low
falls in the streams up which they would run. In fact, as the inlet
is the only stream entering Cayuga Lake in this region which presents
suitable spawning conditions and no obstructions, it can be seen
at once that all the lampreys must spawn in this stream and its
tributaries.

[Illustration: FIG. 297.--Oregon Lamprey, _Entosphenus tridentatus_,
ascending a brook. (Modified from a photograph by Dr. H. M. Smith.
Published by Prof. H. A. Surface.) Willamette River, Oregon.]

"In 'running' they move almost entirely at night, and if they do not
reach a suitable spawning site by daylight, they will cling to roots
or stones during the day and complete their journey the next night.
This has been proven by the positive observation of individuals. Of
the specimens that run up early in the season, about four-fifths are
males. Thus the males do not exactly precede the females, because we
have found the latter sex represented in the stream as early in the
season as the former, but in the earlier part of the season the number
of the males certainly greatly predominates. This proportion of males
gradually decreases, until in the middle of the spawning season the
sexes are about equally represented, and toward the latter part of the
season the females continue to come until they in turn show the greater
numbers. Thus it appears very evident in general that the reproductive
instinct impels the most of the males to seek the spawning ground
before the most of the females do. However, it should be said that
neither the males nor the females show all of the entirely sexually
mature features when they first run up-stream in the beginning of the
season, but later they are perfectly mature and 'ripe' in every regard
when they first appear in the stream. When they migrate, they stop
at the site that seems to suit their fancy, many stopping near the
lake, others pushing on four or five miles farther up-stream. We have
noted, however, that later in the season the lower courses become more
crowded, showing that the late comers do not attempt to push up-stream
as far as those that came earlier. Also it thus follows, from what was
just said about late-running females, that in the latter part of the
season the lower spawning beds are especially crowded with females. In
fact, during the early part of the month of June we have found, not
more than half a mile above the lowest spawning bed, as many as five
females on a spawning nest with but one male; and in that immediate
vicinity many nests indeed were found at that time with two or three
females and but one male.

"Having arrived at a shoal which seems to present suitable conditions
for a spawning nest, the individual or pair commences at once to move
stones with its mouth from the centre to the margin of an area one or
two feet in diameter. When many stones are thus placed, especially
at the upper edge, and they are cleaned quite free of sediment and
algae, both by being moved and by being fanned with the tail, and when
the proper condition of sand is found in the bottom of the basin thus
formed, it is ready to be used as a spawning bed or nest. A great
many nests are commenced and deserted. This has been left as a mystery
in publications on the subject, but we are well convinced that it is
because the lampreys do not find the requisites or proper conditions
of bottom (rocks, sand, etc., as given below) to supply all their
needs and fulfill all conditions for ideal sites. This desertion of
half-constructed nests is just what would be expected and anticipated
in connection with the explanation of 'Requisite Conditions for
Spawning,' given below, because some shallows contain more sand and
fewer stones, and others contain many larger stones but no sand, while
others contain pebbles lying over either rocks or sand. The lampreys
remove some of the material, and if they do not find all the essentials
for a spawning nest, the site is deserted and the creatures move on."

=Requisite Conditions for Spawning with Lampreys.=--"For a spawning
site two conditions are immediately essential--proper conditions of
water and suitable stream bed or bottom. Of course with these it is
essential that no impassable barriers (dam or falls) exist between
the lake and the spawning sites to prevent migration at the proper
'running' season. Lampreys will not spawn where there is no sand lying
on the bottom between the rocks, as sand is essential in covering the
eggs (see remarks on the 'Spawning Process'); neither will they spawn
where the bottom is all sand and small gravel, as they cannot take
hold of this material with their mouths to construct nests or to hold
themselves in the current, and they would not find here pebbles and
stones to carry over the nest while spawning, as described elsewhere.
It can thus be seen that, as suggested above, the reason they do not
spawn in Fall Creek and Cascadilla Creek, between the lake and the
falls, is that the beds of these streams are very rocky, being covered
only with large stones and no sand. There is no doubt that the lampreys
find here suitable conditions of water, but they do not remain to
spawn on account of the absence of the proper conditions of stream
bed. Again, they do not spawn in the lower course of the inlet for
a distance of nearly two miles from the lake, because near the lake
the bed of the stream is composed of silt, while for some distance
above this (up-stream) there is nothing but sand. Farther up-stream
are found pebbles and stones commingled with sand, which combination
satisfies the demands of the lampreys for material in constructing
nests and covering eggs. The accessibility of these sites, together
with their suitable conditions, render the inlet the great and perhaps
the only spawning stream of the lake; and, doubtless, all the mature
lampreys come here to spawn, excepting a few which spawn in the lower
part of Six-mile Creek, a tributary of the inlet.

"As the course of the stream where the beds abound is divided into
pools, separated by stony ripples or shallows, the nests must be made
at the ends of the pools. Of the spawning beds personally observed
during several seasons, nine-tenths of the entire number were formed
just above the shallows at the lower ends of the pools, while only a
few were placed below them. An advantage in forming the nest above the
shoals rather than below it is that in the former place the water runs
more swiftly over the lower and middle parts of such a bed than at its
upper margin, since the velocity decreases in either direction from the
steeper part of the shallows; and any organic material or sediment that
would wash over the upper edge of the nest is thus carried on rather
than left as a deposit. When formed below the shallows, owing to the
decreased velocity at the lower part of the nest compared with that
at the upper, the sediment is likely to settle in the hollow of the
nest, and, through the process of decay of the organic material, prove
disastrous or unfavorable for the developing embryos.

"The necessity of sand in the spawning bed indicates the explanation of
why we see so many shallows which have no spawning lampreys upon them,
while there are others in the same vicinity that are crowded. There
will be no nests formed if there is too little or too much sand, not
enough or too many stones, or stones that are all too small or all too
large. The stones must vary from the size of an egg to the size of a
man's hand, and must be intermingled with sand without mud or rubbish.

"The lampreys choose to make their spawning nests just where the water
flows so swiftly that it will carry the sand a short distance, but
will not sweep it out of the nest. This condition furnishes not only
force to wash the sand over the eggs when laid, but also keeps the
adult lampreys supplied with an abundance of fresh water containing the
dissolved air needed for their very rapid respiration. Of course in
such rapid water the eggs are likely to be carried away down-stream,
but Nature provides against this by the fact that they are adhesive,
and the mating lampreys stir up the sand with their tails, thus
weighing down the freshly laid eggs and holding them in the nest. Hence
the necessity of an abundance of sand at the spawning site."

=The Spawning Process with Lampreys.=--"There is much interest in the
study of the spawning process, as it is for the maintenance of the
race that the lampreys risk and end their lives; and as they are by
far the lowest form of vertebrates found within the United States,
a consideration of their actions and apparent evidences of instinct
becomes of unusual attraction. Let us consider one of those numerous
examples in which the male migrates before the female. When he comes
to that portion of the stream where the conditions named above are
favorable, he commences to form a nest by moving and clearing stones
and making a basin with a sandy bottom about the size of a common
wash-bowl. Several nests may be started and deserted before perfect
conditions are found for the completion of one. The male may be joined
by a female either before or after the nest is completed. There is
at once harmony in the family; but if another male should attempt to
intrude, either before or after the coming of the female, he is likely
to be summarily dealt with and dismissed at once by the first tenant.
As soon as the female arrives she too commences to move pebbles and
stones with her mouth.

"Sometimes the nest is made large enough to contain several pairs, or
often unequal numbers of males and females; or they may be constructed
so closely together as to form one continuous ditch across the
stream, just above the shallows. Many stones are left at the sides
and especially at the upper margin of the nest, and to these both
lampreys often cling for a few minutes as though to rest. While the
female is thus quiet, the male seizes her with his mouth at the back
of her head, clinging as to a fish. He presses his body as tightly as
possible against her side, and loops his tail over her near the vent
and down against the opposite side of her body so tightly that the
sand, accidentally coming between them, often wears the skin entirely
off of either or both at the place of closest contact. In most observed
instances the male pressed against the right side of the female,
although there is no unvarying rule as to position. The pressure of the
male thus aids to force the eggs from the body of the female, which
flow very easily when ripe. The vents of the two lampreys are thus
brought into close proximity, and the conspicuous genital papilla of
the male serves to guide the milt directly to the issuing spawn. There
appears to be no true intromission, although definite observation of
this feature is quite difficult, and, in fact, impossible. During the
time of actual pairing, which lasts but a few seconds, both members
of the pair exhibit tremendous excitement, shaking their bodies in
rapid vibrations and stirring up such a cloud of sand with their
tails that their eggs are at once concealed and covered. As the eggs
are adhesive and non-buoyant, the sand that is stirred up adheres to
them immediately and covers most of them before the school of minnows
in waiting just below the nest can dart through the water and regale
themselves upon the eggs of these enemies of their race; but woe to the
eggs that are not at once concealed. We would suggest that the function
of the characteristic anal fin, which is possessed only by the female,
and only at this time of year, may be to aid in this vastly important
process of stirring up the sand as the eggs are expelled; and the
explanation of the absence of such a fin from the ventral side of the
tail of the male may be found in the fact that it could not be used for
the same purpose at the instant when most needed, since the male is
just then using his tail as a clasping organ to give him an essential
position in pairing. As soon as they shake together they commence to
move stones from one part of the nest to another, to bring more loose
sand down over their eggs. They work at this from one to five minutes,
then shake again, thus making the intervals between mating from one to
five minutes, with a general average of about three and a half minutes.

"Although their work of moving stones does not appear to be systematic
in reference to the placing of the pebbles, or as viewed from the
standpoint of man, it does not need to be so in order to perfectly
fulfill all the purposes of the lampreys. As shown above in the remarks
on the spawning habits of the brook lampreys, the important end which
they thus accomplish is the loosening and shifting of the sand to cover
their eggs; and the more the stones are moved, even in the apparently
indiscriminate manner shown, the better is this purpose achieved. Yet,
in general, they ultimately accomplish the feat of moving to the lower
side of the nest all the stones they have placed or left at the upper
margin. At the close of the spawning season when the nest is seen with
no large pebbles at its upper margin, but quite a pile of stones below,
it can be known that the former occupants completed their spawning
process there; but if many small stones are left at the upper edge and
at the sides, and a large pile is not formed at the lower edge, it can
be known that the nest was forsaken or the lampreys removed before the
spawning process was completed. The stones they move are often twice
as heavy as themselves, and are sometimes even three or four times as
heavy. Since they are not attempting to build a stone wall of heavy
material, there is no occasion for their joining forces to remove
stones of extraordinary size, and they rarely do so, although once
during the past spring (1900) we saw two lake lampreys carrying the
same large stone down-stream across their nest. Although this place
was occupied by scores of brook lampreys, there were but three pairs
of lake lampreys seen here. It is true that one of these creatures
often moves the same stone several times, and many even attempt many
times to move a stone that has already been found too heavy for it; but
sooner or later the rock may become undermined so that the water will
aid them, and they have no way of knowing what they can do under such
circumstances until they try. Also, the repeated moving of one stone
may subserve the same purpose for the lamprey in covering its eggs with
sand as would the less frequent removal of many.

"When disturbed on the spawning nest, either of the pair will return
to the same nest if its mate is to be found there; but if its mate
is in another place, it will go to it, and if its mate is removed or
killed, it is likely to go to any part of the stream to another nest.
When disturbed, they often start up-stream for a short distance, but
soon dart down-stream with a velocity that is almost incredible. They
can swim faster than the true fishes, and after they get a start are
generally pretty sure to make good their escape, although we have seen
them dart so wildly and frantically down-stream that they would shoot
clear out on the bank and become an easy victim of the collector. This
peculiar kind of circumstance is most likely to happen with those
lampreys that are becoming blinded from long exposure to the bright
light over the clear running water. If there is a solitary individual
on a nest when disturbed, it may not return to that nest, but to any
that has been started, or it may stay in the deep pool below the
shallows until evening and then move some distance up-stream. When
the nest is large and occupied by several individuals, those that
are disturbed may return to any other such nest. We have never seen
evidence of one female driving another female out of a spawning-nest;
and from the great number of nests in which we have found the numbers
of the females exceeding those of the males, we would be led to infer
that the former live together in greater harmony than do the males.

"Under the subject of the number of eggs laid, we should have said
that at one shake the female spawns from twenty to forty. We once
caught in fine gauze twenty-eight eggs from a female at one spawning
instant. In accordance with the frequency of spawning stated, and the
number of eggs contained in the body of one female, the entire length
of time given to the spawning process would be from two to four days.
This agrees with the observed facts, although the lampreys spend much
time in moving stones and thoroughly covering the nests with sand. Even
after the work of spawning and moving stones is entirely completed,
they remain clinging to rocks in various parts of the stream, until
they are weakened by fungus and general debility, when they gradually
drift down-stream.

"In forming nests there is a distinct tendency to utilize those sites
that are concealed by overhanging bushes, branches, fallen tree-tops,
or grass or weeds, probably not only for concealment, but also to avoid
the bright sunlight, which sooner or later causes them to go blind, as
it does many fishes when they have to live in water without shade.
Toward the end of the spawning season, it is very common to see blind
lampreys clinging helplessly to any rocks on the bottom, quite unable
to again find spawning-beds. However, at such times they are generally
spent and merely awaiting the inevitable end.

"As with the brook lamprey, the time of spawning and duration of the
nesting period depend upon the temperature of the water, as does also
the duration of the period of hatching or development of the embryo.
They first run up-stream when the water reaches a temperature of 45 deg.
or 48 deg. Fahr., and commence spawning at about 50 deg.. A temperature of
60 deg. finds the spawning process in its height, and at 70 deg. it is fairly
completed. It is thus that the rapidity with which the water becomes
heated generally determines the length of time the lampreys remain in
the stream. This may continue later in the season for those that run
later, but usually it is about a month or six weeks from the time the
first of this species is seen on a spawning-nest until the last is
gone."

=What becomes of Lampreys after Spawning?=--"There has been much
conjecture as to the final end of the lampreys, some writers contending
that they die after spawning, others that they return to deep water
and recuperate, and yet others compromise these two widely divergent
views by saying that some die and others do not. The fact is that the
spawning process completely wears out the lampreys, and leaves them in
a physical condition from which they could never recover. They become
stone-blind; the alimentary canal suffers complete atrophy; their
flesh becomes very green from the katabolic products, which find the
natural outlet occluded; they lose their rich yellow color and plump,
symmetrical appearance; their skin becomes torn, scratched, and worn
off in many places, so that they are covered with sores, and they
become covered with a parasitic or sarcophytic fungus, which forms a
dense mat over almost their entire bodies, and they are so completely
debilitated and worn out that recovery is entirely out of the question.
What is more, the most careful microscopical examination of ovaries and
testes has failed to reveal any evidence of new gonads or reproductive
bodies. This is proof that reproduction could not again ensue without
a practical rebuilding of the animals, even though they should regain
their vitality. A. Mueller, in 1865, showed that all the ova in the
lamprey were of the same size, and that after spawning no small
reproductive bodies remained to be developed later. This is strong
evidence of death after once spawning.

"One author writes that an argument against the theory of their dying
after spawning can be found in the fact that so few dead ones have been
found by him. However, many can be found dead if the investigator only
knows how and where to look for them. We should not anticipate finding
them in water that is shallow enough for the bottom to be plainly seen,
as there the current is strong enough to move them. It is in the deep,
quiet, pools where sediment is depositing that the dead lampreys are
dropped by the running water, and there they sink into the soft ooze.

"The absence of great numbers of dead lampreys from visible portions
of the stream cannot be regarded as important evidence against the
argument that they die soon after spawning once, as the bodies are
very soon disintegrated in the water. In the weir that we maintained
in 1898, a number of old, worn-out, and fungus-covered lampreys were
caught drifting down-stream; some were dead, some alive, and others
dying and already insensible, but none were seen going down that
appeared to be in condition to possibly regain their strength."

[Illustration: FIG. 297_a_.--Brook Lamprey, _Lampetra Wilderi_. (After
Gage.)]




CHAPTER XXIX

THE CLASS ELASMOBRANCHII OR SHARK-LIKE FISHES


=The Sharks.=--The gap between the lancelets and the lampreys is a
very wide one. Assuming the primitive nature of both groups, this
gap must represent the period necessary for the evolution of brain,
skull, and elaborate sense organs. The interspace between the lampreys
and the nearest fish-like forms which follow them in an ascending
scale is not less remarkable. Between the lamprey and the shark we
have the development of paired fins with their basal attachments
of shoulder-girdle and pelvis, the formation of a lower jaw, the
relegation of the teeth to the borders of the mouth, the development
of separate vertebrae along the line of the notochord, the development
of the gill-arches, and of an external covering of enameled points or
placoid scales.

These traits of progress separate the Elasmobranchs from all lower
vertebrates. For those animals which possess them, the class name of
_Pisces_ or fishes has been adopted by numerous authors. If this term
is to be retained for technical purposes, it should be applied to the
aquatic vertebrates above the lampreys and lancelets. We may, however,
regard fish as a popular term only, rather than to restrict the name to
members of a class called _Pisces_. From the bony fishes, on the other
hand, the sharks are distinguished by the much less specialization
of the skeleton, both as regards form and substance, by the lack of
membrane bones, of air-bladder, and of true scales, and by various
peculiarities of the skeleton itself. The upper jaw, for example, is
formed not of maxillary and premaxillary, but of elements which in
the lower fishes would be regarded as belonging to the palatine and
pterygoid series. The lower jaw is formed not of several pieces,
but of a cartilage called Meckel's cartilage, which in higher fishes
precedes the development of a separate dentary bone. These structures
are sometimes called primary jaws, as distinguished from secondary jaws
or true jaws developed in addition to those bones in the _Actinopteri_
or typical fishes. In the sharks the shoulder-girdle is attached, not
to the skull, but to a vertebra at some distance behind it, leaving
a distinct neck, such as is possessed or retained by the vertebrate
higher than fishes. The shoulder-girdle itself is a continuous arch of
cartilage, joining its fellow at the breast of the fish. Other peculiar
traits will be mentioned later.

=Characters of Elasmobranchs.=--The essential character of the
Elasmobranchs as a whole are these: The skeleton is cartilaginous, the
skull without sutures, and the notochord more or less fully replaced
or inclosed by vertebral segments. The jaws are peculiar in structure,
as are also the teeth, which are usually highly specialized and found
on the jaws only. There are no membrane bones; the shoulder-girdle is
well developed, each half of one piece of cartilage, and the ventral
fins, with the pelvic-girdle, are always present, always many-rayed,
and abdominal in position. The skin is covered with placoid scales,
or shagreen, or with bony bucklers, or else it is naked. It is never
provided with imbricated scales. The tail is diphycercal, heterocercal,
or else it degenerates into a whip-like organ, a form which has been
called leptocercal. The gill-arches are 5, 6, or 7 in number, with
often an accessory gill-slit or spiracle. The ventral fins in the
males (except perhaps in certain primitive forms) are provided with
elaborate cartilaginous appendages or claspers. The brain is elongate,
its parts well separated, the optic nerves interlacing. The heart has
a contractile arterial cone containing several rows of valves; the
intestine has a spiral valve; the eggs are large, hatched within the
body, or else deposited in a leathery case.

=Classification of Elasmobranchs.=--The group of sharks and their
allies, rays, and Chimaeras, is usually known collectively as
_Elasmobranchii_ (~elasmos~, blade or plate; ~branchos~, gill). Other
names applied to all or a part of this group are these: _Selachii_
(~selachos~, a cartilage, the name also used by the Greeks for the
gristle-fishes or sharks); _Plagiostomi_ (~plagios~, oblique; ~stoma~,
mouth); _Chondropterygii_ (~chondros~, cartilage; ~pteryx~, fin); and
_Antacea_ (~antakaios~, sturgeon). They represent the most primitive
known type of jaw-bearing vertebrates, or _Gnathostomi_ (~gnathos~,
jaw; ~stoma~, mouth), the Chordates without jaws being sometimes called
collectively _Agnatha_ (~a-gnathos~, without jaws). These higher types
of fishes have been also called collectively _Lyrifera_, the form
of the two shoulder-girdles taken together being compared to that
of a lyre. Through shark-like forms all the higher vertebrates must
probably trace their descent. Sharks' teeth and fin-spines are found
in all rocks from the Upper Silurian deposits to the present time, and
while the majority of the genera are now extinct, the class has had a
vigorous representation in all the seas, later Palaeozoic, Mesozoic, and
Cenozoic, as well as in recent times.

Most of the Elasmobranchs are large, coarse-fleshed, active animals
feeding on fishes, hunting down their prey through superior strength
and activity. But to this there are many exceptions, and the highly
specialized modern shark of the type of the mackerel-shark or man-eater
is by no means a fair type of the whole great class, some of the
earliest types being diminutive, feeble, and toothless.

=Subclasses of Elasmobranchs.=--With the very earliest recognizable
remains it is clear that the Elasmobranchs are already divided into two
great divisions, the sharks and the _Chimaeras_. These groups we may
call subclasses, the _Selachii_ and the _Holocephali_, or Chismopnea.

The _Selachii_, or sharks and rays, have the skull hyostylic, that is,
with the quadrate bone grown fast to the palate which forms the upper
jaw, the hyomandibular, acting as suspensorium to the lower jaw, being
articulated directly to it.

The palato-quadrate apparatus, the front of which forms the upper jaw
in the shark, is not fused to the cranium, although it is sometimes
articulated with it. There are as many external gill-slits as there
are gill-arches (5, 6, or 7), and the gills are adnate to the flesh of
their own arches, without free tips. The cerebral hemispheres are grown
together. The teeth are separated and usually strongly specialized,
being primitively modified from the prickles or other defences of the
skin. There is no frontal holder or bony hook on the forehead of the
male.

The subclass _Holocephali_, or _Chimaeras_, differ from the sharks in
all this series of characters, and its separation as a distinct group
goes back to the Devonian or even farther, the earliest known sharks
having little more in common with Chimaeras than the modern forms have.

=The Selachii.=--There have been many efforts to divide the sharks and
rays into natural orders. Most writers have contented themselves with
placing the sharks in one order (_Squali_ or _Galei_ or _Pleurotremi_)
having the gill-openings on the side, and the rays in another (_Rajae_,
_Batoidei_, _Hypotrema_) having the gill-openings underneath. Of far
more importance than this superficial character of adaptation are the
distinctions drawn from the skeleton. Dr. Gill has used the attachment
of the palato-quadrate apparatus as the basis of a classification. The
_Opistharthri_ (_Hexanchidae_) have this structure articulated with the
postorbital part of the skull. In the _Prosarthri_ (_Heterodontidae_)
it is articulated with the preorbital part of the skull, while in the
other sharks (_Anarthri_) it is not articulated at all. But these
characters do not appear to be always important. _Chlamydoselachus_,
for example, differs in this regard from _Heptranchias_, which in
other respects it closely resembles. Yet, in general, the groups thus
characterized are undoubtedly natural ones.

[Illustration: FIG. 298.--Fin-spine of _Onchus tenuistriatus_ Agassiz.
(After Zittel.)]

=Hasse's Classification of Elasmobranchs.=--In 1882, Professor Carl
Hasse proposed to subdivide the sharks on the basis of the structure
of the individual vertebrae. In the lowest group, a hypothetical
order of _Polyospondyli_, possibly represented by the fossil spines
called _Onchus_, an undivided notochord, perhaps swollen at regular
intervals, is assumed to have represented the vertebral column.
In the _Diplospondyli_ (_Hexanchidae_) the imperfectly segmented
vertebrae are joined in pairs, each pair having two neural arches.
In the _Asterospondyli_ or ordinary sharks each vertebra has its
calcareous lamella radiating star-like from the central axis. In the
_Cyclospondyli_ (_Squalidae_, etc.) the calcareous part forms a single
ring about the axis, and in the _Tectospondyli_ (_Squatina_, rays,
etc.) it forms several rings. These groups again are natural and
correspond fairly with those based on other characters. At the same
time there is no far-reaching difference between _Cyclospondyli_ and
_Tectospondyli_, and the last-named section includes both sharks and
rays.

[Illustration: FIG. 299.--Section of vertebrae of sharks, showing
calcification. (After Hasse.) 1. _Cyclospondyli_ (_Squalus_); 2.
_Tectospondyli_ (_Squatina_); 3. _Asterospondyli_ (_Carcharias_).]

Nothing is known of the _Polyospondyli_, and they may never have
existed at all. The _Diplospondyli_ do not differ very widely from the
earlier _Asterospondyli_ (_Cestraciontes_) which, as a matter of fact,
have preceded the _Diplospondyli_ in point of time, if we can trust our
present knowledge of the geological record.

=Other Classifications of Elasmobranchs.=--Characters more fundamental
may be drawn from the structure of the pectoral fin. In this regard
four distinct types appear. In _Acanthoessus_ this fin consists of
a stout, stiff spine, with a rayless membrane attached behind it.
In _Cladoselache_ the fin is low, with a very long base, like a
fold of skin (_ptychopterygium_), and composed of feeble rays. In
_Pleuracanthus_ it is a jointed axis of many segments, with a fringe
of slender fin-rays, corresponding in structure to all appearance to
the pectoral fin of Dipnoans and Crossopterygians, the type called by
Gegenbaur _archipterygium_ on the hypothesis that it represents the
primitive vertebrate limb.

In most sharks the fin has a fan-shape, with three of the basal
segments larger than the others. Of these the mesopterygium is the
central one, with the propterygium before it and the metapterygium
behind. In the living sharks of the family of _Heterodontidae_, this
form of fin occurs and the teeth of the same general type constitute
the earliest remains distinctly referable to sharks in the Devonian
rocks.

=Primitive Sharks.=--Admitting that these four types of pectoral fin
should constitute separate orders, we have next to consider which form
is the most primitive and what is the line of descent. In this matter
we have, in the phrase of Haeckel, only the "three ancestral documents,
Palaeontology, Morphology, and Ontogeny."

Unfortunately the evidence of these documents is incomplete and
conflicting. So far as Palaeontology is concerned, the fin of
_Cladoselache_, with that of _Acanthoessus_, which may be derived
from it, appears earliest, but the modern type of pectoral fin with
the three basal segments is assumed to have accompanied the teeth of
Psammodonts and Cochliodonts, while the fin of the Chimaera must have
been developed in the Devonian. The jointed fin of _Cladodus_ and
_Pleuracanthus_ may be a modification or degradation of the ordinary
type of shark-fin.

Assuming, however, that the geological record is not perfect and that
the fin of _Cladoselache_ is not clearly shown to be primitive, we have
next to consider the evidence drawn from morphology.

Those who with Balfour and others (see page 69) accept the theory
that the paired fins are derived from a vertebral fold, will regard
with Dean the fin of _Cladoselache_ as coming nearest the theoretical
primitive condition.

The pectoral fin in _Acanthoessus_ Dean regards as a specialized
derivative from a fin like that of _Cladoselache_, the fin-rays being
gathered together at the front and joined together to form the thick
spine characteristic of _Acanthoessus_. This view of the morphology of
the fin of _Acanthoessus_ is not accepted by Woodward, and several
different suggestions have been recorded.

If with Gegenbaur we regard the paired fins as derived from the septa
between the gill-slits, or with Kerr regard them as modified external
gills, the whole theoretical relation of the parts is changed. The
archipterygium of _Pleuracanthus_ would be the nearest approach to
the primitive pectoral limb, and from this group and its allies
all the other sharks are descended. This central jointed axis of
_Pleuracanthus_ is regarded by Traquair as the equivalent of the
metapterygium in ordinary sharks. (See Figs. 44, 45, 46.)

According to Traquair: "The median stern [of the archipterygium],
simplified, shortened up and losing all its radials on the
postaxial side, except in sometimes a few near the tip, becomes the
metapterygium, while the mesopterygium and propterygium are formed by
the fusion into two pieces of the basal joints of a number of preaxial
radials, which have reached and become attached to the shoulder-girdle
in front of the metapterygium."

According to Dr. Traquair, the pectoral fin in _Cladodus neilsoni_, a
shark from the Coal Measures of Scotland, is "apparently a veritable
uniserial archipterygium midway between the truly biserial one of
_Pleuracanthus_ and the pectoral fin of ordinary sharks." Other
authors look on these matters differently, and Dr. Traquair admits
that an opposite view is almost equally probable. Cope and Dean
would derive the tribasal pectoral of ordinary sharks directly
from the ptychopterygium or fan-like fold of _Cladoselache_, while
Fritsch and Woodward would look upon it as derived in turn from the
_Ceratodus_-like fin of _Pleuracanthus_, itself derived from the
ptychopterygium or remains of a lateral fin-fold.

If the Dipnoans are descended from the Crossopterygians, as Dollo
has tried to show, the archipterygium of _Pleuracanthus_ has had a
different origin from the similar-appearing limb of the Dipnoans,
_Dipterus_ and _Ceratodus_.

In such case the archipterygium would not be the primitive pectoral
limb, but a structure which may have been independently evolved in two
different groups.

In the view of Gegenbaur, the Crossopterygians and Dipnoans with all
the higher vertebrates and the bony fishes would arise from the same
primitive stock, ancestors, or allies of the _Ichthyotomi_, which group
would also furnish the ancestors of the _Chimaeras_. In support of this
view, the primitive protocercal or diphycercal tail of _Pleuracanthus_
may be brought in evidence as against the apparently more specialized
heterocercal tail of _Cladoselache_. But this is not conclusive, as
the diphycercal tail may arise separately in different groups through
degeneration, as Dollo and Boulenger have shown.

The matter is one mainly of morphological interpretation, and no final
answer can be given. On page 68 a summary of the various arguments
may be found. Little light is given by embryology. The evidence of
Palaeontology, so far as it goes, certainly favors the view of Balfour.
Omitting detached fin-spines and fragments of uncertain character, the
earliest identifiable remains of sharks belong to the lower Devonian.
These are allies of _Acanthoessus_. _Cladoselache_ comes next in the
Upper Devonian. _Pleuracanthus_ appears with the teeth and spines
supposed to belong to Cestraciont sharks, in the Carboniferous Age.
The primitive-looking _Notidani_ do not appear before the Triassic.
For this reason the decision as to which is the most primitive type of
shark must therefore rest unsettled for the present and perhaps for a
long time to come.

The weight of authority at present seems to favor the view of Balfour,
Wiedersheim, Boulenger, and Dean, that the pectoral limb has arisen
from a lateral fold of skin. But weight of authority is not sufficient
when evidence is confessedly lacking.

For our purpose, without taking sides in this controversy, we may
follow Dean in allowing _Cladoselache_ to stand as the most primitive
of known sharks, thus arranging the Elasmobranchs and rays, recent and
fossil, in six orders of unequal value--_Pleuropterygii_, _Acanthodei_,
_Ichthyotomi_, _Notidani_, _Asterospondyli_, and _Tectospondyli_. Of
these orders the first and second are closely related, as are also
the fourth and fifth, the sixth being not far remote. The true sharks
form the culmination of one series, the rays of another, while from
the _Ichthyotomi_ the Crossopterygians and their descendants may be
descended. But this again is very hypothetical, or perhaps impossible;
while, on the other hand, the relation of the Chimaeras to the sharks
is still far from clearly understood.

=Order Pleuropterygii.=--The order of _Pleuropterygii_ of Dean
(~pleuron~, side; ~pteryx~, fin), called by Parker and Haswell
_Cladoselachea_, consists of sharks in which the pectoral and ventral
fins have each a very wide horizontal base (ptychopterygium), without
jointed axis and without spine. There are no spines in any of the
fins. The dorsal fin is low, and there were probably two of them.
The notochord is persistent, without intercalary cartilage, such as
appear in the higher sharks. The caudal fin is short, broad, and
strongly heterocercal. Apparently the ventral fin is without claspers.
The gill-openings were probably covered by a dermal fold. The teeth
are weak, being modified denticles from the asperities of the skin.
The lateral line is represented by an open groove. The family of
_Cladoselachidae_ consists of a single genus _Cladoselache_ from the
Cleveland shale or Middle Devonian of Ohio. _Cladoselache fyleri_ is
the best-known species, reaching a length of about two feet. Dean
regards this as the most primitive of the sharks, and the position of
the pectorals and ventrals certainly lend weight to Balfour's theory
that they were originally derived from a lateral fold of skin. I am
recently informed by Dr. Dean that he has considerable evidence that
in _Cladoselache_ the anus was _subterminal_. If this statement is
verified, it would go far to establish the primitive character of
_Cladoselache_.

[Illustration: FIG. 300.--_Cladoselache fyleri_ (Newberry), restored.
Upper Devonian of Ohio. (After Dean.)]

=Order Acanthodei.=--Near the _Pleuropterygii_, although much more
highly developed, we may note the strange group of _Acanthodei_
(~akanthodes~, spinous). These armed fishes were once placed among
the Crossopterygians, but there seems no doubt that Woodward is right
in regarding them as a highly specialized aberrant offshoot of the
primitive sharks. In this group the paired fins consist each of a
single stout spine, nearly or quite destitute of other rays. A similar
spine is placed in front of the dorsal fin and one in front of the
anal. According to Dean these spines are each produced by the growing
together of all the fin-rays normally belonging to the fin, a view of
their morphology not universally accepted.

[Illustration: FIG. 301.--_Cladoselache fyleri_ (Newberry), restored.
Ventral view. (After Dean.)]

[Illustration: FIG. 302.--Teeth of _Cladoselache fyleri_ (Newberry).
(After Dean.)]

[Illustration: FIG. 303.--_Acanthoessus wardi_ (Egerton).
Carboniferous. Family _Acanthoessidae_. (After Woodward.)]

The dermal covering is highly specialized, the shagreen denticles being
much enlarged and thickened, often set in little squares suggesting a
checker-board. The skull is covered with small bony plates and membrane
bones form a sort of ring about the eye. The teeth are few, large, and
"degenerate in their fibrous structure." Some of the species have
certainly no teeth at all. The tail is always heterocercal, or bent
upward at tip as in the _Cladoselache_, not diphycercal, tapering and
horizontal as in the _Ichthyotomi_.

The lower Acanthodeans, according to Woodward, "are the only
vertebrates in which there are any structures in the adult apart
from the two pairs of fins which may be plausibly interpreted as
remnants of once continuous lateral folds. In _Climatius_, one of
the most primitive genera (see Fig. 305), there exists, according
to Woodward, and as first noticed by Cope, between the pectoral and
pelvic (or ventral) fins a close and regular series of paired spines,
in every respect identical with those supporting the appendages
that presumably correspond to the two pairs of fins in the higher
genera. They may even have supported fin membranes, though specimens
sufficiently well preserved to determine this point have not yet been
discovered. However, it is evident that dermal calcifications attained
a greater development in the _Acanthodei_ than in any of the more
typical Elasmobranchs, and we may look for much additional information
on the subject when the great fishes to which the undetermined
_Ichthyodorulites_ pertained became known." (See Fig. 305.)

The _Acanthodei_ constitute three families. In the _Acanthoessidae_
there is but one short dorsal fin opposite the anal, and clavicular
bones are absent. The gill-openings being provided with "frills" or
collar-like margins, perhaps resembled those of the living genus
_Chlamydoselachus_, the frilled shark. The pectoral spine is very
strong, and about the eye is a ring of four plates. The body is
elongate, tapering, and compressed. _Acanthoessus_ of Agassiz, the name
later changed by its author to _Acanthodes_, is the principal genus,
found in the Devonian and Carboniferous.

The species of _Acanthoessus_ are all small fishes rarely more than a
foot long, with very small teeth or none, and with the skin well armed
with a coat-of-mail. _Acanthoessus bronni_ is the one longest known.
In the earliest species known, from the Devonian, the ventral fins are
almost as large as the pectorals and nearly midway between pectorals
and anal. In the later species the pectoral fins become gradually
larger and the ventrals move forward. In the Permian species the
pectorals are enormous.

_Traquairia pygmaea_, from the Permian of Bohemia, is a diminutive
sharklet three or four inches long with large scales, slender spines,
and apparently no ventral fins.

In the genus _Cheiracanthus_ the dorsal fin is placed before the anal.
In _Acanthodopsis_ the teeth are few, large, and triangular, and the
fin-spines relatively large.

The _Ischnacanthidae_ have no clavicles, and two dorsal fins.
_Ischnacanthus gracilis_ of the Devonian has a few large conical teeth
with small cusps between them.

The _Diplacanthidae_, with two dorsal fins, possess bones interpreted as
clavicles. The teeth are minute or absent. In _Diplacanthus striatus_
and _Diplacanthus longispinus_ of the Lower Devonian stout spines are
attached to the shoulder-girdle between the pectoral spines below.

[Illustration: FIG. 304.--_Diplacanthus crassissimus_ Duff. Devonian.
Family _Diplacanthidae_. (After Nicholson). (Restoration of jaws and
gill-openings; after Traquair.)]

In the very small sharks called _Climatius_ the fin-spines are very
strong, and a series of several free spines occurs, as above stated, on
each side between the pectoral and ventral fins, a supposed trace of a
former lateral fold. In _Paraxus_ the first dorsal spine is enormously
enlarged in size, the other spines remaining much as in _Climatius_.

=Dean on Acanthodei.=--In his latest treatise on these fishes,
"The Devonian Lamprey," Dr. Dean unites the _Pleuropterygii_ and
_Acanthodei_ in a single order under the former name, regarding
_Acanthoessus_ as an ally and perhaps descendant of the primitive
_Cladoselache_. Dr. Dean observes:

"In the foregoing classification it will be noted that the
Acanthodia are regarded as included under the first order of sharks,
_Pleuropterygii_. To this arrangement Smith Woodward has already
objected that the spines of Acanthodians cannot be regarded as the
homologues of the radial elements of the Cladoselachian fin (which by
a process of concrescence have become fused in its interior margin),
since he believes the structure to be entirely dermal in origin. His
criticism, however, does not seem to me to be well grounded, for,
although all will admit that Acanthodian spines have become incrusted,
and deeply incrusted, with a purely dermal calcification, it does
not follow that the interior of the spine has not had primitively
a non-dermal core. That the concrescence of the radial supporting
elements of the fin took place _pari passu_ with the development of a
strengthening dermal support of the fin margin was the view expressly
formulated in my previous paper on this subject. It is of interest in
this connection to recall that the earliest types of Acanthodian spines
were the widest, and those which, in spite of their incasing dermal
calcification, suggest most clearly the parallel elements representing
the component radial supports. There should also be recalled the
many features in which the Acanthodians have been shown to resemble
_Cladoselache_."

[Illustration: FIG. 305.--_Climatius scutiger_ Egerton, restored.
Family _Diplacanthidae_. (After Powrie, per Zittel.)]

From these primitive extinct types of shark we may proceed to
those forms which have representatives among living fishes. From
_Cladoselache_ a fairly direct series extends through the _Notidani_
and _Cestraciontes_, culminating in the Lamnoid and Galeoid sharks.

Still another series, destitute of anal fin, probably arising near the
_Acanthodei_, reaches its highest development in the side branch of the
_Batoidei_ or rays. The _Holocephali_ and _Dipneusti_ must also find
their origin in some of these primitive types, certainly not in any
form of more highly specialized sharks.

[Illustration: FIG. 306.--_Pleuracanthus decheni_ Goldfuss. Family
_Pleuracanthidae_. (After Roemer, per Zittel.)]

Woodward prefers to place the _Tectospondyli_ next to the
_Ichthyotomi_, leaving the specialized sharks to be treated later.
There is, however, no linear system which can interpret natural
affinities, and we follow custom in placing the dogfishes and rays at
the end of the shark series.

[Illustration: FIG. 307.--_Pleuracanthus decheni_, restored. (After
Brongniart.) The anterior anal very hypothetical.]

[Illustration: FIG. 308.--Head-bones and teeth of _Pleuracanthus
decheni_ Goldfuss. (After Davis, per Dean.)]

[Illustration: FIG. 309.--Teeth of _Didymodus bohemicus_ Quenstadt.
Carboniferous. Family _Pleuracanthidae_. (After Zittel)]

=Order Ichthyotomi.=--In the order _Ichthyotomi_ (~ichthys~, fish;
~tomos~, cutting; named by Cope from the supposed segmentation of
the cranium; called by Parker and Haswell _Pleuracanthea_) the very
large pectoral fins are developed each as an archipterygium. Each fin
consists of a long segmented axis fringed on one or both sides with
fin-rays. The notochord is very simple, scarcely or never constricted,
the calcifications of its sheath "arrested at the most primitive or
rhachitomous stage, except in the tail." This is the best defined of
the orders of sharks, and should perhaps rank rather as a subclass,
as the _Holocephali_. Two families of _Ichthyotomi_ are recognized
by Woodward, the _Pleuracanthidae_ and the _Cladodontidae_. In the
_Pleuracanthidae_ the dorsal fin is long and low, continuous from head
to tail, and the pectoral rays are in two rows. There is a long barbed
spine with two rows of serrations at the nape. The body is slender,
not depressed, and probably covered with smooth skin. The teeth have
two or more blunt cusps, sometimes with a smaller one between and a
blunt button behind. The interneural cartilages are more numerous than
the neural spines. The genera are imperfectly known, the skeleton of
_Pleuracanthus decheni_ only being well preserved. This is the type
of the genus called _Xenacanthus_ which, according to Woodward, is
identical with _Pleuracanthus_, a genus otherwise known from spines
only. The denticles on the spine are straight or hooked backward,
in _Pleuracanthus_ (_laevissimus_), the spine being flattened. In
_Orthacanthus_ (_cylindricus_), the spine is cylindrical in section.
The species called _Dittodus_ and _Didymodus_ are known from the
teeth only. These resemble the teeth of _Chlamydoselachus_. It is not
known that _Dittodus_ possesses the nuchal spine, although detached
spines like those of _Pleuracanthus_ lie about in remains called
_Didymodus_ in the Permian rocks of Texas. In _Dicranodus texensis_
the palato-quadrate articulates with the postorbital process of the
cranium, as in the _Hexanchidae_, and the hyomandibular is slender.

[Illustration: FIG. 310.--Shoulder-girdle and pectoral fins of
_Cladodus neilsoni_ Traquair.]

A genus, _Chondrenchelys_, from the sub-Carboniferous of Scotland, is
supposed to belong to the _Pleuracanthidae_, from the resemblance of
the skeleton. It has no nuchal spine, and no trace of paired fins is
preserved.

The _Cladodontidae_ differ in having the "pectoral fin developed in
the form of a uniserial archipterygium intermediate between the truly
biserial one of _Pleuracanthus_ and the pectoral fin of modern sharks."
The numerous species are known mainly from detached teeth, especially
abundant in America, the earliest being in the Lower Carboniferous. One
species, _Cladodus nelsoni_ (Fig. 310), described by Traquair, from
the sub-Carboniferous of Scotland shows fairly the structure of the
pectoral fin.

[Illustration: FIG. 311.--Teeth of _Cladodus striatus_ Agassiz. (After
Davis.) Carboniferous.]

In _Cladodus mirabilis_ the teeth are very robust, the crown consisting
of a median principal cone and two or three large lateral cones on each
side. The cones are fairly striate. In _Lambdodus_ from Illinois there
are no lateral cones. Other genera are _Dicentrodus_, _Phoebodus_,
_Carcharopsis_, and _Hybocladodus_.




CHAPTER XXX

THE TRUE SHARKS


=Order Notidani.=--We may recognize as a distinct order, a primitive
group of recent sharks, a group of forms finding its natural place
somewhere between the _Cladoselachidae_ and _Heterodontidae_, both of
which groups long preceded it in geological time.

The name _Notidani_ (_Notidanus_, ~notidanos~, dry back, an old name of
one of the genera) may be retained for this group, which corresponds
to the _Diplospondyli_ of Hasse, the _Opistharthri_ of Gill, and the
_Protoselachii_ of Parker and Haswell. The _Notidani_ are characterized
by the primitive structure of the spinal column, which is without
calcareous matter, the centra being imperfectly developed. There
are six or seven branchial arches, and in the typical forms (not in
_Chlamydoselachus_) the palato-quadrate or upper jaw articulates
with the postorbital region of the skull. The teeth are of primitive
character, of different forms in the same jaw, each with many cusps.
The fins are without spines, the pectoral fin having the three basal
cartilages (mesopterygium with propterygium and metapterygium) as usual
among sharks.

[Illustration: FIG. 312.--Griset or Cow-shark, _Hexanchus griseus_
(Gmelin). Currituck Inlet, N. C.]

The few living forms are of high interest. The extinct species are
numerous, but not very different from the living species.

=Family Hexanchidae.=--The majority of the living Notidanoid sharks
belong to the family of _Hexanchidae_. These sharks have six or seven
gill-openings, one dorsal fin, and a relatively simple organization.
The bodies are moderately elongate, not eel-shaped, and the
palato-quadrate articulates with the postorbital part of the skull.
The six or eight species are found sparsely in the warm seas. The two
genera, _Hexanchus_, with six, and _Heptranchias_, with seven vertebrae,
are found in the Mediterranean. The European species are _Hexanchus
griseus_, the cow-shark, and _Heptranchias cinereus_. The former
crosses to the West Indies. In California, _Heptranchias maculatus_
and _Hexanchus corinus_ are occasionally taken, while _Heptranchias
deani_ is the well known Aburazame or oil shark of Japan. _Heptranchias
indicus_, a similar species, is found in India.

[Illustration: FIG. 313.--Teeth of _Heptranchias indicus_ Gmelin.]

Fossil _Hexanchidae_ exist in large numbers, all of them referred
by Woodward to the genus _Notidanus_ (which is a later name than
_Hexanchus_ and _Heptranchias_ and intended to include both these
genera), differing chiefly in the number of gill-openings, a character
not ascertainable in the fossils. None of these, however, appear before
Cretaceous time, a fact which may indicate that the simplicity of
structure in _Hexanchus_ and _Heptranchias_ is a result of degeneration
and not altogether a mark of primitive simplicity. The group is
apparently much younger than the Cestraciontes and little older than
the Lamnoids, or the Squaloid groups. _Heptranchias microdon_ is common
in English Cretaceous rocks, and _Heptranchias primigenius_ and other
species are found in the Eocene.

=Family Chlamydoselachidae.=--Very great interest is attached
to the recent discovery by Samuel Garman of the frilled shark,
_Chlamydoselachus anguineus_, the sole living representative of the
_Chlamydoselachidae_.

[Illustration: FIG. 314.--Frill-shark, _Chlamydoselachus anguineus_
Garman. From Misaki, Japan. (After Guenther.)]

This shark was first found on the coast of Japan, where it is rather
common in deep water. It has since been taken off Madeira and off
the coast of Norway. It is a long, slender, eel-shaped shark with
six gill-openings and the palato-quadrate not articulated to the
cranium. The notochord is mainly persistent, in part replaced
by feeble cyclospondylic vertebral centra. Each gill-opening is
bordered by a broad frill of skin. There is but one dorsal fin. The
teeth closely resemble those of _Dittodus_ or _Didymodus_ and other
extinct _Ichthyotomi_. The teeth have broad, backwardly extended
bases overlapping, the crown consisting of three slender curved
cusps, separated by rudimentary denticles. Teeth of a fossil species,
_Chlamydoselachus lawleyi_, are recorded by J. W. Davis from the
Pliocene of Tuscany.

=Order Asterospondyli.=--The order of _Asterospondyli_ comprises
the typical sharks, those in which the individual vertebrae are
well developed, the calcareous lamellae arranged so as to radiate,
star-fashion, from the central axis. All these sharks possess two
dorsal fins and one anal fin, the pectoral fin is normally developed,
with the three basal cartilages; there are five gill-openings, and the
tail is heterocercal.

[Illustration: FIG. 315.--Bullhead-shark, _Heterodontus francisci_
(Girard). San Pedro, Cal.]

=Suborder Cestraciontes.=--The most ancient types may be set off as a
distinct suborder under the name of _Cestraciontes_ or _Prosarthri_.

[Illustration: FIG. 316.--Lower jaw of _Heterodontus philippi_. From
Australia. Family _Heterodontidae_. (After Zittel.)]

These forms find their nearest allies in the _Notidani_, which they
resemble to some extent in dentition and in having the palato-quadrate
articulated to the skull although fastened farther forward than in the
_Notidani_. Each of the two dorsal fins has a strong spine.

[Illustration: FIG. 317.--Teeth of Cestraciont Sharks. (After
Woodward.) _d_, _Synechodus dubrisianus_ Mackie; _e_, _Heterodontus
canaliculatus_ Egerton; _f_, _Hybodus striatulus_ Agassiz. (After
Woodward.)]

[Illustration: FIG. 318.--Egg of Port Jackson Shark, _Heterodontus
philippi_ (Lacepede). (After Parker & Haswell.)]

=Family Heterodontidae.=--Among recent species this group contains only
the family of _Heterodontidae_, the bullhead sharks, or Port Jackson
sharks. In this family the head is high, with usually projecting
eyebrows, the lateral teeth are pad-like, ridged or rounded, arranged
in many rows, different from the pointed anterior teeth, the fins are
large, the coloration is strongly marked, and the large egg-cases are
spirally twisted. All have five gill-openings. The living species
of _Heterodontidae_ are found only in the Pacific, the Port Jackson
shark of Australia, _Heterodontus philippi_, being longest known.
Other species are _Heterodontus francisci_, common in California,
_Heterodontus japonicus_, in Japan, and _Heterodontus zebra_, in China.
These small and harmless sharks at once attract attention by their
peculiar forms. In the American species the jaws are less contracted
than in the Asiatic species, called _Heterodontus_. For this reason Dr.
Gill has separated the former under the name of _Gyropleurodus_. The
differences are, however, of slight value. The genus _Heterodontus_
first appears in the Jurassic, where a number of species are known, one
of the earliest being _Heterodontus falcifer_.

Three families of _Cestraciontes_ are recognized by Hay. The most
primitive of these is the group of _Orodontidae_. _Orodus_, from the
Lower Carboniferous, has the teeth with a central crown, its surface
wrinkled. Of the _Heterodontidae_, _Hybodus_, of the Carboniferous and
Triassic, is one of the earliest and largest genera, characterized
by elongate teeth of many cusps, different in different parts of
the jaw, somewhat as in the _Hexanchidae_, the median points being,
however, always longest. The dorsal fins are provided with long spines
serrated behind. The vertebrae with persistent notochord show qualities
intermediate between those of _Hexanchidae_ and _Heterodontidae_, and the
same relation is shown by the teeth. In this genus two large hooked
half-barbed dermal spines occur behind each orbit.

[Illustration: FIG. 319.--Tooth of _Hybodus delabechei_ Charlesworth.
(After Woodward.)]

[Illustration: FIG. 320.--Fin-spine of _Hybodus basanus_ Egerton.
Cretaceous. Family _Heterodontidae_. (After Nicholson.)]

[Illustration: FIG. 321.--Fin-spine of _Hybodus reticulatus_ Agassiz.
(After Zittel.)]

_Palaeospinax_, with short stout spines and very large pectoral fins,
formerly regarded as a dogfish, is placed near _Heterodontus_ by
Woodward. _Acrodus_, from the Triassic, shows considerable resemblance
to _Heterodontus_. Its teeth are rounded and without cusps.

Most of these species belong to the Carboniferous, Triassic, and
Jurassic, although some fragments ascribed to Cestraciont sharks occur
in the Upper Silurian. _Asteracanthus_, known only from fin-spines in
the Jura, probably belongs here.

It is a singular fact first noted by Dr. Hay, that with all the
great variety of sharks, ten families in the Carboniferous age,
representatives of but one family, _Heterodontidae_, are found in the
Triassic. This family may be the parent of all subsequent sharks and
rays, six families of these appearing in the Jurassic and many more in
the Cretaceous.

=Edestus and its Allies.=--Certain monstrous structures, hitherto
thought to be fin-spines, are now shown by Dr. Eastman and others to be
coalescent teeth of Cestraciont sharks.

[Illustration: Fig. 322.--Fin-spine of _Hybodus canaliculatus_ Agassiz.]

[Illustration: Fig. 323.--Teeth of Cestraciont Sharks. (After
Woodward.) _a_, _Hybodus laevis_ Woodward (after Woodward); _b_,
_Heterodontus rugosus_ Agassiz; _c_, _Hybodus delabechei_ Charlesworth.]

[Illustration: Fig. 324.--_Edestus vorax_ Leidig, supposed to be a
whorl of teeth. (After Newberry.)]

These remarkable _Ichthyodorulites_ are characteristic structures of
sharks of unknown nature, but probably related to the _Heterodontidae_.
Of these the principal genera are _Edestus_, _Helicoprion_, and
_Campyloprion_. Karpinsky regards these ornate serrated spiral
structures as whorls of unshed teeth cemented together and extending
outside the mouth, "sharp, piercing teeth which were never shed but
became fused in whorls as the animals grew."

Dr. Eastman has, however, shown that these supposed teeth of _Edestus_
are much like those of the _Cochliodontidae_, and the animals which bore
them should doubtless find their place among the Cestraciont sharks,
perhaps within the family of _Heterodontidae_.

[Illustration: FIG. 325.--_Helicoprion bessonowi_ Karpinsky. Teeth from
the Permian of Krasnoufimsk, Russia. (After Karpinsky.)]

=Onchus.=--The name _Onchus_ was applied by Agassiz to small laterally
compressed spines, their sides ornamented with smooth or faintly
crenulated longitudinal ridges, and with no denticles behind.
Very likely these belonged to extinct Cestraciont sharks. _Onchus
murchisoni_ and _Onchus tenuistriatus_ occur in the Upper Silurian
rocks of England, in the lowest strata in which sharks have been found.

To a hypothetical group of primitive sharks Dr. Hasse has given the
name of _Polyospondyli_. In these supposed ancestral sharks the
vertebrae were without any ossification, a simple notochord, possibly
swollen at intervals. The dorsal fin was single and long, a fold of
skin with perhaps a single spine as an anterior support. The teeth
must have been modified dermal papillae, each probably with many
cusps. Probably seven gill-openings were developed, and the tail was
diphycercal, ending in a straight point. The finely striated fin-spines
not curved upward at tip, called _Onchus_ from the Upper Silurian of
the Ludlow shales of England and elsewhere, are placed by Hasse near
his Polyspondylous sharks. Such spines have been retained by the group
of _Chimaeras_, supposed to be derived from the ancestors of _Onchus_,
as well as by the _Heterodontidae_ and _Squalidae_.

=Family Cochliodontidae.=--Another ancient family known from teeth
alone is that of _Cochliodontidae_. These teeth resemble those of
the _Heterodontidae_, but are more highly specialized. The form of
the body is unknown, and the animals may have been rays rather than
sharks. Eastman leaves them near the _Petalodontidae_, which group
of supposed rays shows a similar dentition. The teeth are convex in
form, strongly arched, hollowed at base, and often marked by ridges or
folds, being without sharp cusps. In each jaw is a strong posterior
tooth with smaller teeth about. The elaborate specialization of these
ancient teeth for crushing or grinding shells is very remarkable.
The species are chiefly confined to rocks of the Carboniferous age.
Among the principal genera are _Helodus_, _Psephodus_, _Sandalodus_,
_Venustodus_, _Xystrodus_, _Deltodus_, _Poecilodus_, and _Cochliodus_.

[Illustration: FIG. 326.--Lower jaw of _Cochliodus contortus_ Agassiz.
Carboniferous. (After Zittel.)]

Concerning the teeth of various fossil sharks, Dr. Dean observes:
"Their general character appears to have been primitive, but in
structural details they were certainly specialized. Thus their
dentition had become adapted to a shellfish diet, and they had evolved
defensive spines at the fin margins, sometimes at the sides of the
head. In some cases the teeth remain as primitive shagreen cusps on
the rim of the mouth, but become heavy and bluntish behind; in other
forms the fusion of tooth clusters may present the widest range in
their adaptations for crushing; and the curves and twistings of the
tritoral surfaces may have resulted in the most specialized forms of
dentition which are known to occur, not merely in sharks but among all
vertebrates."

In this neighborhood belongs, perhaps, the family of _Tamiobatidae_,
known from the skull of a single specimen, called _Tamiobatis
vetustus_, from the Devonian in eastern Kentucky. The head has the
depressed form of a ray, but it is probably a shark and one of the very
earliest known.

=Suborder Galei.=--The great body of recent sharks belong to
the suborder _Galei_, or _Euselachii_, characterized by the
asterospondylous vertebrae, each having a star-shaped nucleus, and
by the fact that the palato-quadrate apparatus or upper jaw is not
articulated with the skull. The sharks of this suborder are the most
highly specialized of the group, the strongest and largest and, in
general, the most active and voracious. They are of three types
and naturally group themselves about the three central families
_Scyliorhinidae_, _Lamnidae_, and _Carchariidae_ (_Galeorhinidae_).

The _Asterospondyli_ are less ancient than the preceding groups, but
the modern families were well differentiated in Mesozoic times.

Among the _Galei_ the dentition is less complex than with the ancient
forms, although the individual teeth are more highly specialized. The
teeth are usually adapted for biting, often with knife-like or serrated
edges; only the outer teeth are in function; as they are gradually
lost, the inner teeth are moved outward, gradually taking the place of
these.

We may place first, as most primitive, the forms without nictitating
membrane.

=Family Scyliorhinidae.=--The most primitive of the modern families
is doubtless that of the _Scyliorhinidae_, or cat-sharks. This group
includes sharks with the dorsal fins both behind the ventrals, the tail
not keeled and not bent upward, the spiracles present, and the teeth
small and close-set. The species are small and mostly spotted, found
in the warm seas. All of them lay their eggs in large cases, oblong,
and with long filaments or strings at the corners. The cat-sharks, or
roussettes, _Scyliorhinus canicula_ and _Catulus stellaris_, abound
in the Mediterranean. Their skin is used as shagreen or sandpaper in
polishing furniture. The species of swell-sharks (_Cephaloscylium_)
(_C. uter_, in California; _C. ventriosus_, in Chile; _C. laticeps_,
in Australia; _C. umbratile_, in Japan) are short, wide-bodied sharks,
which have the habit of filling the capacious stomach with air, then
floating belly upward like a globefish. Other species are found in the
depths of the sea. _Scyliorhinus_, _Catulus_, and numerous other genera
are found fossil. The earliest is _Palaeoscyllium_, in the Jurassic, not
very different from _Scyliorhinus_, but the fins are described as more
nearly like those of _Ginglymostoma_.

Close to the _Scyliorhinidae_ is the Asiatic family, _Hemiscylliidae_,
which differs in being ovoviviparous, the young, according to Mr.
Edgar R. Waite, hatched within the body. The general appearance is
that of the _Scyliorhinidae_, the body being elongate. _Chiloscyllium_
is a well-known genus with several species in the East Indies.
_Chiloscyllium modestum_ is the dogfish of the Australian fishermen.
The _Orectolobidae_ are thick-set sharks, with large heads provided with
fleshy fringes. _Orectolobus barbatus_ (_Crossorhinus_ of authors)
abounds from Japan to Australia.

Another family, _Ginglymostomidae_, differs mainly in the form of
the tail, which is long and bent abruptly upward at its base. These
large sharks, known as nurse-sharks, are found in the warm seas.
_Ginglymostoma cirrhatum_ is the common species with _Orectolobus_.
_Stegostoma tigrinum_, of the Indian seas and north to Japan, one of
several genera called tiger-sharks, is remarkable for its handsome
spotted coloration. The extinct genus _Pseudogaleus_ (_voltai_) is said
to connect the _Scyliorhinoid_ with the _Carcharioid_ sharks.

=The Lamnoid or Mackerel Sharks.=--The most active and most ferocious
of the sharks, as well as the largest and some of the most sluggish,
belong to a group of families known collectively as Lamnoid, because
of a general resemblance to the mackerel-shark, or _Lamna_, as
distinguished from the blue sharks and white sharks allied to
_Carcharias_ (_Carcharhinus_).

The Lamnoid sharks agree with the cat-sharks in the absence of
nictitating membrane or third eyelid, but differ in the anterior
insertion of the first dorsal fin, which is before the ventrals. Some
of these sharks have the most highly specialized teeth to be found
among fishes, most effective as knives or as scissors. Still others
have the most highly specialized tails, either long and flail-like, or
short, broad, and muscular, fitting the animal for swifter progression
than is possible for any other sharks. The Lamnoid families are
especially numerous as fossils, their teeth abounding in all suitable
rock deposits from Mesozoic times till now. Among the Lamnoid sharks
numerous families must be recognized.

The most primitive is perhaps that of the _Odontaspididae_ (called
_Carchariidae_ by some recent authors), now chiefly extinct, with the
tail unequal and not keeled, and the teeth slender and sharp, often
with smaller cusps at their base. _Odontaspis_ and its relatives of the
same genus are numerous, from the Cretaceous onward, and three species
are still extant, small sharks of a voracious habit, living on sandy
shores. _Odontaspis littoralis_ (also known as _Carcharias littoralis_)
is the common sand-shark of our Atlantic coast. _Odontaspis taurus_ is
a similar form in the Mediterranean.

=Family Mitsukurinidae, the Goblin-sharks.=--Closely allied to
_Odontaspis_ is the small family of _Mitsukurinidae_, of which a single
living species is known. The teeth are like those of _Odontaspis_, but
the appearance is very different.

The goblin-shark, or Tenguzame, _Mitsukurina owstoni_, is a very large
shark rarely taken in the Kuro Shiwo, or warm "Black Current" of Japan.
It is characterized by the development of the snout into a long flat
blade, extending far beyond the mouth, much as in _Polyodon_ and in
certain Chimaeras. Several specimens are now known, all taken by Capt.
Alan Owston of Yokohama in Sagami Bay, Japan. The original specimen,
a young shark just born, was presented by him to Professor Kakichi
Mitsukuri of the University of Tokyo. From this our figure was taken.
The largest specimen now known is in the United States National Museum
and is fourteen feet in length. In the Upper Cretaceous is a very
similar genus, _Scapanorhynchus_ (_lewisi_, etc.), which Professor
Woodward thinks may be even generically identical with _Mitsukurina_,
though there is considerable difference in the form of the still longer
rostral plate, and the species of _Scapanorhynchus_ differ among
themselves in this regard.

[Illustration: FIG. 327.--Goblin-shark (Tenguzame), _Mitsukurina
owstoni_ Jordan. From a young specimen in the Imperial University of
Tokyo.]

_Mitsukurina_, with _Heterodontus_, _Heptranchias_, and
_Chlamydoselache_, is a very remarkable survival of a very ancient
form. It is an interesting fact that the center of abundance of all
these relics of ancient life is in the Black Current, or Gulf Stream,
of Japan.

[Illustration: FIG. 328.--_Scapanorhynchus lewisi_ Davis. Family
_Mitsukurinidae_. Under side of snout. (After Woodward.)]

=Family Alopiidae, or Thresher Sharks.=--The related family of
_Alopiidae_ contains probably but one recent species, the great
fox-shark, or thresher, found in all warm seas. In this species,
_Alopias vulpes_, the tail is as long as the rest of the body and bent
upward from the base. The snout is very short, and the teeth are small
and close-set. The species reaches a length of about twenty-five feet.
It is not especially ferocious, and the current stories of its attacks
on whales probably arise from a mistake of the observers, who have
taken the great killer, _Orca_, for a shark. The killer is a mammal,
allied to the porpoise. It attacks the whale with great ferocity,
clinging to its flesh by its strong teeth. The whale rolls over and
over, throwing the killer into the air, and sailors report it as a
thresher. As a matter of fact the thresher very rarely if ever attacks
any animal except small fish. It is said to use its tail in rounding
up and destroying schools of herring and sardines. Fossil teeth of
thresher-sharks of some species are found from the Miocene.

=Family Pseudotriakidae.=--The _Pseudotriakidae_ consist of two species.
One of these is _Pseudotriakis microdon_, a large shark with a long
low tail, long and low dorsal fin, and small teeth. It has been only
twice taken, off Portugal and off Long Island. The other, the mute
shark, _Pseudotriakis acrales_, a large shark with the body as soft as
a rag, is in the museum of Stanford University, having been taken by
Mr. Owston off Misaki.

=Family Lamnidae.=--To the family of _Lamnidae_ proper belong the
swiftest, strongest, and most voracious of all sharks. The chief
distinction lies in the lunate tail, which has a keel on either side
at base, as in the mackerels. This form is especially favorable
for swift swimming, and it has been independently developed in the
mackerel-sharks, as in the mackerels, in the interest of speed in
movement.

[Illustration: FIG. 329.--Tooth of _Lamna cuspidata_ Agassiz.
Oligocene. Family _Lamnidae_. (After Nicholson.)]

The porbeagle, _Lamna cornubica_, known as salmon-shark in Alaska,
has long been noted for its murderous voracity. About Kadiak Island
it destroys schools of salmon, and along the coasts of Japan, and
especially of Europe and across to New England, it makes its evil
presence felt among the fishermen. Numerous fossil species of _Lamna_
occur, known by the long knife-like flexuous teeth, each having one or
two small cusps at its base.

[Illustration: FIG. 330.--Mackerel-shark, _Isuropsis dekayi_ Gill.
Pensacola, Fla.]

In the closely related genus, _Isurus_, the mackerel-sharks, this cusp
is wanting, while in _Isuropsis_ the dorsal fin is set farther back.
In each of these genera the species reach a length of 20 to 25 feet.
Each is strong, swift, and voracious. _Isurus oxyrhynchus_ occurs
in the Mediterranean, _Isuropsis dekayi_, in the Gulf of Mexico, and
_Isuropsis glauca_, from Hawaii and Japan westward to the Red Sea.

=Man-eating Sharks.=--Equally swift and vastly stronger than these
mackerel-sharks is the man-eater, or great white shark, _Carcharodon
carcharias_. This shark, found occasionally in all warm seas, reaches a
length of over thirty feet and has been known to devour men. According
to Linnaeus, it is the animal which swallowed the prophet Jonah. "Jonam
Prophetum," he observes, "ut veteris Herculem trinoctem, in hujus
ventriculo tridui spateo baesisse, verosimile est."

[Illustration: FIG. 331.--Tooth of _Isurus hastalis_ (Agassiz).
Miocene. Family _Lamnidae_. (After Nicholson.)]

It is beyond comparison the most voracious of fish-like animals. Near
Soquel, California, the writer obtained a specimen in 1880, with a
young sea-lion (_Zalophus_) in its stomach. It has been taken on the
coasts of Europe, New England, Carolina, California, Hawaii, and Japan,
its distribution evidently girdling the globe. The genus _Carcharodon_
is known at once by its broad, evenly triangular, knife-like teeth,
with finely serrated edges, and without notch or cusp of any kind. But
one species is now living. Fossil teeth are found from the Eocene. One
of these, _Carcharodon megalodon_ (Fig. 332), from fish-guano deposits
in South Carolina and elsewhere, has teeth nearly six inches long. The
animal could not have been less than ninety feet in length. These huge
sharks can be but recently extinct, as their teeth have been dredged
from the sea-bottom by the _Challenger_ in the mid-Pacific.

Fossil teeth of _Lamna_ and _Isurus_ as well as of _Carcharodon_ are
found in great abundance in Cretaceous and Tertiary rocks. Among the
earlier species are forms which connect these genera very closely.

The fossil genus _Otodus_ must belong to the _Lamnidae_. Its massive
teeth with entire edges and blunt cusps at base are common in
Cretaceous and Tertiary deposits. The teeth are formed much as
in _Lamna_, but are blunter, heavier, and much less effective as
instruments of destruction. The extinct genus _Corax_ is also placed
here by Woodward.

[Illustration: FIG. 332.--_Carcharodon megalodon_ Charlesworth.
Miocene. Family _Lamnidae_. (After Zittel.)]

=Family Cetorhinidae, or Basking Sharks.=--The largest of all living
sharks is the great basking shark (_Cetorhinus maximus_), constituting
the family of _Cetorhinidae_. This is the largest of all fishes,
reaching a length of thirty-six feet and an enormous weight. It is a
dull and sluggish animal of the northern seas, almost as inert as a
sawlog, often floating slowly southward in pairs in the spring and
caught occasionally by whalers for its liver. When caught, its huge
flabby head spreads out wide on the ground, its weight in connection
with the great size of the mouth-cavity rendering it shapeless.
Although so clumsy and without spirit, it is said that a blow with its
tail will crush an ordinary whaleboat. The basking shark is known on
all northern coasts, but has most frequently been taken in the North
Sea, and about Monterey Bay in California. From this locality specimens
have been sent to the chief museums of Europe. In its external
characters the basking shark has much in common with the man-eater.
Its body is, however, relatively clumsy forward; its fins are lower,
and its gill-openings are much broader, almost meeting under the
throat. The great difference lies in the teeth, which in _Cetorhinus_
are very small and weak, about 200 in each row. The basking shark,
also called elephant-shark and bone-shark, does not pursue its prey,
but feeds on small creatures to be taken without effort. Fossil teeth
of _Cetorhinus_ have been found from the Cretaceous, as also fossil
gill-rakers, structures which in this shark are so long as to suggest
whalebone.

[Illustration: FIG. 333.--Basking Shark, _Cetorhinus maximus_ (Gunner).
France.]

=Family Rhineodontidae.=--The whale-sharks, _Rhineodontidae_, are
likewise sluggish monsters with feeble teeth and keeled tails. From
_Cetorhinus_ they differ mainly in having the last gill-opening
above the pectorals. There is probably but one species, _Rhineodon
typicus_, of the tropical Pacific, straying northward to Florida, Lower
California, and Japan.

=The Carcharioid Sharks, or Requins.=--The largest family of recent
sharks is that of _Carchariidae_ (often called _Galeorhinidae_, or
_Galeidae_), a modern offshoot from the Lamnoid type, and especially
characterized by the presence of a third eyelid, the nictitating
membrane, which can be drawn across the eye from below. The
heterocercal tail has no keel; the end is bent upward; both dorsal
fins are present, and the first is well in front of the ventral fins;
the last gill-opening over the base of the pectoral, the head normally
formed; these sharks are ovoviviparous, the young being hatched in a
sort of uterus, with or without placental attachment.

Some of these sharks are small, blunt-toothed, and innocuous. Others
reach a very large size and are surpassed in voracity only by the
various _Lamnidae_.

The genera _Cynias_ and _Mustelus_, comprising the soft-mouthed or
hound-sharks, have the teeth flat and paved, while well-developed
spiracles are present. These small, harmless sharks abound on almost
all coasts in warm regions, and are largely used as food by those
who do not object to the harsh odor of shark's flesh. The best-known
species is _Cynias canis_ of the Atlantic. By a regular gradation of
intermediate forms, through such genera as _Rhinotriacis_ and _Triakis_
with tricuspid teeth, we reach the large sharp-toothed members of
this family. _Galeus_ (or _Galeorhinus_) includes large sharks having
spiracles, no pit at the root of the tail, and with large, coarsely
serrated teeth. One species, the soup-fin shark (_Galeus zyopterus_),
is found on the coast of California, where its fins are highly valued
by the Chinese, selling at from one to two dollars for each set. The
delicate fin-rays are the part used, these dissolving into a finely
flavored gelatine. The liver of this and other species is used in
making a coarse oil, like that taken from the dogfish. Other species
of _Galeus_ are found in other regions, _Galeus galeus_ being known in
England as tope, _Galeus japonicus_ abounding in Japan.

[Illustration: FIG. 334.--Soup-fin Shark, _Galeus zyopterus_ (Jordan &
Gilbert). Monterey.]

_Galeocerdo_ differs mainly in having a pit at the root of the tail.
Its species, large, voracious, and tiger-spotted, are found in warm
seas and known as tiger-sharks (_Galeocerdo maculatus_ in the Atlantic,
_Galeocerdo tigrinus_ in the Pacific).

The species of _Carcharias_ (_Carcharhinus_ of Blainville) lack the
spiracles. These species are very numerous, voracious, armed with sharp
teeth, broad or narrow, and finely serrated on both edges. Some of
these sharks reach a length of thirty feet. They are very destructive
to other fishes, and often to fishery apparatus as well. They are
sometimes sought as food, more often for the oil in their livers, but,
as a rule, they are rarely caught except as a measure for getting
rid of them. Of the many species the best known is the broad-headed
_Carcharias lamia_, or cub-shark, of the Atlantic. This the writer has
taken with a great hook and chain from the wharves at Key West. These
great sharks swim about harbors in the tropics, acting as scavengers
and occasionally seizing arm or leg of those who venture within their
reach. One species (_Carcharias nicaraguensis_) is found in Lake
Nicaragua, the only fresh-water shark known, although some run up the
brackish mouth of the Ganges and into Lake Pontchartrain. _Carcharias
japonicus_ abounds in Japan.

[Illustration: FIG. 335.--Cub-shark, _Carcharias lamia_ Rafinesque.
Florida.]

A closely related genus is _Prionace_, its species _Prionace glauca_,
the great blue shark, being slender and swift, with the dorsal
farther back than in _Carcharias_. Of the remaining genera the most
important is _Scoliodon_, small sharks with oblique teeth which have
no serrature. One of these, _Scoliodon terrae-novae_, is the common
sharp-nosed shark of our Carolina coast. Fossil teeth representing
nearly all of these genera are common in Tertiary rocks.

Probably allied to the _Carchariidae_ is the genus _Corax_, containing
large extinct sharks of the Cretaceous with broadtriangular serrate
teeth, very massive in substance, and without denticles. As only the
teeth are known, the actual relations of the several species of _Corax_
are not certainly known, and they may belong to the _Lamnidae_.

[Illustration: FIG. 336.--Teeth of _Corax pristodontus_.]

=Family Sphyrnidae, or Hammer-head Sharks.=--The _Sphyrnidae_, or
hammer-headed sharks, are exactly like the _Carchariidae_ except that
the sides of the head are produced, so as to give it the shape of a
hammer or of a kidney, the eye being on the produced outer edge. The
species are few, but mostly widely distributed; rather large, voracious
sharks with small sharp teeth.

The true hammer-head, _Sphyrna zygaena_, Fig. 337, is common from the
Mediterranean to Cape Cod, California, Hawaii, and Japan. The singular
form of its head is one of the most extraordinary modifications
shown among fishes. The bonnet-head (_Sphyrna tiburo_) has the head
kidney-shaped or crescent-shaped. It is a smaller fish, but much the
same in distribution and habits. Intermediate forms occur, so that
with all the actual differences we must place the _Sphyrnidae_ all in
one genus. Fossil hammer-heads occur in the Miocene, but their teeth
are scarcely different from those of _Carcharias_. _Sphyrna prisca_,
described by Agassiz, is the primeval species.

=The Order of Tectospondyli.=--The sharks and rays having no anal
fin and with the calcareous lamellae arranged in one or more rings
around a central axis constitute a natural group to which, following
Woodward, we may apply the name of _Tectospondyli_. The _Cyclospondyli_
(_Squalidae_, etc.) with one ring only of calcareous lamellae may be
included in this order, as also the rays, which have tectospondylous
vertebrae and differ from the sharks as a group only in having the
gill-openings relegated to the lower side by the expansion of the
pectoral fins. The group of rays and Hasse's order of _Cyclospondyli_
we may consider each as a suborder of _Tectospondyli_. The origin of
this group is probably to be found in or near the _Cestraciontes_,
as the strong dorsal spines of the _Squalidae_ resemble those of the
_Heterodontidae_.

[Illustration: FIG. 337.--Hammer-head Shark, _Sphyrna zygaena_ L.
Hindustan. (After Day.)]

=Suborder Cyclospondyli.=--In this group the vertebrae have the
calcareous lamellae arranged in a single ring about the central axis.
The anal fin, as in all the tectospondylous sharks and rays, is
wanting. In all the asterospondylous sharks, as in the _Ichthyotomi_,
_Acanthodei_, and _Chimaeras_, this fin is present. It is present in
almost all of the bony fishes. All the species have spiracles, and
in all are two dorsal fins. None have the nictitating membrane, and
in all the eggs are hatched internally. Within the group there is
considerable variety of form and structure. As above stated, we have
a perfect gradation among _Tectospondyli_ from true sharks, with the
gill-openings lateral, to rays, which have the gill-opening on the
ventral side, the great expansion of the pectoral fins, a character of
relatively recent acquisition, having crowded the gill-openings from
their usual position.

=Family Squalidae.=--The largest and most primitive family of
_Cyclospondyli_ is that of the _Squalidae_, collectively known as
dogfishes or skittle-dogs. In the _Squalidae_ each dorsal fin has a
stout spine in front, the caudal is bent upward and not keeled, and the
teeth are small and varied in form, usually not all alike in the same
jaw.

[Illustration: FIG. 338.--Dogfish, _Squalus acanthias_ L. Gloucester,
Mass.]

The genus _Squalus_ includes the dogfishes, small, greedy sharks
abundant in almost all cool seas and in some tropical waters. They
are known by the stout spines in the dorsal fins and by their sharp,
squarish cutting teeth. They are largely sought by fishermen for
the oil in their livers, which is used to adulterate better oils.
Sometimes 20,000 have been taken in one haul of the net. They are very
destructive to herrings and other food-fishes. Usually the fishermen
cut out the liver, throwing the shark overboard to die or to be cast
on the beach. In northern Europe and New England _Squalus acanthias_
is abundant. _Squalus sucklii_ replaces it in the waters about Puget
Sound, and _Squalus mitsukurii_ in Japan and Hawaii. Still others
are found in Chile and Australia. The species of _Squalus_ live near
shore and have the gray color usual among sharks. Allied forms perhaps
hardly different from _Squalus_ are found in the Cretaceous rocks and
have been described as _Centrophoroides_. Other genera related to
_Squalus_ live in greater depths, from 100 to 600 fathoms, and these
are violet-black. Some of the deep-water forms are the smallest of
all sharks, scarcely exceeding a foot in length. _Etmopterus spinax_
lives in the Mediterranean, and teeth of a similar species occur in the
Italian Pliocene rocks. _Etmopterus lucifer_,[150] a deep-water species
of Japan, has a brilliant luminous glandular area along the sides of
the belly. Other small species of deeper waters belong to the genera
_Centrophorus_, _Centroscymnus_, and _Deania_. In some of these species
the scales are highly specialized, pedunculate, or having the form of
serrated leaves. Some species are Arctic, the others are most abundant
about Misaki in Japan and the Madeira Islands, two regions especially
rich in semi-bathybial types. Allied to the _Squalidae_ is the small
family of _Oxynotidae_ with short bodies and strong dorsal spine.
_Oxynotus centrina_ is found in the Mediterranean, and its teeth occur
in the Miocene.

[Illustration: FIG. 339.--_Etmopterus lucifer_ Jordan & Snyder. Misaki,
Japan.]

=Family Dalatiidae.=--The _Dalatiidae_, or scymnoid sharks, differ from
the _Squalidae_ almost solely in the absence of dorsal spines. The
smaller species belonging to _Dalatias_ (_Scymnorhinus_, or _Scymnus_),
_Dalatias licha_, etc., are very much like the dogfishes.

They are, however, nowhere very common. The teeth of _Dalatias major_
exist in Miocene rocks. In the genus _Somniosus_ the species are of
very much greater size, _Somniosus microcephalus_ attaining the length
of about twenty-five feet. This species, known as the sleeper-shark or
Greenland shark, lives in all cold seas and is an especial enemy of
the whale, from which it bites large masses of flesh with a ferocity
hardly to be expected from its clumsy appearance. From its habit of
feeding on fish-offal, it is known in New England as "gurry-shark." Its
small quadrate teeth are very much like those of the dogfish, their
tips so turned aside as to form a cutting edge. The species is stout
in form and sluggish in movement. It is taken for its liver in the
north Atlantic on both coasts in Puget Sound and Bering Sea, and I have
seen it in the markets of Tokyo. In Alaska it abounds about the salmon
canneries feeding on the refuse.

=Family Echinorhinidae.=--The bramble-sharks, _Echinorhinidae_, differ
in the posterior insertion of the very small dorsal fins, and in the
presence of scattered round tubercles, like the thorns of a bramble
instead of shagreen. The single species, _Echinorhinus spinosus_
reaches a large size. It is rather scarce on the coasts of Europe,
and was once taken on Cape Cod. The teeth of an extinct species,
_Echinorhinus richardi_, are found in the Pliocene.

[Illustration: FIG. 340.--Brain of Monkfish, _Squatina squatina_ L.
(After Dumeril.)]

=Suborder Rhinae.=--The suborder _Rhinae_ includes those sharks having
the vertebrae tectospondylous, that is, with two or more series of
calcified lamellae, as on the rays. They are transitional forms, as
near the rays as the sharks, although having the gill-openings rather
lateral than inferior, the great pectoral fins being separated by a
notch from the head.

The principal family is that of the angel-fishes, or monkfishes
(_Squatinidae_). In this group the body is depressed and flat like that
of a ray. The greatly enlarged pectorals form a sort of shoulder in
front alongside of the gill-openings, which has suggested the bend
of the angel's wing. The dorsals are small and far back, the tail
is slender with small fins, all these being characters shared by the
rays. But one genus is now extant, widely diffused in warm seas. The
species if really distinct are all very close to the European _Squatina
squatina_. This is a moderate-sized shark of sluggish habit feeding on
crabs and shells, which it crushes with its small, pointed, nail-shaped
teeth. Numerous fossil species of _Squatina_ are found from the
Triassic and Cretaceous, _Squatina alifera_ being the best known.

[Illustration: FIG. 341.--Saw-shark, _Pristiophorus japonicus_ Guenther.
Specimen from Nagasaki.]

=Family Pristiophoridae, or Saw-sharks.=--Another highly aberrant family
is that of the sawsharks, _Pristiophoridae_. These are small sharks,
much like the _Dalatiidae_ in appearance, but with the snout produced
into a long flat blade, on either side of which is a row of rather
small sharp enameled teeth. These teeth are smaller and sharper than
in the sawfish (_Pristis_), and the whole animal is much smaller than
its analogue among the rays. This saw must be an effective weapon
among the schools of herring and anchovies on which the sawsharks
feed. The true teeth are small, sharp, and close-set. The few species
of sawsharks are marine, inhabiting the shores of eastern Asia and
Australia. _Pristiophorus japonicus_ is found rather sparsely along the
shores of Japan. The vertebrae in this group are also tectospondylous.
Both the _Squatina_ and _Pristiophorus_ represent a perfect transition
from the sharks and rays. We regard them as sharks only because the
gill-openings are on the side, not crowded downward to the under side
of the body-disk. As fossil, _Pristiophorus_ is known only from a few
detached vertebrae found in Germany.

=Suborder Batoidei, or Rays.=--The suborder of _Batoidei_, _Rajae_,
or _Hypotrema_, including the skates and rays, is a direct modern
offshoot from the ancestors of tectospondylous sharks, its characters
all specialized in the direction of life on the bottom with a food of
shells, crabs, and other creatures less active than fishes.

The single tangible distinctive character of the rays as a whole lies
in the position of the gill-openings, which are directly below the disk
and not on the side of the neck in all the sharks. This difference
in position is produced by the anterior encroachment of the large
pectoral fins, which are more or less attached to the side of the
head. By this arrangement, which aids in giving the body the form of a
flat disk, the gill-openings are limited and forced downward. In the
_Squatinidae_ (angel-fishes) and the _Pristiophoridae_ (sawsharks) the
gill-openings have an intermediate position, and these families might
well be referred to the _Batoidei_, with which group they agree in the
tectospondylous vertebrae.

Other characters of the rays, appearing progressively, are the widening
of the disk, through the greater and greater development of the fins,
the reduction of the tail, which in the more specialized forms becomes
a long whip, the reduction, more and more posterior insertion, and the
final loss of the dorsal fins, which are always without spine, the
reduction of the teeth to a tessellated pavement, then finally to flat
plates and the retention of the large spiracle. Through this spiracle
the rays breathe while lying on the bottom, thus avoiding the danger of
introducing sand into their gills, as would be done if they breathed
through the mouth. In common with the cyclospondylous sharks, all the
rays lack the anal fin. The rays rarely descend to great depths in
the sea. The different members have varying relations, but the group
most naturally divides into thick-tailed rays or skates (_Sarcura_)
and whip-tailed rays or sting-rays (_Masticura_). The former are much
nearer to the sharks and also appear earliest in geological times.

=Pristididae, or Sawfishes.=--The sawfishes, _Pristididae_, are long,
shark-like rays of large size, having, like the sawsharks, the snout
prolonged into a very long and strong flat blade, with a series of
strong enameled teeth implanted in sockets along either side of it.
These teeth are much larger and much less sharp than in the sawsharks,
but they are certainly homologous with these, and the two groups must
have a common descent, distinct from that of the other rays. Doubtless
when taxonomy is a more refined art they will constitute a small
suborder together. This character of enameled teeth on the snout would
seem of more importance than the position of the gill-openings or
even the flattening and expansion of the body. The true teeth in the
sawfishes are blunt and close-set, pavement-like as befitting a ray.
(See Fig. 152.)

[Illustration: FIG. 342.--Sawfish, _Pristis pectinatus_ Latham.
Pensacola, Fla.]

The sawfishes are found chiefly in river-mouths of tropical America
and West Africa: _Pristis pectinatus_ in the West Indies; _Pristis
zephyreus_ in western Mexico; and _Pristis pectinatus_ in the Senegal.
They reach a length of ten to twenty feet, and with their saws they
make great havoc among the schools of mullets and sardines on which
they feed. The stories of their attacks on the whale are without
foundation. The writer has never found any of the species in the
open sea. They live chiefly in the brackish water of estuaries and
river-mouths.

Fossil teeth of sawfishes occur in abundance in the Eocene. Still
older are vertebrae from the Upper Cretaceous at Maestricht. In
_Propristis schweinfurthi_ the tooth-sockets are not yet calcified.
In _Sclerorhynchus atavus_, from the Upper Cretaceous, the teeth
are complex in form, with a "crimped" or stellate base and a sharp,
backward-directed enameled crown.

=Rhinobatidae, or Guitar-fishes.=--The _Rhinobatidae_ (guitar-fishes)
are long-bodied, shovel-nosed rays, with strong tails; they are
ovoviviparous, hatching the eggs within the body. The body, like that
of the shark or sawfish, is covered with nearly uniform shagreen.
The numerous species abound in all warm seas; they are olive-gray
in color and feed on small animals of the seabottoms. The length
of the snout differs considerably in different species, but in all
the body is relatively long and strong. Most of the species belong
to _Rhinobatus_. The best-known American species are _Rhinobatus
lentiginosus_ of Florida and _Rhinobatus productus_ of California.
The names guitar-fish, fiddler-fish, etc., refer to the form of the
body. Numerous fossil species, allied to the recent forms, occur from
the Jurassic. Species much like _Rhinobatus_ occur in the Cretaceous
and Eocene. _Tamiobatis vetustus_, lately described by Dr. Eastman
from a skull found in the Devonian of eastern Kentucky, the oldest
ray-like fish yet known, is doubtless the type of a distinct family,
_Tamiobatidae_. It is more likely a shark however than a ray, although
the skull has a flattened ray-like form.

[Illustration: FIG. 343.--Guitar-fish, _Rhinobatus lentiginosus_
Garman. Charleston, S. C.]

Closely related to the _Rhinobatidae_ are the _Rhinidae_
(_Rhamphobatidae_), a small family of large rays shaped like the
guitar-fishes and found on the coast of Asia. _Rhina ancylostoma_
extends northward to Japan.

In the extinct family of _Astrodermidae_, allied to the _Rhinobatidae_,
the tail has two smooth spines and the skin is covered with tubercles.
In _Belemnobatis sismondae_ the tubercles are conical; in _Astrodermus
platypterus_ they are stellate.

=Rajidae, or Skates.=--The _Rajidae_, skates, or rays, inhabit the colder
waters of the globe and are represented by a large number of living
species. In this family the tail is stout, with two-rayed dorsal fins
and sometimes a caudal fin. The skin is variously armed with spines,
there being always in the male two series of specialized spinous hooks
on the outer edge of the pectoral fin. There is no serrated spine or
"sting," and in all the species the eggs are laid in leathery cases,
which are "wheelbarrow-shaped," with a projecting tube at each of
the four angles. The size of this egg-case depends on the size of the
species, ranging from three to about eight inches in length. In some
species more than one egg is included in the same case.

Most of the species belong to the typical genus _Raja_, and these are
especially numerous on the coasts of all northern regions, where they
are largely used as food. The flesh, although rather coarse and not
well flavored, can be improved by hot butter, and as "raie au beurre
noir" is appreciated by the epicure. The rays of all have small rounded
teeth, set in a close pavement.

[Illustration: FIG. 344.--Common Skate, _Raja erinacea_ Mitchill. Woods
Hole, Mass.]

Some of the species, known on our coasts as "barn-door skates," reach
a length of four or five feet. Among these are _Raja laevis_ and _Raja
ocellata_ on our Atlantic coast, _Raja binoculata_ in California,
and _Raja tengu_ in Japan. The small tobacco-box skate, brown with
black spots, abundant on the New England coast, is _Raja erinacea_.
The corresponding species in California is _Raja inornata_, and in
Japan _Raja kenojei_. Numerous other species, _Raja batis_, _clavata_,
_circularis_, _fullonica_, etc., occur on the coasts of Europe. Some
species are variegated in color, with eye-like spots or jet-black
marblings. Still others, living in deep waters, are jet-black with
the body very soft and limp. For these Garman has proposed the
generic name _Malacorhinus_, a name which may come into general use
when the species are better known. In the deep seas rays are found
even under the equator. In the south-temperate zone the species are
mostly generically distinct, _Psammobatis_ being a typical form,
differing from _Raja_. _Discobatus sinensis_, common in China and
Japan, is a shagreen-covered form, looking like a _Rhinobatus_. It
is, however, a true ray, laying its eggs in egg-cases, and with the
pectorals extending on the snout. Fossil _Rajidae_, known by the teeth
and bony tubercles, are found from the Cretaceous onward. They belong
to _Raja_ and to the extinct genera _Dynatobatis_, _Oncobatis_, and
_Acanthobatis_. The genus _Arthropterus_ (_rileyi_) from the Lias,
known from a large pectoral fin, with distinct cylindrical-jointed
rays, may have been one of the _Rajidae_, or perhaps the type of a
distinct family, _Arthropteridae_.

[Illustration: FIG. 345.--Numbfish, _Narcine brasiliensis_ Henle,
showing electric cells. Pensacola, Fla.]

=Narcobatidae, or Torpedoes.=--The torpedoes, or electric rays
(_Narcobatidae_), are characterized by the soft, perfectly smooth skin,
by the stout tail with rayed fins, and by the ovoviviparous habit,
the eggs being hatched internally. In all the species is developed
an elaborate electric organ, muscular in its origin and composed of
many hexagonal cells, each filled with soft fluid. These cells are
arranged under the skin about the back of the head and at the base of
the pectoral fin, and are capable of benumbing an enemy by means of a
severe electric shock. The exercise of this power soon exhausts the
animal, and a certain amount of rest is essential to recovery.

The torpedoes, also known as crampfishes or numbfishes, are peculiarly
soft to the touch and rather limp, the substance consisting largely of
watery or fatty tissues. They are found in all warm seas. They are not
often abundant, and as food they have not much value.

Perhaps the largest species is _Tetronarce occidentalis_, the crampfish
of our Atlantic coast, black in color, and said sometimes to weigh 200
pounds. In California _Tetronarce californica_ reaches a length of
three feet and is very rarely taken, in warm sandy bays. _Tetronarce
nobiliana_ in Europe is much like these two American species. In the
European species, _Narcobatus torpedo_, the spiracles are fringed
and the animal is of smaller size. To _Narcine_ belong the smaller
numbfish, or "entemedor," of tropical America. These have the spiracles
close behind the eyes, not at a distance as in _Narcobatus_ and
_Tetronarce_. _Narcine brasiliensis_ is found throughout the West
Indies, and _Narcine entemedor_ in the Gulf of California. _Astrape_,
a genus with but one dorsal fin, is common in southern Japan. Fossil
_Narcobatus_ and _Astrape_ occur in the Eocene, one specimen of the
former nearly five feet long. Vertebrae of _Astrape_ occur in Prussia in
the amber-beds.

[Illustration: FIG. 346.--Teeth of _Janassa linguaeformis_ Atthey.
Carboniferous. Family _Petalodontidae_. (After Nicholson.)]

=Petalodontidae.=--Near the _Squatinidae_, between the sharks and the
rays, Woodward places the large extinct family of _Petalodontidae_, with
coarsely paved teeth each of which is elongate with a central ridge and
one or more strong roots at base. The best-known genera are _Janassa_
and _Petalodus_, widely distributed in Carboniferous time. _Janassa_ is
a broad flat shark, or, perhaps, a skate, covered with smooth shagreen.
The large pectoral fins are grown to the head; the rather large ventral
fins are separated from them. The tail is small, and the fins, as in
the rays, are without spines. The teeth bear some resemblance to those
of _Myliobatis_. _Janassa_ is found in the coal-measures of Europe
and America, and other genera extend upward from the Subcarboniferous
limestones, disappearing near the end of Carboniferous time.
_Petalodus_ is equally common, but known only from the teeth. Other
widely distributed genera are _Ctenoptychius_ and _Polyrhizodus_.

[Illustration: FIG. 347.--_Polyrhizodus radicans_ Agassiz. Family
_Petalodontidae_. Carboniferous of Ireland. (After McCoy.)]

These forms may be intermediate between the skates and the sting-rays.
In dentition they resemble most the latter.

Similar to these is the extinct family of _Pristodontidae_ with one
large tooth in each jaw, the one hollowed out to meet the other. It is
supposed that but two teeth existed in life, but that is not certain.
Nothing is known of the rest of the body in _Pristodus_, the only genus
of the group.

=Dasyatidae, or Sting-rays.=--In the section _Masticura_ the tail is
slender, mostly whip-like, without rayed dorsal or caudal fins, and
it is usually armed with a very long spine with saw-teeth projecting
backward. In the typical forms this is a very effective weapon, being
wielded with great force and making a jagged wound which in man rarely
heals without danger of blood-poisoning. There is no specific poison,
but the slime and the loose cuticle of the spine serve to aggravate
the irregular cut. I have seen one sting-ray thrust this spine through
the body of another lying near it in a boat. Occasionally two or
three of these spines are present. In the more specialized forms of
sting-rays this spine loses its importance. It becomes very small and
not functional, and is then occasionally or even generally absent in
individuals.

The common sting-rays, those in which the caudal spine is most
developed, belong to the family of _Dasyatidae_. This group is
characterized by the small skate-like teeth and by the non-extension
of the pectoral rays on the head. The skin is smooth or more or less
rough. These animals lie flat on the sandy bottoms in nearly all seas,
feeding on crabs and shellfish. All hatch the eggs within the body.
The genus _Urolophus_ has a rounded disk, and a stout, short tail
with a caudal fin. It has a strong spine, and for its size is the
most dangerous of the sting-rays. _Urolophus halleri_, the California
species, was named for a young man who was stung by the species at
the time of its first discovery at San Diego in 1863. _Urolophus
jamaicensis_ abounds in the West Indies, _Urolophus mundus_ at Panama,
and _Urolophus fuscus_ in Japan. None of the species reach Europe. The
true sting-ray (stingaree, or clam-cracker), _Dasyatis_, is more widely
diffused and the species are very closely related. In these species
the body is angular and the tail whip-like. Some of the species reach
a length of ten or twelve feet. None have any economic value, and all
are disliked by fishermen. _Dasyatis pastinaca_ is common in Europe,
_Dasyatis centrura_ along our Atlantic coast, _Dasyatis sabina_ ascends
the rivers of Florida, and _Dasyatis dipterura_ abounds in the bay of
San Diego. Other species are found in tropical America, while still
others (_Dasyatis akajei_, _kuhlii_, _zugei_, etc.) swarm in Japan and
across India to Zanzibar.

[Illustration: FIG. 348.--Sting-ray, _Dasyatis sabina_ Le Sueur.
Galveston.]

_Pteroplatea_, the butterfly-ray, has the disk very much broader than
long, and the trivial tail is very short, its little spine more often
lost than present. Different species of this genus circle the globe:
_Pteroplatea maclura_, on our Atlantic coast; _Pteroplatea marmorata_,
in California; _Pteroplatea japonica_, in Japan; and _Pteroplatea
altavela_, in Europe. They are all very much alike, olive, with the
brown upper surface pleasingly mottled and spotted.

Sting-rays of various types, _Taeniura_, _Urolophus_, etc., occur as
fossils from the Eocene onward. A complete skeleton called _Xiphotrygon
acutidens_, distinguished from _Dasyatis_ by its sharp teeth, is
described by Cope from the Eocene of Twin Creek in Wyoming. Vertebrae of
_Urolophus_ are found in German Eocene. _Cyclobatis_ (_oligodactylus_),
allied to _Urolophus_, with a few long pectoral rays greatly produced,
extending over the tail and forming a rayed wreath-like projection over
the snout, is known from the Lower Cretaceous.

=Myliobatidae.=--The eagle-rays, _Myliobatidae_, have the pectoral fins
extended to the snout, where they form a sort of rayed pad. The teeth
are very large, flat, and laid in mosaic. The whip-like tail is much
like that in the _Dasyatidae_, but the spine is usually smaller. The
eagle-like appearance is suggested by the form of the skull. The eyes
are on the side of the head with heavy eyebrows above them. The species
are destructive to clams and oysters, crushing them with their strong
flat teeth.

In _Aetobatus_ the teeth are very large, forming but one row. The
species _Aetobatus narinari_ is showily , brown with yellow
spots, the body very angular, with long whip-like tail. It is found
from Brazil to Hawaii and is rather common.

In _Myliobatis_ the teeth are in several series. The species are many,
and found in all warm seas. _Myliobatis aquila_ is the eagle-ray of
Europe, _Myliobatis californicus_ is the batfish of California, and
_Myliobatis tobijei_ takes its place in Japan.

In _Rhinoptera_ the snout is notched and cross-notched in front so that
it appears as if ending in four lobes at the tip. These "cow-nosed
rays," or "whipparees," root up the soft bottoms of shallow bays in
their search for clams, much as a drove of hogs would do it. The common
American species is _Rhinopterus bonasus_. _Rhinoptera steindachneri_
lives in the Gulf of California.

Teeth and spines of all these genera are common as fossils from the
Eocene onwards, as well as many of the extinct genus, _Ptychodus_,
with cyclospondylous vertebrae. _Ptychodus mammilaris_, _rugosus_,
and _decurrens_ are characteristic of the Cretaceous of England.
_Myliobatis dixoni_ is common in the European Eocene, as is also
_Myliobatis toliapicus_ and _Aetobatis irregularis_. _Apocopodon
seriacus_ is known from the Cretaceous of Brazil.

[Illustration: FIG. 349.--Eagle-ray, _Aetobatis narinari_ (Euphrasen).
Cedar Keys, Fla.]

=Family Psammodontidae.=--The _Psammodontidae_ are known only from the
teeth, large, flat, or rounded and finely dotted or roughened on the
upper surface, as the name _Psammodus_ (~psammos~, sand; ~odous~,
tooth) would indicate. The way in which the jaws lie indicates that
these teeth belonged to rays rather than sharks. Numerous species
have been described, mostly from the Subcarboniferous limestones.
_Archaeobatis gigas_, perhaps, as its name would indicate, the primeval
skate, is from the Subcarboniferous limestone of Greencastle, Indiana.
Teeth of numerous species of _Psammodus_ and _Copodus_ are found
in many rocks of Carboniferous age. _Psammodus rugosus_ common in
Carboniferous rocks of Europe.

[Illustration: FIG. 350.--Devil-ray or Sea-devil, _Manta birostris_
(Walbaum). Florida.]

=Family Mobulidae.=--The sea-devils, _Mobulidae_, are the mightiest of
all the rays, characterized by the development of the anterior lobe
of the pectorals as a pair of cephalic fins. These stand up like
horns or cars on the upper part of the head. The teeth are small and
flat, tubercular, and the whip-like tail is with or without spine.
The species are few, little known, and inordinately large, reaching a
width of more than twenty feet and a weight, according to Risso, of
1250 pounds. When harpooned it is said that they will drag a large boat
with great swiftness. The manta, or sea-devil, of tropical America is
_Manta birostris_. It is said to be much dreaded by the pearl-fishers,
who fear that it will devour them "after enveloping them in its vast
wings." It is not likely, however, that the manta devours anything
larger than the pearl-oyster itself. _Manta hamiltoni_ is a name given
to a sea-devil of the Gulf of California. The European species _Mobula
edentula_ reaches a similarly enormous size, and _Mobula hypostoma_
has been scantily described from Jamaica and Brazil. _Mobula japonica_
occurs in Japan. A foetus in my possession from a huge specimen taken
at Misaki is nearly a foot across. In _Mobula_ (_Cephaloptera_) there
are teeth in both jaws, in _Manta_ (_Ceratoptera_) in the lower jaw
only. In _Ceratobatis_ from Jamaica (_C. robertsi_) there are teeth in
the upper jaw only. Otherwise the species of the three genera are much
alike, and from their huge size are little known and rarely seen in
collections. Of _Mobulidae_ no extinct species are known.

FOOTNOTES:

[150] Dr. Peter Schmidt has made a sketch of this little shark at night
from a living example, using its own light.




CHAPTER XXXI

THE HOLOCEPHALI, OR CHIMAERAS


=The Chimaeras.=--Very early in geological times, certainly as early as
the middle Silurian, the type of _Chimaeras_ diverged from that of the
sharks. Hasse derives them directly from his hypothetical primitive
_Polyospondyli_, by way of the _Acanthodei_ and _Ichthyotomi_. In
any event the point of divergence must be placed very early in the
evolution of sharks, and this suggestion is as likely as any other.
The chief character of Chimaeras is found in the autostylic skull,
which is quite different from the hyostylic skull of the sharks. In
the sharks and in all higher fishes the mandible is joined to the
skull by a suspensorium of bones or cartilages (quadrate, symplectic,
and hyomandibular bones in the Teleost fishes). To this arrangement
the name hyostylic is given. In the Chimaera there is no suspensorium,
the mandible being directly attached to the cranium, of which the
hyomandibular and quadrate elements form an integral part, this
arrangement being called autostylic. The palato-quadrate apparatus, of
which the upper jaw is the anterior part, is immovably fused with the
cranium, instead of being articulated with it. This fact, gives the
name to the subclass _Holocephali_ (~holos~, whole or solid; ~kephale~,
head). Other characters are found in the incomplete character of the
back-bone, which consists of a scarcely segmented notochord differing
from the most primitive condition imagined only in being surrounded
by calcareous rings, no lime entering into the composition of the
notochord itself. The tail is diphycercal and usually prolonged in
a filament (leptocercal). The shoulder-girdle, as in the sharks, is
free from the skull. The pectoral fins are short and broad, without
segmented axis or archipterygium and without recognizable analogue
of the three large cartilages seen in the sharks, the propterygium,
mesopterygium, and metapterygium. In the mouth, instead of teeth, are
developed flat, bony plates called tritors or grinders, set endwise in
the front of the jaws. The gills are fringe-like, free at the tips as
in ordinary fishes, and there is a single external opening for them
all as in true fishes, and they are covered with a flap of skin. These
structures are, however, quite different from those of the true fishes
and are doubtless independently developed. There is no spiracle. The
skin is smooth or rough. In the living forms and most of the extinct
species there is a strong spine in the dorsal fin. The ventral fin
in the male has complex, usually trifid, claspers, and an analogous
organ, the cephalic holder, is developed on the front of the head,
in the adult male. This is a bony hook with a brush of glistening
enameled teeth at the end. The eggs are large, and laid in oblong or
elliptical egg-cases, provided with silky filaments. The eggs are
fertilized after they are extruded. Mucous channels and lateral line
are highly developed, being most complex about the head. The brain
is essentially shark-like, the optic nerves form a chiasma, and the
central hemispheres are large.

The teeth of the Chimaeras are thus described by Woodward, vol. 2, pp.
36, 37:

"In all the known families of Chimaeroids, the dentition consists
of a few large plates of vascular dentine, of which certain areas
('tritors') are specially hardened by the deposition of calcareous
salts within and around groups of medullary canals, which rise at right
angles to the functional surface. In most cases there is a single pair
of such plates in the lower jaw, meeting at the symphysis, while two
pairs are arranged to oppose these above. As a whole, the dentition
thus closely resembles that of the typical Dipnoi (as has often been
pointed out); and the upper teeth may be provisionally named palatine
and vomerine until further discoveries shall have revealed their
precise homologies. The structures are sometimes described as 'jaws,'
and regarded as dentaries, maxillae, and premaxillae, but the presence
of a permanent pulp under each tooth is conclusive proof of their
bearing no relation to the familiar membrane-bones thus named in higher
fishes."

=Relationship of Chimaeras.=--As to the origin of the Chimaeras and
their relation to the sharks, Dr. Dean has this recent ("The Devonian
Lamprey") and interesting word:

"The Holocephali have always been a doubtful group, anatomy and
palaeontology contributing but imperfect evidence as to their
position in the gnathostome phylum. Their embryology, however, is
still undescribed, except in a brief note by T. J. Parker, and it
is reasonably looked to to contribute evidence as to their line of
descent. The problem of the relationships of the Chimaeroids has long
been of especial interest to me, and it has led me to obtain embryonic
material of a Pacific species of one of these forms. It may be of
interest in this connection to state that the embryology of this form
gives the clearest evidence that the wide separation of the Selachii
and Holocephali is not tenable. The entire plan of development in
_Chimaera colliei_ is clearly like that of a shark. The ovulation
is closely like that of certain of the rays and sharks: the eggs
are large, the segmentation is distinctly shark-like; the circular
blastoderm overgrows the yolk in an elasmobranchian manner. The early
embryos are shark-like; and the later ones have, as T. J. Parker has
shown, external gills, and I note further that these arise, precisely
as in shark-embryos, from the posterior margin of the gill-bar. A
spiracle also is present. A further and most interesting developmental
feature is the fact that the autostylism in _Chimaera_ is purely of
secondary nature and is at the most of ordinal value. It is found that
in a larva of _Chimaera_ measuring 45 mm. in length, the palato-quadrate
cartilage is still separated from the skull by a wide fissure. This
becomes gradually reduced by the confluence of the palato-quadrate
cartilage with the skull, the fusion taking place at both the anterior
and posterior ends of the mesal rim of the cartilage. The remains of
the fissure are still well marked in the young _Chimaera_, four inches
in length; and a rudiment of it is present in the adult skull as a
passage-way for a nerve. Regarding the dentition: it may also be noted
in the present connection that the growth of the dental plates in
_Chimaera_ suggests distinctly elasmobranchian conditions. Thus on the
roof of the mouth the palatine plates are early represented by a series
of small more or less conical elements which resemble outwardly, at
least, the 'anlagen' of the pavement teeth in cestraciont sharks."

[Illustration: FIG. 351.--Skeleton of _Chimaera monstrosa_ Linnaeus.
(After Dean.)]

=Family Chimaeridae.=--The existing Chimaeras are known also as
spookfishes, ratfishes, and elephant-fishes. These are divided
by Garman into three families, and in the principal family, the
_Chimaeridae_, the snout is blunt, the skin without plates, and the
dorsal fin is provided with a long spine. The flat tritors vary in the
different genera. The single genus represented among living fishes
is _Chimaera_, found in cold seas and in the oceanic depths. The
best-known species, _Chimaera colliei_, the elephant-fish, or chimaera
of California, abounds in shallow waters of ten to twenty fathoms from
Sitka to San Diego. It is a harmless fish, useless except for the oil
in its liver, and of special interest to anatomists as the only member
of the family to be found when desired for dissection. This species
was first found at Monterey by Mr. Collie, naturalist of Captain
Beechey's ship, the _Blossom_. It is brown in color, with whitish
spots, and reaches a length of 2-1/2 feet. As a shallow-water form,
with certain differences in the claspers and in the tail, _Chimaera
colliei_ is sometimes placed in a distinct genus, _Hydrolagus_. Other
species inhabit much greater depths and have the tail produced into
a long filament. Of these, _Chimaera monstrosa_, the sea-cat of the
north Atlantic, has been longer known than any other Chimaera. _Chimaera
affinis_ has been dredged in the Gulf Stream and off Portugal. _Chimaera
phantasma_ and _Chimaera mitsukurii_ are frequently taken in Japan, and
the huge jet-black _Chimaera purpurascens_ in Hawaii and Japan. None of
these species are valued as food, but all impress the spectator with
their curious forms.

[Illustration: FIG. 352.--Elephant-fish, _Chimaera colliei_ Lay &
Bennett. Monterey.]

The fossil _Chimaeridae_, although numerous from Triassic times and
referred to several genera, are known chiefly by their teeth with
occasional fin-spines, frontal holders, or impressions of parts of
the skeleton. The earliest of _chimaeroid_ remains has been described
by Dr. Charles D. Walcott[151] from Ordovician or Lower Silurian
rocks at Canon City, Colorado. Of the species called _Dictyorhabdus
priscus_, only parts supposed to be the sheath of the notochord have
been preserved. Dr. Dean thinks this more likely to be part of the axis
of a cephalopod shell. The definitely known _Chimaeridae_ are mainly
confined to the rocks of the Mesozoic and subsequent eras. _Ischyodus
priscus_ (_avitus_) of the lower Jura resembles a modern chimaera.
_Granodus oweni_ is another extinct chimaera, and numerous fin-spines,
teeth, and other fragments in the Cretaceous and Eocene of America
and Europe are referred to _Edaphodon_. A species of _Chimaera_ has
been recorded from the Pliocene of Tuscany, and one of _Callorhynchus_
from the greensand of New Zealand. Other American Cretaceous genera of
chimaeroids are _Mylognathus_, _Bryactinus_, _Isotaenia_, _Leptomylus_,
and _Sphagepoea_. Dental plates called _Rhynchodus_ are found in the
Devonian.

=Rhinochimaeridae.=--The most degenerate of existing chimaeras belong
to the family of _Rhinochimaeridae_, characterized by the long flat
soft blade in which the snout terminates. This structure resembles
that seen in the deep-sea shark, _Mitsukurina_, and in _Polyodon_.
In _Rhinochimaera pacifica_ of Japan the teeth in each jaw form but
a single plate. In _Harriotta raleighana_, of the Gulf Stream, they
are more nearly as in _Chimaera_. Both are bathybial fishes, soft in
texture, and found in great depths. The family of _Callorhynchidae_, or
Antarctic Chimaeras, includes the bottle-nosed Chimaera (_Callorhynchus
callorhynchus_) of the Patagonian region. In this species the snout is
also produced, a portion being turned backward below in front of the
mouth, forming a sensory pad well supplied with nerves.

=Extinct Chimaeroids.=--According to Woodward, three other families are
recognizable among the extinct forms.

The _Ptyctodontidae_ are known from the teeth only, a single pair of
large, laterally compressed dental plates in each jaw, with a few hard
tritoral areas. These occur in Silurian and Devonian rocks. _Ptyctodus
obliquus_ from the Devonian of Russia is the best-known species. Other
genera are _Rhynchodus_ and _Palaeomylus_.

The _Squalorajidae_ have the head depressed and the snout produced in
a flat rostrum, as in _Harriotta_. There is no dorsal spine, and the
teeth are a few thin curved plates. The frontal holder of the male
is well developed. The few species occur in the Lias. _Squaloraja
dolichognathos_ is known from numerous fragments from the Triassic in
England and Scotland. _Chalcodus permianus_ is found in German Permian.

The _Myriacanthidae_ have the body elongate, with dermal plates on the
head and a long straight spine in the dorsal fin. The frontal holder
is large. The species, few in number, are found in Mesozoic rocks.
_Myriacanthus paradoxus_ is the best-known species. Of another species,
_Chimaeropsis paradoxa_, a skeleton about three feet long has been found
which shows a number of peculiar traits. The skin is covered with
ribbed shagreen scales. The dorsal fin has a large spine with retrorse
serrations behind. The tail is slim, and the pectoral and ventral fins
are very large. Bony plates with conical spines protect the neck. The
teeth are large and angular, of peculiar form.

=Ichthyodorulites.=--The term ichthyodorulite (~ichthys~, fish; ~dory~,
lance; ~lithos~, stone) is applied to detached fin-spines, dermal
spines, and tubercles belonging to unrecognized species of sharks and
chimaeras. Some of these are serrated, others entire, some straight,
some curved, and some with elaborate armature or sculpture. Some
doubtless belong to _Cestraciontes_, others to _Pleuracanthidae_; some
to _Squalidae_, some to chimaeras, and others, perhaps, to forms still
altogether unknown.

FOOTNOTES:

[151] Bulletin Geol. Soc. America, 1892.




CHAPTER XXXII

THE CLASS OSTRACOPHORI[152]


=Ostracophores.=--Among the earliest vertebrates actually recognized
as fossils belongs the group known as _Ostracophori_ (~ostrakos~,
a box; ~phoreo~, to bear). These are most extraordinary creatures,
jawless, apparently limbless, and enveloped in most cases anteriorly
in a coat of mail. In typical forms the head is very broad, bony, and
horseshoe-shaped, attached to a slender body, often scaly, with small
fins and ending in a heterocercal tail. What the mouth was like can
only be guessed, but no trace of jaws has yet been found in connection
with it. The most remarkable distinctive character is found in the
absence of jaws and limbs in connection with the bony armature. The
latter is, however, sometimes obsolete. The back-bone, as usual in
primitive fishes, is developed as a persistent notochord imperfectly
segmented. The entire absence of jaw structures, as well as the
character of the armature, at once separates them widely from the
mailed _Arthrodires_ of a later period. But it is by no means certain
that these structures were not represented by soft cartilage, of which
no traces have been preserved in the specimens known.

=Nature of the Ostracophores.=--The Ostracophores are found in the
Ordovician or Lower Silurian rocks, in the Upper Silurian, and in
the Devonian. After the latter period they disappear. The species
are very numerous and varied. Their real affinities have been much
disputed. Zittel leaves them with the Ganoids, where Agassiz early
placed them, but they show little homology in structure with the true
Ganoids. Some have regarded them as aberrant Teleosts, possibly as
freakish catfishes. Cope saw in them a huge mailed group of archaic
Tunicates, while Patten has soberly and with considerable plausibility
urged their affinity[153] to the group of spiders, especially to
the horseshoe-crabs (_Limulus_) and their palaeozoic ancestors, the
_Eurypteridae_ and _Merostomata_.

The best guess as to the affinities of the Ostracophores is perhaps
that given by Dr. Ramsey H. Traquair ("Fossil Fishes of the Silurian
Rocks of the South of Scotland," 1899). Traquair regards them as highly
aberrant sharks, or, more exactly, as being derived, like the Chimaeras,
from a primitive Elasmobranch stock. In favor of this view is the
character of their armature, the bony plates themselves to be regarded
as formed by the fusion of shagreen grains or scales. According to
Traquair: "Specialization from the most specialized form, _Lanarkia_,
has been accompanied by (1) fusion of the spinelets (_Lanarkia_) or
shagreen grains (_Thelodus_) into plates, scutes, and rhombic scales,
supported by hard matter developed in a deeper layer of skin, and (2)
alterations in the pectoral fin-flaps, which, becoming covered up by
the postero-lateral plates in _Drepanaspis_, are finally no longer
recognizable in the _Pteraspidae_."

[Illustration: FIG. 353.--_Odontotodus schrencki_ (Pander)
(_Tremataspis_), ventral side. Island of Oesel. (After Patten.)]

[Illustration: FIG. 354.--_Odontotodus schrencki_ (Pander)
(_Tremataspis_), dorsal side. Island of Oesel. (After Patten.)]

Woodward leaves their exact relationship undefined, while others have
regarded them as mailed lampreys, at any rate to be excluded from
the _Gnathostomi_, or jaw-bearing series. The apparent absence of
true jaws, true limbs, and limb-girdles certainly seems to separate
them widely from true fishes, but these characters are negative
only, perhaps due to degeneration, and at any rate they are not yet
absolutely determined. Certainly they offer no positive proof of
affinity with the modern Cyclostomes.

Dr. Traquair regards the _Heterostraci_ or most primitive
_Ostracophores_ as most certainly derived from the Elasmobranchs. Other
writers have attacked the integrity of the group of Ostracophores,
questioning the mutual relationship of its component parts. Reiss,
for example, regards the association of the _Osteostraci_ with the
_Heterostraci_ as "unbegruendet" and "unheilvoll," while Ray Lankester,
as quoted by Traquair, affirms that "there is absolutely no reason for
regarding _Cephalaspis_ as allied to _Pteraspis_ beyond that the two
genera occur in the same rocks, and still less for concluding that
either has any connection with _Pterichthys_." Elsewhere Lankester
states that the _Heterostraci_ are associated at present with the
_Osteostraci_, "because they have, like _Cephalaspis_, a large
head-shield, and because there is nothing else with which to associate
them." Patten, on the other hand, seems inclined to deny the rank of
_Heterostraci_ and _Osteostraci_ as even separate orders, regarding
them as very closely related to each other as also to their supposed
spider-like ancestors.

[Illustration: FIG. 355.--Head of _Odontotodus schrencki_ Pander, from
the side. (After Patten.)]

But the consensus of opinion favors the belief that the four orders
usually included under this head are distinct and at the same time are
really related one to another. For our purposes, then, we may regard
the _Ostracophori_ as a distinct class of vertebrates. By placing it
after the Elasmobranchs we may indicate its probable descent from a
primitive shark-like stock.

[Illustration: FIG. 356.--The Horseshoe Crab or King-crab, _Limulus
polyphemus_ Linnaeus. Supposed by Professor Patten to be an ally of the
Ostracophores; usually regarded as related to the Spiders.]

On this subject Dr. Dean remarks: "The entire problem of the homology
of the dermal plates and 'scales' in the Ostracophores and Arthrognaths
is to the writer by no means as clear as previous writers have
conceded. From the histological standpoint, admitting the craniote
nature of the vasodentine and cancellous layers in the dermal plates,
it nevertheless does not follow that they have been derived from the
actual conditions of the dermal denticles of the ancestral Gnathostome,
as were unquestionably the dermal plates of Teleostomes and Dipnoans.
It seems equally if not more probable, on the other hand, that the
dermal armoring of the distinct groups may have had an altogether
different mode of origin, the product of a crude evolution which aimed
to strengthen the skin by a general deposition of calcareous matter
throughout its entire thickness. The tuberculation of plates thus
acquired might have become an important step in the development of a
more superficial type of armoring which is most preferably represented
by the dermal denticles of Selachians. Nor, in passing, need the
presence of a mucus-canal system in the early plated forms be of
greater morphological importance than a foreshadowing of the conditions
of Gnathostomes, for this system of organs might serve as well as
evidence, in a general way, of relationship with Marsipobranchs. Nor
is this evidence the more conclusive when we reflect that _no known
type of Gnathostome, recent or fossil, possesses open sensory grooves
in distinct dermal plates_. The presence, furthermore, of a dorsal
fin and a 'truly piscine heterocercal tail,' as noted by Traquair, is
by no means as Gnathostome-like as these structures at first glimpse
appear. For they lack not merely the characteristic radial supports of
fishes, but even actinotrichia. Their mode of support, on the other
hand, as Smith Woodward points out, is of a more generalized nature,
bent scales, homologous with those of the adjacent body region, taking
the place of the piscine external supports." The actual position in the
system to be finally assigned to the Ostracophores is therefore still
uncertain.

=Orders of Ostracophores.=--Four orders of _Ostracophori_ are now
usually recognized, known in the systems of Woodward and Traquair as
_Heterostraci_, _Osteostraci_, _Antiarcha_, and _Anaspida_. The former
is the most primitive and perhaps the most nearly allied to the sharks,
the second is not very remote from it, the last two aberrant in very
different directions. Hay places the _Antiarcha_ with the _Arthrodira_
under the superorder of _Placodermi_.

=Order Heterostraci.=--The _Heterostraci_ (~heteros~], different;
~ostrakos~, box) have no bone-corpuscles in the coat of mail. This
typically consists of a few pieces above, firmly united and traversed
by dermal sense-organs or "lateral lines." The ventral shield is
simple. Four families are recognized by Traquair as constituting the
_Heterostraci_, these forming a continuous series from shark-like forms
to the carapace-covered _Pteraspis_. In the most primitive family,
the _Thelodontidae_,[154] the head and trunk are covered with small
scales or tubercles of dentine and not fused into large plates. The
tail is slender and heterocercal, the caudal fin deeply forked. Until
lately these tubercles were regarded as belonging to sharks, and they
are still regarded by Traquair as evidence of the affinity of the
_Heterostraci_ with the _Acanthodei_. Dr. Traquair thinks that a flap
or lappet-like projection behind the head may be a pectoral fin.
The three known genera are _Thelodus_, _Lanarkia_, and _Ateleaspis_.
In _Thelodus_ the scales consist of a base and a crown separated by
a constriction or neck. _Thelodus scoticus_, _Thelodus pagei_, and
_Thelodus planus_ are found in the Silurian rocks of Scotland. Other
species, as _Thelodus tulensis_ of Russia, extend to the Upper Devonian.

In _Lanarkia_ the large sharp scales have an expanded base like the
mouth of a trumpet. _Lanarkia horrida_ and _L. spinulosa_ are found
in the shire of Lanark in Scotland. In _Ateleaspis_ (_tesselatus_)
the skin is covered with small polygonal plates. The lateral flaps or
possibly fins take the form of flat rhombic sculptured scales. In this
genus the eyes seem to be on the top of the head.

[Illustration: FIG. 357.--_Lanarkia spinosa_ Traquair. Upper Silurian.
Family _Thelodontidae_. (After Traquair.)]

In the _Psammosteidae_ of the Devonian the head is covered with large
plates which are not penetrated by the sense-organs. These plates are
covered with minute, close-set tubercles, covered with brilliant ganoid
enamel and with finely crimped edges. According to Dr. Traquair, these
tubercles are shagreen granules which have coalesced and become united
to plates formed in a deeper layer of the skin, as in _Ateleaspis_ the
minute scales have run together into polygonal plates. These creatures
have been considered as "armored sharks," and Dr. Traquair regards them
as really related to the acanthodean sharks. Nevertheless they are not
really sharks at all, and they find their place with the _Pteraspis_
and other longer known Heterostracans.

The family of _Drepanaspidae_ consists of a single recently known
species, _Drepanaspis gmundenensis_, found in a pyritized condition
in purple roofing-slate in Gmuenden, Germany. This fish, which reaches
a length of about two feet, has a broad head, with eyes on its outer
margin, with a slender body and heterocercal tail. The head has a
broad median plate and smaller polygonal ones. The flaps, supposed
to represent the pectoral fins, are here cased in immovable bone. No
trace of internal skeleton is found by Traquair, who has given the
restoration of this species, but the mouth has been outlined.

[Illustration: FIG. 358.--_Drepanaspis gmundenensis_ Schlueter. Upper
Silurian, Gmuenden, Germany. (After Traquair.)]

The best known of the Heterostracan families is that of _Pteraspidae_.
In this family the plates of the head are coalesced in a large carpace,
the upper part originally formed of seven coalesced pieces. A stout
dorsal spine fits into a notch of the carapace. The slender body is
covered with small scales and ends in a heterocercal tail. The dermal
sense-organs are well developed. _Pteraspis rostrata_ occurs in the
Lower Devonian. Other genera are _Palaeaspis_ and _Cyrthaspis_.

[Illustration: FIG. 359.--_Pteraspis rostrata_ Agassiz. Devonian.
Family _Pteraspidae_. (After Nicholson.)]

=Order Osteostraci.=--The Osteostraci (~osteon~, bone; ~ostrakos~, box)
(called _Aspidocephali_ by Rohon) have bone-corpuscles in the shields,
and the shield of the back is in one piece without lateral-line
channels or sense-organs. Ventral shield single. The order includes
three families. The _Cephalaspidae_ have the shields tuberculate,
the one between the eyes fixed, and the anterior body-shields are
not fused into a continuous plate. The best known of the numerous
species is _Cephalaspis lyelli_ from the Lower Devonian of England.
_Hemicyclaspis murchisoni_ occurs in the Upper Silurian of England, and
the extraordinary _Cephalaspis dawsoni_ in the Lower Devonian of Gaspe,
Canada. _Eukeraspis pustulifera_ has the head-shield very slender and
armed with prickles. In the _Thyestidae_ the anterior body-scales are
fused into a continuous plate. _Thyestis_ and _Didymaspis_ are genera
of this type. The _Odontotodontidae_ (_Tremataspidae_) have the shield
truncate behind, its surface finely punctate, and the piece between
the eyes not fixed. _Odontotodus[155] schrenki_ is found in the Upper
Silurian of the Island of Oesel in company with species of _Thyestes_.
The _Euphaneropidae_ are represented in the Devonian of Quebec.

[Illustration: FIG. 360.--_Cephalaspis lyelli_ Agassiz, restored.
(After Agassiz.)]

=Order Antiarcha.=--The Antiarcha (~anti~, opposite; ~archos~, anus)
have also bone-corpuscles in the plates, which are also enameled. The
sense-organs occupy open grooves, and the dorsal and ventral shields
are of many pieces. The head is jointed on the trunk, and jointed to
the head are paddle-like appendages, covered with bony plates and
resembling limbs. There is no evidence that these erectile plates
are real limbs. They seem to be rather jointed appendages of the
head-plate, erectile on a hinge like a pectoral spine. There are traces
of ear-cavities, gill-arches, and other fish-like structures, but
nothing suggestive of mouth or limbs.

This group contains one family, the _Asterolepidae_, with numerous
species, mostly from Devonian rocks. The best known genus is
_Pterichthyodes_,[156] in which the anterior median plate of the back
is overlapped by the posterior dorso-lateral. _Pterichthyodes milleri_
from the Lower Devonian, named by Agassiz for Hugh Miller, is the best
known species, although numerous others, mostly from Scottish quarries,
are in the British Museum. _Asterolepis maximus_ is a very large
species from the same region, known from a single plate. _Bothriolepis
canadensis_ is from the Upper Devonian of Scaumenac Bay near Quebec,
numerous specimens and fragments finely preserved having been found.

[Illustration: FIG. 361.--_Cephalaspis dawsoni_ Lankester. Lower
Devonian of Canada. Family _Cephalaspidae_. (After Woodward.) In the
square a portion of the tubercular surface is shown.]

_Microbrachium dicki_ with the pectoral appendages small occurs in the
Devonian of Scotland.

The earliest remains of _Ostracophori_ are found in Ordovician or
Lower Silurian rocks of the Trenton horizon at Canon City, Colorado.
These consist of enormous numbers of small fragments of bones
mixed with sand. With these is a portion of the head carapace of a
small Ostracophore which has been named by Dr. Walcott _Asteraspis
desiderata_ and referred provisionally to the family of _Asterolepidae_,
which belongs otherwise to the Lower Devonian.

[Illustration: FIG. 362.--_Pterichthyodes testudinarius_ (Agassiz),
restored. Lower Devonian Family _Asterolepidae_. (After Traquair and
others.)]

With these remains are found also scales possibly belonging to a
Crossopterygian fish (_Eriptychius_). These remains make it evident
that the beginning of the fish series lies far earlier than the rocks
called Silurian, although fishes in numbers are not elsewhere known
from rocks earlier than the Ludlow shales of the Upper Silurian,
corresponding nearly to the Niagara period in America.

In the Ludlow shales we find the next appearance of the Ostracophores,
two families, _Thelodontidae_ and _Birkeniidae_, being there represented.

[Illustration: FIG. 363.--_Pterichthyodes testudinarius_ Agassiz, side
view. (After Zittel, etc.)]

[Illustration: FIG. 364.--_Birkenia elegans_ Traquair. Upper Silurian.
(After Traquair.)]

=Order Anaspida.=--Recently a fourth order, _Anaspida_ (~a~, without;
~aspis~, shield), has been added to the _Ostracophori_ through the
researches of Dr. Traquair. This group occurs in the Upper Silurian
in the south of Scotland. It includes the single family _Birkeniidae_,
characterized by the fusiform body, bluntly rounded head, bilobate,
heterocercal tail, and a median row of hooked spinous scales along the
ventral margin. No trace of jaws, teeth, limbs, or internal skeleton
has been found. Unlike other Ostracophores, _Birkenia_ has no cranial
buckler with orbits on the top, nor have the scales and tubercles
the microscopic structure found in other Ostracophores. In the genus
_Birkenia_ the head and body are completely covered by tubercular
scutes. The gill-openings seem to be represented by a series of
small perforations on the sides. A dorsal fin is present. _Birkenia
elegans_ is from the Ludlow and Downstonian rocks of southern Scotland.
_Lasianius problematicus_ from the same rocks is very similar, but is
scaleless. It has a row of ventral plates like those of _Birkenia_, the
only other hard parts it possesses being a number of parallel rods
behind the head, homologous with the lateral series of _Birkenia_.
_Lasianius_ is therefore a specialized and degenerate representation
of _Birkenia_, differing somewhat as "the nearly naked _Phanerosteon_
differs from other _Palaeoniscidae_ whose bodies are covered with osseous
scales."

[Illustration: FIG. 365.--_Lasianius problematicus_ Traquair. Upper
Silurian. (After Traquair.)]

FOOTNOTES:

[152] This group was first called by Cope _Ostracodermi_--a name
preoccupied for the group of bony trunkfishes, _Ostracidae_. The still
earlier name of _Placodermi_, chosen by McCoy (1848), was intended
to include Arthrodires as well as Ostracophores. Rohon (1892) calls
the group _Protocephali_, and to the two orders he assigns the
names _Aspidorhini_ and _Aspidocephali_. These groups correspond to
_Heterostraci_ and _Osteostraci_ of Woodward. Another name of early
date is that of _Aspidoganoidei_, given by Professor Gill in 1876, but
not defined until 1896. These fishes are, however, not "Ganoids" and
the name _Ostracophori_ seems to receive general preference. The group
_Peltacephalata_ of Patten corresponds essentially to _Ostracophori_,
as does also the order _Hypostomata_ of Gadow.

[153] According to Professor Patten's view, the close resemblance of
the shields of _Pteraspis_ to those of contemporaneous _Eurypterids_
indicates real affinity. But the _Eurypterids_ are related to the
spiders and to _Limulus_. The only reason for thinking that _Pteraspis_
is a fish at all lies in its resemblance to _Cephalaspis_, which is
in several ways fish-like, although its head shield is much like that
of _Limulus_. All these resemblances in Patten's view indicate real
affinity. Patten considers the _Pteraspids_ as derived from primitive
arachnid or spider-like forms having a bony carapace as _Limulus_ has.
From _Pteraspis_ he derives the other Ostracophores, and from these
the sharks and other vertebrates, all of which appear later in time
than the earliest Ostracophores. This view of the origin of vertebrates
is recently urged with much force by Professor Patten (Amer. Nat.,
1904, 1827). Most naturalists regard such resemblances in specialized
structures on the outside of an animal as parallelisms due to likeness
in conditions of life. The external structure in forms of really
different nature is often similarly modified. Thus certain catfishes,
pipefishes, sea-moths, and agonoid fishes are all provided with bony
plates not unlike those of ganoid fishes, although indicative of no
real affinity with them. Commonly the ancestry of vertebrates is traced
through enteropneustans to soft-bodied worms which have left no trace
in the rocks.

In the same connection, Professor Patten suggests that the lateral fold
from which many writers have supposed that the limbs or paired fins
of vertebrates is evolved is itself a resultant of the fusion of the
fringing appendages on the sides of the body. Such appendages are found
in the primitive mailed arachnoids and in _Limulus_. They are shown
very plainly in Patten's restoration of _Cephalaspis_. About thirty
of them of a bony nature and jointed to the body occur on either side
between the gill opening and the vent.

[154] Called _Coelolepidae_ by Pander and Traquair, but _Coelolepis_ is
a later synonym of _Thelodus_.

[155] This name, inappropriate or meaningless, is older than
_Tremataspis_.

[156] The earlier name of _Pterichthys_ has been already used for a
genus of living fishes.




CHAPTER XXXIII

ARTHRODIRES


=The Arthrodires.=--Another large group of extinct fishes mailed and
helmeted is included under the general name of _Arthrodira_[157]
(~arthros~, joint; ~deira~, neck), or _Arthrognathi_ (~arthros~,
~gnathos~, jaw), the latter term recently framed by Dr. Dean with a
somewhat broader application than the former.

These fishes differ from the Ostracophores, on the one hand, in the
possession of jaws and in the nature of their armored covering. On the
other hand, the nature of these jaws, the lack of differentiation of
the skeleton, and the uncertain character of the limbs separate them
still more widely from the true fishes. Their place in the system is
still unknown, but their origin seems as likely to be traceable to
Ostracophores as to any other group.

The head in all the species is covered with a great bony helmet.
Behind this on the nape is another large shield, and between the two
is usually a huge joint which Dr. Dean compares to the hinge of a
spring-beetle (_Elater_).

As to the presence of limbs, no trace of pectoral fin or anterior
limb has been found. Dean denies the existence of any structures
corresponding to either limb, but Woodward figures a supposititious
posterior limb in _Coccosteus_, finding traces of basal bones which may
belong to it.

These monstrous creatures have been considered by Woodward and others
as mailed Dipnoans, but their singular jaws are quite unlike those
of the _Dipneusti_, and very remote from any structures seen in the
ordinary fish. The turtle-like mandibles seem to be formed of dermal
elements, in which there lies little homology to the jaws of a fish and
not much more with the jaws of Dipnoan or shark.

The relations with the Ostracophores are certainly remote, though
nothing else seems to be any nearer. They have no affinity with the
true Ganoids, to which vaguely limited group many writers have attached
them. Nor is there any sure foundation to the view adopted by Woodward,
that they are to be considered as armored offshoots of the Dipnoans.

According to Dean we might as well refer the Arthrodires to the sharks
as to the Dipnoans. Dean further observes ("Fishes Living and Fossil"):

[Illustration: FIG. 366.--_Coccosteus cuspidatus_ Agassiz, restored.
Lower Devonian. (After Traquair, per Woodward.)]

"The puzzling characters of the Arthrodirans do not seem to be lessened
by a more definite knowledge of their different forms. The tendency, as
already noted, seems to be at present to regard the group provisionally
as a widely modified offshoot of the primitive Dipnoans, basing
this view upon their general structural characters, dermal plates,
dentition, autostylism. But only in the latter regard could they have
differed more widely from the primitive Elasmobranch or Teleostome,
if it be admitted that in the matter of dermal structures they may
be clearly separated from the Chimaeroid. It certainly is difficult
to believe that the articulation of the head of Arthrodirans could
have been evolved after dermal bones had come to be formed, or that
a Dipnoan could become so metamorphosed as to lose not only its body
armoring, but its pectoral appendages as well. The size of the pectoral
girdle is, of course, little proof that an anterior pair of fins must
have existed, since this may well have been evolved in relation to
the muscular supports of plastron, carapace, trunk, and head. The
intermovement of the dental plates, seen especially in _Dinichthys_,
is a further difficulty in accepting their direct descent from the
Dipnoans."

[Illustration: FIG. 367.--Jaws of _Dinichthys hertzeri_ Newberry. Upper
Devonian. Ohio. (After Newberry.)]

=Occurrence of Arthrodires.=--These fishes occur in abundance from the
Silurian times to the Mesozoic. In the Devonian their gigantic size and
thick armor gave them the leading position among the hosts of the sea.
Among the genera there occurred "series of forms most interesting as to
their evolution." "It is found more and more evident," says Dr. Dean
("Fishes, Living and Fossil," pp. 135, 136) "that the Arthrodirans may
have represented the dominant group in the Devonian period, as were the
sharks in the Carboniferous, or as are the Teleosts in modern times.
There were forms which, like _Coccosteus_, had eyes at the notches of
the head-buckler; others, like _Macropetalichthys_, in which orbits
were well centralized; some, like _Dinichthys_ and _Titanichthys_,
with the pineal foramen present; some with pectoral spines(?); some
with elaborately sculptured dermal plates. Among their forms appear
to have been those whose shape was apparently subcylindrical, adapted
for swift swimming; others (_Mylostoma_) whose trunk was depressed to
almost ray-like proportions. In size they varied from that of the perch
to that of a basking shark. In dentition they presented the widest
range in variation, from the formidable shear-like jaws of _Dinichthys_
to the lip-like mandibles of _Titanichthys_, the tearing teeth of
_Trachosteus_, the wonderfully forked tooth-bearing jaw-tips of
_Diplognathus_, to the Cestraciont type, _Mylostoma_. The latter form
has hitherto been known only from its dentition, but now proves to be,
as Newberry and Smith Woodward suggested, a typical Arthrodiran."

=Classification of Arthrodira.=--Our knowledge of the systematic
relations of the Arthrodira is mostly of recent origin. Woodward
refers most of the remains to the best known genus _Coccosteus_,
and recognizes as families the _Coccosteidae_, _Mylostomidae_,
_Asterosteidae_, and _Phyllolepidae_.

[Illustration: FIG. 368.--An Arthrodire, _Dinichthys intermedius_
Newberry, restored. Devonian, Ohio. (After Dean.)]

Dr. Bashford Dean in different papers has treated these fishes
in great detail. In a recent paper on the "Relationships of the
_Arthrognathi_"[158] he recognizes the group as a class coordinate
with _Cyclostomi_ and _Elasmobranchii_. This class, which he calls
_Arthrognathi_, is first divided into two suborders, _Anarthrodira_,
without joint at the neck, and _Arthrodira_, with such a joint. The
former comprises one order, _Stegothalami_, and the latter two orders,
_Temnothoraci_ and _Arthrothoraci_. The following is Dr. Dean's
definition of these orders and their component families:

=Arthrognathi.=--"Chordates whose anterior body region is encased in
dermal elements, and divisible by a more or less definite partition
into head and trunk. Dermal plates which surround the mouth function
as jaws. No evidence of branchial arches. Column notochordal, showing
no traces of centra; well-marked neural and haemal elements. Paired
limbs [absent or uncertain]. Dermal plates consisting typically of two
layers, the superficial tuberculate, the inner bony with radiating
lamellae. Orbits situated near or at the margin of the head-shield
and separated from one another by fixed integumental plates. A pineal
funnel present situated in a fixed plate. A mucous system whose canals
radiate from the preoccipital region."

=Anarthrodira.=--"Arthrognaths in which the cranial and dorsal regions
are separated by a fixed partition whose dorsal rim is overlapped and
concealed by superficial plates. Of these a large median dorsal element
is present which extends backward superficially from the region near
the pineal funnel. Also a pair of elements which overlie the position
of the external occipital joint. Suborbital plates apparently absent.
Jaw elements undescribed."

=Stegothalami= (~stegos~, roof; ~thalamos~, chamber).--"Anarthrodires
in which the cranio-dorsal septum is vertical and deep, its height
equal apparently to that of the arch of the head-shield. By this deep
partition the latter appears to inclose two chambers (whence the
ordinal name). Orbits inclosed by pre- and postorbital plates. Mucous
system lacks a postorbital canal."

One family, the _Macropetalichthyidae_, thus defined:

"Stegothalami with large orbits and well-arched cranio-dorsal
shield. Dorso-central shield long, wide, gomphoidal, extending
backward to the hinder margin of the shield and bordered by all
plates save the postorbitals and marginals. Pineal funnel small and
obscure." _Macropetalichthys sullivanti_ from Ohio Devonian rocks,
and _Macropetalichthys agassizi_ from the Devonian of Germany, are
important species of this group.

The _Asterosteidae_ perhaps constitute a second family in this order.
The single species _Asterosteus stenocephalus_ is from the Devonian of
Ohio.

=Arthrodira.=--"Arthrognaths in which the dorsal armoring is separated
into dorsal and cranial elements, the latter attached to the former
movably by means of a pair of peg-and-socket joints. The interval
lying between cranial and dorsal armoring does not appear to have
been protected by plates, and in the median line, instead of the
cranio-central of the Anarthrodires, there are separate elements,
median occipital, median dorsal, and perhaps others. Suborbital plates
present. Jaws of three pairs of elements. Ventral armoring of two
pairs of lateral and two median elements."

=Temnothoraci= (~temno~, to cut; ~thorax~, thorax).--"Arthrodires
whose cranial and dorsal shields are closely apposed, separated only
by a transverse fissure-like interval (whence the ordinal name);
interarticulation of cranial and dorsal shields little developed.
Head-shield elliptical in outline as far as the line of the transverse
division. The anterior rim of the shoulder-shield flattened at
its sides, suggesting a rudiment of the vertical partition of the
Anarthrodira. Suborbital plate is present, but takes no part,
apparently, in the ventral boundary of the orbit, this being formed,
as in the Anarthrodira, by the pre- and postorbital elements. Jaws,
ventral armoring, and endoskeleton not definitely known."

One family, _Chelonichthyidae_, thus defined:

"Temnothoraci with orbits relatively small in size and situated
well forward in the head-shield. Occipital elements produced
antero-posteriorly, the external occipital forming the posterior
lateral angle of the head, no projection of the head occurring in the
region of the marginal plate. Median occipital trapezoidal. Centrals
take part in the median boundary of the orbits, and embrace the pineal
plate. Median dorsal with poorly developed keel and terminal process."

_Heterosteus asmussi_ (perhaps to be called _Ichthyosauroides
spinosus_) is a gigantic species from the Lower Devonian of Livonia.

Allied to this species is _Homostius milleri_ from Scotland, celebrated
as the "Asterolepis of Stromness" in Hugh Miller's "Footsteps of the
Creator." Another notable species is _Homostius formosissimus_ from the
Lower Devonian of Russia.

=Arthrothoraci.=--"Arthrodires whose dorsal shield articulates with
the head-roof by a conspicuous and movable peg-and-socket joint, and
leaves a definite interval (unprotected?) between the two armorings.
Orbits marginal, bounded inferiorly not by the suborbital element.
In the head-shield the postero-lateral angles formed by the marginal
plate (_Phlyctaenaspis?_), the occipital border concave. A dorsal fin is
present, supported by endoskeletal elements." Five families, the most
important being the _Coccosteidae_, thus defined:

"Arthrothoraci with head-shield hexagonal in outline. Median occipital
trapezoidal, margins underlapped conspicuously by the external
occipitals. Prefrontals meet below pineal plates, thus occluding this
element from contact with centrals. The median dorsal plate elongated,
terminating in an acute heavy point; no definite ventral keel; its
anterior border approaches the head-shield more closely than in related
families. Cranio-dorsal joint relatively small. Postero-dorso-lateral
large." (?A pair of spines occurs in the pectoral region.) The
best-known species is _Coccosteus cuspidatus_ (_decipiens_) of the
Lower Red Sandstone or Devonian of Scotland.

The family of _Dinichthyidae_ consists of "Arthrothoraci with stout
trenchant jaws, whose cutting surfaces have worn away marginal teeth.
Plates heavy. Head-shield with conspicuous lateral indentation to form
dorsal border of orbit. Preorbitals separated by rostral and pineal
elements, the latter passing backward between the anterior ends of the
centrals. Cranio-dorsal joint conspicuous. Median dorsal shovel-shaped,
nearing a stout keel with a large neck and with heavy gouge-shaped
terminal. Postero-dorso-lateral relatively small in size." _Dinichthys
hertzeri_ and numerous other species are described from the Devonian
and Carboniferous rocks of Ohio.

The _Titanichthyidae_ are "Arthrothoraci with slender edentulous jaws
bearing a longitudinal sulcus. Plates squamous. Head-shield wide,
with indentations to form dorsal border of orbit. Cranio-dorsal joint
complete, but of relatively small size. Median dorsal with lateral
border indented with rudimentary keel and with flat and rounded
terminal. Antero-dorso-lateral with an area of overlap on median
border." _Titanichthys agassizi_ is a gigantic mailed fish from the
Lower Carboniferous of Cleveland, Ohio.

The _Mylostomidae_ are "Arthrothoraci with dental elements in the
character of crushing plates. Cranial shield wide, rounded anteriorly,
deeply indented in nuchal margin; orbital rim not apparent in dorsal
aspect. Central separated from marginal." _Mylostoma terrelli_ is based
on jaws from Cleveland, Ohio.

The _Selenosteidae_ are "Arthrothoraci with jaws studded with cuspidate
teeth; the mandibular rami rounding out anteriorly or presenting
diverging tips, bearing teeth in the symphysis. Cranial shield deeply
concave on lateral margins, no orbital rim here apparent. Nuchal border
deeply indented. (Centrals separate from marginals.) Cranio-dorsal
hinges large in size. Dorsal armoring reduced antero-posteriorly,
giving an almost zone-like appearance. Dorso-median crescent-shaped,
with feeble keel and knob." _Selenosteus glaber_ is described by Dean
from the Cleveland shales.

=Relations of Arthrodires.=--To complete our account of the Arthrodira
we may here summarize Dr. Dean's reasons for separating its members
from true fishes on the one hand and from the Ostracophores on the
other.

"FIRST. The Arthrodira cannot be strictly included among the Pisces.
According to the definition of the latter class its members are
Craniotes possessing the following characters: _a_, dermal defenses
which in their simplest terms can be reduced to the shagreen
denticles of the Elasmobranch; _b_, a series of definite gill-arches
whose foremost elements are metamorphosed into hyoid and mandibular
apparatus; _c_, paired fins, or their equivalents. In the first of
these regards I think it can be shown that the remarkable character
of the dermal plates in the Arthrognaths approaches rather that
of the Ostracophores than that of the Pisces. In certain of these
forms, _Trachosteus_, for example, the tuberculated plates are made
up of inner and outer elements, each with tubercles, which denote a
distinctly different mode of origin from that of any known type of
fish. The absence of remains of gill-arches in the Arthrognaths would
be not a serious objection to including these forms among Pisces,
especially in view of the fact that cartilaginous gill-arches are
rarely preserved even in favorable fossils. But that their presence
is more than doubtful is indicated by the peculiar character of the
'jaws' in these forms. For the character of these structures is such
as to suggest that they are not homologous with the branchial arch
jaws of the true fishes, but are rather parallel structures which
owe their origin to distinctly exoskeletal elements, i.e., that they
were derived from dermal plates surrounding the mouth, which became
mobile, and whose edges became apposed as sectorial structures. I would
in this connection call attention to the fact that the 'mandibles,'
'premaxillary,' and 'maxillary' dental plates[159] were not fixed
in the sense in which these elements are in the true Pisces. On the
evidence of several types, _Dinichthys_, _Titanichthys_, _Mylostoma_,
_Trachosteus_, _Diplognathus_, and other of the American forms,
_Macropetalichthys_[160] excepted, there is the clearest proof that
each element of the jaws had a considerable amount of independent
movement. On account of the mobility of these elements the name
Arthrognathi is suggested. Thus the mandibular rami could change the
angle of inclination towards each other, as well as their plane with
reference to the vertical axis. So, too, could the 'premaxillae' be
protracted like a pair of bent fingers, and it is more than probable
that the 'maxillae' had a considerable amount of independent movement.
In connection with these characters it is also important to note
that the blades of the 'mandible' show nowhere the faintest trace
of an articular facet for attachment to the cranium. In short, the
entire plan of the mandibular apparatus in these forms is strikingly
unfish-like, although one will frankly confess that it is remarkable
that these forms should have paralleled so strikingly the piscine
conditions, to the extent of producing mandibular rami margined with
teeth, and an arrangement of toothed elements on the 'upper jaw' which
resembles superficially the premaxillary and maxillary structures of
teleostomes, or the vomero-palatine structures of lung-fishes and
chimaeras.

"In the matter of paired fins there seems little evidence to conclude
that either pectoral or pelvic fins were present. In spite of the
researches upon these forms during the past half-century, no definite
remains of pectoral fins have been described. The so-called pectoral
spines described for _Dinichthys_ by Newberry, whatever they may be,
certainly are not, as far as the present evidence goes, pterygial, nor
are the similar structures in _Brachydirus_.[161] The sigmoid element,
described as a 'pelvic girdle' by Smith Woodward, in Coccosteus, a
structure which appears to occur in a small species of _Dinichthys_(?),
may as reasonably be interpreted as a displaced element of the
armor-plates of the trunk. In _Coccosteus_, as far as I am aware, it
occurs in well-preserved condition in but a single specimen.

"In referring to the singular joint between the shoulder-plates and the
hinder margin of the cranium Smith Woodward has called attention to
one of the striking features of the group. It is one, however, which,
as a functional structure, i.e., a joint, characterizes only a portion
of its members; and in these the region in which vestiges of the joint
are sought is overlaid and concealed by dermal plates. Such are the
conditions in _Macropetalichthys_ (with transitional characters in
_Trachosteus_ and in _Mylostoma_). For this form a special subclass (or
order) may be created which we may term Anarthrodira.

"SECOND. The _Arthrognathi_ cannot well be included in any other class.
It would certainly be more convenient to retain the Arthrognaths
among the Ostracophores, regarding them as a fourth subclass, were it
not that they differ from them in so marked a way in the presence of
well-marked vertebral arches, of supports for the unpaired fin, and in
the possession of 'jaws.' In these regards--add to them the (probable
if not certain) absence of the paired paddle-like 'spines'--they stand
certainly further from the Antiarcha than these from the Osteostraci,
or than the latter from the Heterostraci. It appears to me desirable,
therefore, that the Arthrodira and the Anarthrodira be brought together
as a separate class. Should subsequent researches demonstrate a closer
affinity with the Ostracophores, the Arthrognathi can be regarded as of
rank as a subclass, with the orders Anarthrodira and Arthrodira."[162]

In a recent paper Dr. Otto Jaekel unites Arthrodires and Ostracophores
under the name _Placodermi_. He regards _Pteraspis_ as a larval type,
_Asterolepis_ as one more specialized. In _Coccosteus_ he claims to
find a pelvic girdle as well as a more segmented skeleton. He regards
all of these as true fishes, the _Coccosteidae_ as ancestral, related
on the one hand to the _Crossopterygians_, and on the other to the
_Stegocephali_ and other ancestral Amphibians.

=Suborder Cycliae.=--We may append to the _Arthrodira_ as a possible
suborder the group called _Cycliae_ by Dr. Gill, based on a single
imperfectly known species. Few organisms discovered in recent times
have excited as much interest as this minute fish-like creature, called
_Palaeospondylus gunni_, discovered in 1890 by Dr. R. H. Traquair in the
flagstones of Caithness in Scotland. Many specimens have been obtained,
none more than an inch and a half long. Its structure and systematic
position have been discussed by Dr. R. H. Traquair, by Woodward, Gill,
Gegenbaur, and recently by Dean, from whose valuable memoir on "The
Devonian Lamprey" we make several quotations.

[Illustration: FIG. 369.--_Palaeospondylus gunni_ Traquair. Devonian.
(After Traquair and Dean.)]

=Palaeospondylus.=--According to Dr. Traquair: "The _Palaeospondylus
gunni_ is a very small organism, usually under one inch in length,
though exceptionally large specimens occasionally measure one inch and
a half.... It has a head and vertebral column, but no trace of jaws or
limbs; and, strange to say, all the specimens are seen only from the
ventral aspect, as is shown by the relation of the neural arches to the
vertebral centra.

"The head is in most cases much eroded.... It is divided by a
notch ... into two parts.... The _anterior part_ shows a groove the
edges of which are elevated, while the surface on each side shows two
depressions, like fenestrae, though perhaps they are not completely
perforated, and also a groove partially divided off, posteriorly
and externally, a small lobe. In front there is a ring-like
opening ... surrounded by small pointed cirri, four ventrally, at least
five dorsally, and two long lateral ones which seem to arise inside
the margin of the ring instead of from its rim like the others. The
_posterior part_ of the cranium is flattened, but the median groove is
still observable. Connected with the posterior or occipital aspect of
the skull are two small narrow plates which lie closely alongside the
first half-dozen vertebrae.

"The bodies of the vertebrae are hollow or ring-like, and those
immediately in front are separated from each other by perceptible
intervals; their surfaces are marked with a few little longitudinal
grooves, of which one is median. They are provided with neural arches,
which are at first short and quadrate, but towards the caudal extremity
lengthen out into slender neural spines, which form the dorsal
expansion of a caudal fin, while shorter haemal ones are also developed
on the ventral aspect."

Dr. Traquair concludes that "there seems to be no escape from
the conclusion that the little creature must be classed as a
Marsipobranch." "If _Palaeospondylus_ is not a Marsipobranch, it
is quite impossible to refer it to any other existing group of
vertebrates."

=Gill on Palaeospondylus.=--In 1896 Dr. Gill proposed to regard
_Palaeospondylus_ provisionally as the type of a distinct order of
Cyclostomes to be called _Cycliae_ (~kyklos~, circle), from the median
ring on the head, whether nostril or mouth. Dr. Gill observes:

"Assuming the correctness of Dr. Traquair's description and figures, we
certainly have a remarkable combination of characters. On the one hand,
if the 'median opening or rim' is indeed nasal, the animal certainly
cannot be referred to the class of Selachians or of Teleostomes.
On the other hand, the cranium and the segmental vertebral column
indicate a more advanced stage of development of the vertebrate line
than that from the living Marsipobranchs must have originated. We
may, therefore, with propriety isolate it as the representative not
only of a peculiar family (_Palaeospondylidae_), but of an order or even
subclass (Cycliae) of vertebrates which may provisionally (and only
provisionally) be retained in the class of Marsipobranchs.

"The group may be defined as Monorrhines with a continuous (?) cranium,
a median nasal (?) ring, and a segmented vertebral column.

"The differences between the _Hyperoartia_ and _Hyperotreta_ are very
great, and Prof. Lankester did not go much too far when he elevated
those groups to class rank. Among the numerous distinctive characters
are the great differences in the auditory organs. Perhaps the organs
of _Palaeospondylus_ might be worked out in some specimen and throw
light on the subject of affinities. At present even the region of
the auditory organs is not exactly known and we are now at a loss to
orient the several parts of the cranium. In fact, the question of the
relations of _Palaeospondylus_ is a very open one."

=Views as to the Relationships of Palaeospondylus.=--Dr. Dean thus
summarizes in a convenient and interesting fashion the views of
different students of fossil fishes in regard to _Palaeospondylus_:

=Huxley.=--A "baby _Coccosteus_."

=Traquair, 1890.=--"Certainly not a Placoderm, its resemblance to a
supposed 'baby Coccosteus' being entirely deceptive. The appearance
of the head does remind us in a strange way of the primitive skull
of _Myxine_, a resemblance which is rendered still more suggestive
by the apparent complete absence of the lower jaw, or of limbs or
limb-girdles."

=Traquair, 1893.=--"It seems, indeed, impossible to refer the organism
to any existing vertebrate class, unless it be the Marsipobranchs or
Cyclostomata." Does not believe it a larval form, because the possible
adult is unknown, and because of the highly differentiated vertebrae.
Granting his interpretation of the parts of the fossil, "there seems no
escape from the conclusion that the little creature must be classed as
a Marsipobranch."

=Traquair, 1897.=--"The question of the affinities of _Palaeospondylus_
is left precisely where it was after I had written my last paper on the
subject."

=Smith Woodward, 1892.=--"It seems to possess an unpaired nose, lip
cartilages in place of functional jaws, and no paired limbs; thus
agreeing precisely with the lampreys and hagfishes, of which the fossil
representatives have long been sought. It is extremely probable,
therefore, that _Palaeospondylus_ belongs to this interesting category."

=Dawson, 1893.=--_Palaeospondylus_ suggests "the smaller snake-like
Batrachians of the Carboniferous and Permian; and I should not be
surprised if it should come to be regarded as either a forerunner of
the Batrachians or as a primitive tadpole."

=Gill, 1896.=--"The group to which _Palaeospondylus_ belongs may be
defined as Monorrhines with a continuous (?) cranium, a median nasal
(?) ring, and a segmented vertebral column." "The cranium and segmented
vertebral column indicate a more advanced stage of development of the
vertebrate line than that from which the living Marsipobranchs must
have originated. We may, therefore, with propriety isolate it as the
representative not only of a peculiar family (_Palaeospondylidae_),
but of an order or even subclass (_Cycliae_) of vertebrates which may
provisionally (and only provisionally) be retained in the class of
Marsipobranchs."

=Dean, 1896.=--"Place it with the Ostracoderms among the curiously
specialized offshoots of the early Chordates, but this position would
be at the best unsatisfactory."

=Dean, 1898.=--"_Palaeospondylus_ should not be given a place--even a
provisional one--among the Marsipobranchs." To be accepted "as the
representative of the new subclass (or class) Cycliae constituted for it
by Professor Gill."

=Parker & Haswell, 1897.=--"There is some reason to regard that
_Palaeospondylus_ is referable to the Cyclostomes." "A distinctly higher
type than recent forms."

=Gegenbaur, 1898.=--"Discovery of _Palaeospondylus_ one of the highest
importance. If this organism stands in no way near the Cyclostomes, the
tentacles lose their higher importance, since they also occur in other
groups." "Through _Palaeospondylus_ came also the attempt (Pollard) to
deduce the presence of the tentacular condition in the higher forms."
(_Mem._--In this Gegenbaur has not consulted the literature accurately.
At the time of founding his "Cirrhostomal Theory" Pollard was unaware
of the discovery of _Palaeospondylus_). "Ich muss sagen, das die
positive Behauptung der einen wie der anderen Deutung mir sehr unsicher
scheint, da auch an den uebrigen Resten des Kopfskelets keine bestimmten
Uebereinstimmungen mit anderen Organismen erweisbar sind. Es ist daher
auch nicht zu vermuthen, dass sogar an Beziehung zu Froschlarven
gedacht ward. Unter diesen Umstaenden moechte ich jene im Verhaeltniss
zum Kopfe wie zum gesammten Koerper bedeutende, von Cirren umstellte
Eingangsoeffnung als nicht einer Nase, sondern einem Munde oder beiden
zugleich angehoerig betrachten. Zu einem dem Cyclostomenriechorgan
vergleichbaren Verhalten fehlen alle Bedingungen."

=Relationships of Palaeospondylus.=--The arguments for and against the
supposition that _Palaeospondylus_ is a Cyclostome may be here summed up
after Professor Dean.

The vertebral column agrees with that of the lamprey in having the
notochord in part persistent. On the other hand, the vertebrae have
continuous centra, showing definite processes. Those of the different
regions are differentiated. These conditions are quite unlike those
seen in the lamprey.

The cranium is massive, over twice as large proportionally as that
of the lamprey. In the latter type the cranium forms but a small
portion of the bulk of the head; in _Palaeospondylus_, on the other
hand, the cranium bears every sign of having filled the contour of the
head. Moreover, if the region adjacent to the structure is admitted
to be that of the eye, and few, I believe, will doubt it, then the
brain-cavity must, by many analogies, have been much larger than that
of a Marsipobranch. Also the auditory capsules must have been of
extraordinary size. In short, there is very little about the cranium to
suggest the structures of Cyclostomes.

The "oral cirri" suggest somewhat the barbels of the nose and mouth
of a hagfish. They, however, resemble even as much in arrangement
and greater number the buccal cirri of _Amphioxus_. On the other
hand, similar mouth-surrounding tentacles are evolved independently
in many groups of fishes, siluroids, sharks, forms like _Pogonias_,
_Hemitripterus_. A possibility further exists that the "cirri" may
turn out to be remnants of cranial or facial structures of an entirely
different nature. In fact the very uncertain preservation of these
parts renders their evidence of little definite value. In but one
specimen, as far as I am aware, is there any evidence of the presence
of ventral cirri.

The jaw parts in _Palaeospondylus_ are unknown. It is possible that the
ventral rim of the "nasal ring" may prove to be the remains of the
Meckelian cartilage (the cartilaginous core of the lower jaw).

It is possible that certain very faint ray-like markings noted by
Professor Dean may be the basalia of paired fins. In such case
_Palaeospondylus_ can have no affinity with the lampreys. Dr. Dean
asserts that the presence of these, in view of the wide dissimilarity
in other and important structures, is sufficient to remove
_Palaeospondylus_ from its provisional position among the Cyclostomes.
The postoccipital plates may represent a pectoral arch. It is, however,
much more likely, as Dr. Traquair has insisted, that the supposed rays
are due to the reflection of light from striations on the stone, and
that the creature had no pectoral limbs.

The caudal fin, with its dichotomous rays, is essentially like the
tail of a lamprey. This condition is, however, found in other groups
of fishes, as among sharks and lung-fishes. It is, moreover, doubtful
whether the rays are really dichotomous.

It is possible that _Palaeospondylus_ may be, as Huxley suggests,
a larval Arthrodire. It is not probable that this is the case,
but, on the other hand, _Palaeospondylus_ seems to be an immature
form. According to Dr. Dean, it is more likely to prove a larval
_Coccosteus_, or the young of some other Arthrodire, than a
lamprey. Against this view must be urged the fact that the tail of
_Palaeospondylus_ is not heterocercal, a fact verified by Dr. Traquair
on all of his many specimens. It is more like the tail of a lamprey
than that of _Coccosteus_. It is, however, certain that it cannot be
placed in the same class with the living _Cyclostomes_, and that it is
far more highly specialized than any of them. In a still later paper
(1904) Dr. Dean shows that the fossil might as easily be considered a
Chimaera as a lamprey, and repeats his conviction that it is a larval
form of which the adult is still unrecognized.

We cannot go much farther than Dr. Dean's statement in 1896, that
it belongs "among the curiously specialized offshoots of the early
Chordates."

FOOTNOTES:

[157] "The name _Arthrodira_ as given to Coccosteans, as distinguished
from the _Antiarcha_, is not altogether a satisfactory one, since at
least from the time of Pander the head of Pterichthys (_Asterolepis_)
is known to be articulated with the armoring of the trunk in a way
closely resembling that of _Coccosteus_. This term may, however,
be retained as a convenient one for the order of Coccosteans, in
which, together with other differentiating features, this structure
is prominently evolved. A renewed examination of the subject has
caused me to incline strongly to the belief, as above expressed, that
_Pterichthys_ and Coccosteans are not as widely separated in phylogeny
as Smith Woodward, for example, has maintained. But, as far as present
evidence goes, they appear to me certainly as distinct as fishes are
from amphibia, or as reptiles are from birds or from mammals." (DEAN.)

The name _Placodermi_ used by McCoy in 1848 was applied to the
_Ostracophores_ as well as to the _Arthrodires_. Hay revives it as the
name of a superorder to include the _Antiarcha_ and the _Arthrodira_,
the former being detached from the _Ostracophores_. This superorder is
equivalent to the subclass _Azygostei_ of Hay.

[158] Memoirs New York Academy of Sciences, 1901.

[159] It will be recalled that there is no ground for concluding
that the "mandibular rami" possessed an endoskeletal core, and were
comparable, therefore, to the somewhat mobile jaws of Elasmobranchs.
On the other hand, there is the strongest evidence that they are
entirely comparable to adjacent dermal plates. Histologically they are
identical, and in certain cases their exposed surfaces bear the same
tuberculation.

[160] The similarity of _Macropetalichthys_ to Dinichthyids in the
general matter of the dermal plates is so complete that I have had no
hesitation in associating it with the Arthrognaths. (_Cf._ Eastman.)
The circumstance that its "jaws" have not yet been found has to a large
degree been due to the lack of energy on the part of local collectors.
In the corniferous quarries near Delaware, Ohio, this fossil is stated
to be relatively abundant.

[161] It is by no means impossible that there may ultimately be found
pectoral elements to correspond in a general way with the paddle-like
"spines" of the Antiarcha.

[162] The group Placodermi, created by McCoy (1848) as a "family"
for the reception of Coccosteus and Pterichthys might then be justly
elevated to rank as a class, superseding the Ostracophori of Cope
(1891). The latter group might, however, be retained as a subclass, and
include the Heterostraci and Osteostraci as ordinal divisions.




CHAPTER XXXIV

THE CROSSOPTERYGII


=Class Teleostomi.=--We may unite the remaining groups of fishes into a
single class, for which the name _Teleostomi_ (~teleos~, true; ~stoma~,
mouth), proposed by Bonaparte in 1838, may be retained. The fishes of
this class are characterized by the presence of a suspensorium to the
mandible, by the existence of membrane-bones (opercles, suborbitals,
etc.) on the head, by a single gill-opening leading to gill-arches
bearing filamentous gills, and by the absence of claspers on the
ventral fins. The skeleton is at least partly ossified in all the
_Teleostomi_. More important as a primary character, distinguishing
these fishes from the sharks, is the presence typically and primitively
of the air-bladder. This is at first a lung, arising as a diverticulum
from the ventral side of the oesophagus, but in later forms it becomes
dorsal and is, by degrees, degraded into a swim-bladder, and in very
many forms it is altogether lost with age.

This group comprises the vast majority of recent fishes, as well as a
large percentage of those known only as fossils. In these the condition
of the lung can be only guessed.

The _Teleostomi_ are doubtless derived from sharks, their relationship
being possibly nearest to the _Ichthyotomi_ or to the primitive
_Chimaeras_. The Dipnoans among _Teleostomi_ retain the shark-like
condition of the upper jaw, made of palatal elements, which may be, as
in the _Chimaera_, fused with the cranium. In the lower forms also the
primitive diphycercal or protocercal form of tail is retained, as also
the archipterygium or jointed axis of the paired fins, fringed with
rays on one or both sides.

We may divide the Teleostomes, or true fishes, into three subclasses:
the _Crossopterygii_, or fringe-fins; the _Dipneusti_, or lung-fishes;
_Actinopteri_, or ray-fins, including the _Ganoidei_ and the
_Teleostei_, or bony fishes. Of these many recent writers are disposed
to consider the _Crossopterygii_ as most primitive, and to derive from
it by separate lines each of the remaining subclasses, as well as the
higher vertebrates. The _Ganoidei_ and _Teleostei_ (constituting the
_Actinopteri_) are very closely related, the ancient group passing by
almost imperceptible degrees into the modern group of bony fishes.

=Subclass Crossopterygii.=--The earliest Teleostomes known belong to
the subclass or group called after Huxley, _Crossopterygii_ (~krossos~,
fringe; ~pteryx~, fin). A prominent character of the group lies in
the retention of the jointed pectoral fin or archipterygium, its axis
fringed by a series of soft rays. This character it shares with the
_Ichthyotomi_ among sharks, and with the _Dipneusti_. From the latter
it differs in the hyostylic cranium, the lower jaw being suspended from
the hyomandibular, and by the presence of distinct premaxillary and
maxillary elements in the upper jaw. In these characters it agrees with
the ordinary fishes. In the living Crossopterygians the air-bladder is
lung-like, attached by a duct to the ventral side of the oesophagus.
The lung-sac, though specialized in structure, is simple, not cellular
as in the Dipnoans. The skeleton is more or less perfectly ossified.
Outside the cartilaginous skull is a bony coat of mail. The skin is
covered with firm scales or bony plates, the tail is diphycercal,
straight, and ending in a point, the shoulder-girdle attached to
the cranium is cartilaginous but overlaid with bony plates, and the
branchiostegals are represented by a pair of gular plates.

In the single family represented among living fishes the heart has
a muscular arterial bulb with many series of valves on its inner
edge, and the large air-bladder is divided into two lobes, having the
functions of a lung, though not cellular as in the lung-fishes.

The fossil types are very closely allied to the lung-fishes, and the
two groups have no doubt a common origin in Silurian times. It is now
usually considered that the Crossopterygian is more primitive than the
lung-fish, though at the same time more nearly related to the Ganoids,
and through them to the ordinary fishes.

=Origin of Amphibians.=--From the primitive _Crossopterygii_ the
step to the ancestral _Amphibia_, which are likewise mailed and
semi-aquatic, seems a very short one. It is true that most writers
until recently have regarded certain Dipneustans as the _Dipteridae_
as representing the parents of the Amphibians. But the weight of
recent authority, Gill, Pollard, Boulenger, Dollo, and others, seems
to place the point of separation of the higher vertebrates with
the Crossopterygians, and to regard the lobate pectoral member of
_Polypterus_ as a possible source of the five-fingered arm of the frog.
This view is still, however, extremely hypothetical and there is still
much to be said in favor of the theory of the origin of Amphibia from
Dipnoans and in favor of the view that the Dipnoans are also ancestors
of the Crossopterygians.

[Illustration: FIG. 370.--Shoulder-girdle of _Polypterus bichir_.
Specimen from the White Nile.]

In the true Amphibians the lungs are better developed than in the
Crossopterygian or Dipnoan, although the lungs are finally lost in
certain salamanders which breathe through epithelial cells. The
gills lose, among the Amphibia, their primitive importance, although
in _Proteus anguineus_ of Austria and _Necturus maculosus_, the
American "mud-puppy" or water-dog, these persist through life. The
archipterygium, or primitive fin, gives place to the chiropterygium,
or fingered arm. In this the basal segment of the archipterygium
gives place to the humerus, the diverging segments seen in the most
specialized type of archipterygium (_Polypterus_) become perhaps radius
and ulna, the intermediate quadrate mass of cartilage possibly becoming
carpal bones, and from these spring the joints called metacarpals and
phalanges. In the Amphibians and all higher forms the shoulder-girdle
retains its primitive insertion at a distance from the head, and the
posterior limbs remain abdominal.

The Amphibians are therefore primarily fishes with fingers and toes
instead of the fringe-fins of their ancestors. Their relations are
really with the fishes, as indicated by Huxley, who unites the
amphibians and fishes in a primary group, _Ichthyopsida_, while
reptiles and birds form the contrasting group of _Sauropsida_.

[Illustration: FIG. 371.--Arm of a frog.]

The reptiles differ from the Amphibians through acceleration of
development, passing through the gill-bearing stages within the
egg. The birds bear feathers instead of scales, and the mammals
nourish their young by means of glandular secretions. Through a
reptile-amphibian ancestry the birds and mammals may trace back their
descent from palaeozoic Crossopterygians. In the very young embryo
of all higher vertebrates traces of double-breathing persist in all
species, in the form of rudimentary gill-slits.

=The Fins of Crossopterygians.=--Dollo and Boulenger regard the
heterocercal tail as a primitive form, the diphycercal form being a
result of degradation, connected with its less extensive use as an
organ of propulsion. Most writers who adopt the theory of Gegenbaur
that the archipterygium is the primitive form of the pectoral fin are
likely, however, to consider the diphycercal tail found associated
with it in the _Ichthyotomi_, _Dipneusti_, _Crossopterygii_ as the
more primitive form of the tail. From this form the heterocercal tail
of the higher sharks and Ganoids may be derived, this giving way in
the process of development to the imperfectly homocercal tail of the
salmon, the homocercal tail of the perch, and the isocercal tail of the
codfish and its allies, the gephyrocercal and the leptocercal tail,
tapering or whip-like, representing various stages of degeneration.
Boulenger draws a distinction between the protocercal tail, the one
primitively straight, and the diphycercal tail modified, like the
homocercal tail, from an heterocercal ancestry.

[Illustration: FIG. 372.--_Polypterus congicus_, a Crossopterygian fish
from the Congo River. Young, with external gills. (After Boulenger.)]

=Orders of Crossopterygians.=--Cope and Woodward divide the
_Crossopterygia_ into four orders or suborders, _Haplistia_,
_Rhipidistia_, _Actinistia_, and _Cladistia_. To the latter belong the
existing species, or the family of _Polypteridae_, alone. Boulenger
unites the three extinct orders into one, which he calls _Osteolepida_.
In all three of these the pectorals are narrow with a single basal
bone, and the nostrils, as in the Dipneustans, are below the snout.
The differences are apparently such as to justify Cope's division into
three orders.

=Haplistia.=--In the _Haplistia_ the notochord is persistent, and
the basal bones of dorsal and anal fins are in regular series, much
fewer in number than the fin-rays. The single family _Tarrassiidae_
is represented by _Tarrasius problematicus_, found by Traquair in
Scotland. This is regarded as the lowest of the Crossopterygians, a
small fish of the Lower Carboniferous, the head mailed, the body with
small bony scales.

=Rhipidistia.=--In the _Rhipidistia_ the basal bones of the median fins
("axonosts and baseosts") are found in a single piece, not separate
as in the _Haplistia_. Four families are recognized, _Holoptychiidae_,
_Megalichthyidae_, _Osteolepidae_, and _Onychodontidae_, the first of
these being considered as the nearest approach of the Crossopterygians
to the Dipnoans.

The _Holoptychiidae_ have the pectoral fins acute, the scales cycloid,
enameled, and the teeth very complex. _Holoptychius nobilissimus_ is a
very large fish from the Devonian. _Glyptolepis leptopterus_ from the
Lower Devonian is also a notable species. _Dendrodus_ from the Devonian
is known from detached teeth.

[Illustration: FIG. 373.--Basal bone of dorsal fin, _Holoptychius
leptopterus_ (Agassiz). (After Woodward.)]

In the Ordovician rocks of Canon City, Colorado, Dr. Walcott finds
numerous bony scales with folded surfaces and stellate ornamentation,
and which he refers with some doubt to a Crossopterygian fish of the
family _Holoptychiidae_. This fish he names _Eriptychius americanus_. If
this identification proves correct, it will carry back the appearance
of Crossopterygian fishes, the earliest of the Teleostome forms, to the
beginning of the Silurian, these Canon City shales being the oldest
rocks in which remains of fishes are known to occur. In the same rocks
are found plates of Ostracophores and other fragments still more
doubtful. It is certain that our records in palaeontology fall far short
of disclosing the earliest sharks, as well as the earliest remains of
Ostracophores, Arthrodires, or even Ganoids.

=Megalichthyidae.=--The _Megalichthyidae_ (wrongly called
"_Rhizodontidae_") have the pectoral fins obtuse, the teeth relatively
simple, and the scales cycloid, enameled. There are numerous species
in the Carboniferous rocks, largely known from fragments or from
teeth. _Megalichthys_, _Strepsodus_, _Rhizodopsis_, _Gyroptychius_,
_Tristichopterus_, _Eusthenopteron_, _Cricodus_, and _Sauripterus_ are
the genera; _Rhizodopsis sauroides_ from the coal-measures of England
being the best-known species.

The _Osteolepidae_ differ from the _Megalichthyidae_ mainly in the
presence of enameled rhomboid scales, as in _Polypterus_ and
_Lepisosteus_. In _Glyptopomus_ these scales are sculptured, in
the others smooth. In _Osteolepis_, _Thursius_, _Diplopterus_, and
_Glyptopomus_ a pineal foramen is present on the top of the head.
This is wanting in _Parabatrachus_ (_Megalichthys_ of authors). In
_Osteolepis_, _Thursius_, and _Parabatrachus_ the tail is heterocercal,
while in _Diplopterus_ and _Glyptopomus_ it is diphycercal. _Osteolepis
macrolepidotus_ and numerous other species occur in the Lower Devonian.
_Diplopterus agassizii_ is common in the same horizon. _Megalichthys
hibberti_ is found in the coal-measures, and _Glyptopomus minimus_ in
the Upper Devonian. _Palaeosteus_ is another genus recently described.

[Illustration: FIG. 374.--_Gyroptychius microlepidotus_ Agassiz.
Devonian. Family _Megalichthyidae_. (After Pander.)]

The _Onychodontidae_ are known from a few fragments of _Onychodus
sigmoides_ from the Lower Devonian of Ohio and _Onychodus anglicus_
from England.

[Illustration: FIG. 375.--_Coelacanthus elegans_ Newberry. From the
Ohio Carboniferous, showing air-bladder. (After Dean.)]

=Order Actinistia.=--In the _Actinistia_ there is a single fin-ray to
each basal bone, the axonosts of each ray fused in a single piece. The
notochord is persistent, causing the back-bone in fossils to appear
hollow, the cartilaginous material leaving no trace in the rocks. The
genera and species are numerous, ranging from the Subcarboniferous to
the Upper Cretaceous, many of them belonging to _Coelacanthus_, the
chief genus of the single family _Coelacanthidae_. In _Coelacanthus_
the fin-rays are without denticles. _Coelacanthus granulatus_ is found
in the European Permian. _Coelacanthus elegans_ of the coal-measures
is found in America also. In _Undina_ the anterior fin-rays are marked
with tubercles. _Undina penicillata_ and _Undina gulo_ from the
Triassic are well-preserved species. In _Macropoma_ (_lewesiensis_)
the fin-rays are robust, long, and little articulated. Other genera
are _Heptanema_, _Coccoderma_, _Libys_, _Diplurus_, and _Graphiurus_.
_Diplurus longicaudatus_ was found by Newberry in the Triassic of New
Jersey and Connecticut.

[Illustration: FIG. 376.--_Undina gulo_ Egerton; Lias. Family
_Coelacanthidae_. (After Woodward.)]

=Order Cladistia.=--In the _Cladistia_ the axis of the pectoral limb
is fan-shaped, made of two diversified bones joined by cartilage.
The notochord is restricted and replaced by ossified vertebrae. The
axonosts of the dorsal and anal are in regular series, each bearing
a fin-ray. The order contains the single family _Polypteridae_. In
this group the pectoral fin is formed differently from that of the
other Crossopterygians, being broad, its base of two diverging bones
with cartilage between. This structure, more specialized than in
any other of the Crossopterygians or _Dipneusti_, has been regarded
by Gill and others, as above stated, as the origin of the fingered
hand (chiropterygium) of the frogs and higher vertebrates. The base
of the diverging bones has been identified as the antecedent of the
humerus, the bones themselves as radius and ulna, while the intervening
non-ossified cartilage breaks up into carpal bones, from which
metacarpals and digits ultimately diverge. This hypothesis is open to
considerable doubt. The nostrils, as in true fishes, are superior.
The body in these fishes is covered with rhombic enameled scales, as
in the garpike; the head is similarly mailed, but, in distinction from
the garpike, the anterior rays of the dorsal are developed as isolated
spines.

The young have a bushy external gill with a broad scaly base. The
air-bladder is double, not cellular, with a large air-duct joining the
ventral surface of the oesophagus. The intestine has a spiral valve.

The cranium, according to Boulenger ("Poissons du Bassin du Congo," p.
11), is remarkable for its generalized form, this character forming
a trait of union between the Ganoids and the primitive _Amphibia_ or
_Stegocephali_. Without considering _Polypterus_, it is not possible
to interpret the homologies of the cranium of the amphibians and the
sharks.

The jaws are similar to those of the vertebrates higher than fishes.
Tooth-bearing premaxillaries and dentaries are solidly joined at the
front of the cranium, and united by a suture to the toothed maxillaries
which form most of the edge of the mouth. Each half of the lower jaw
consists of four elements, covering Meckel's cartilage, which is
ossified at the symphysis. These are the articular, angular, dentary,
and splenial (coronoid). Most of these bones are armed with teeth. The
palato-suspensory consists of hyomandibular, quadrate, ectopterygoid,
entopterygoid, metapterygoid, and palatine elements, the pterygoid
elements bearing teeth. In _Erpetoichthys_ only the opercle is distinct
among the gill-covers. In _Polypterus_ there is a subopercle also; the
suborbital chain is represented by two small bones.

The gill-arches are four, but without lower pharyngeals. The teeth are
conic and pointed, and in structure, according to Agassiz, they differ
largely from those of bony fishes, approaching the teeth of reptiles.

[Illustration: FIG. 377.--Lower jaw of _Polypterus bichir_, from below.]

The external gill of the young, first discovered by Steindachner in
1869, consists of a fleshy axis bordered above and below by secondary
branches, themselves fringed. In form and structure this resembles the
external gills of amphibians. It is inserted, not on the gill-arches,
but on the hyoid arch. Its origin is from the external skin. It can
therefore not be compared morphologically with the gills of other
fishes, nor with the pseudobranchiae, but rather with the external gills
of larval sharks. The vertebrae are very numerous and biconcave as in
ordinary fishes. Each of the peculiar dorsal spines is primitively a
single spine, not a finlet of several pieces, as some have suggested.
The enameled, rhomboid scales are in movable oblique whorls, each scale
interlocked with its neighbors.

[Illustration: FIG. 378.--_Polypterus congicus_, a Crossopterygian fish
from the Congo River. Young, with external gills. (After Boulenger.)]

[Illustration: FIG. 379.--_Polypterus delhezi_ Boulenger. Congo River.]

The shoulder-girdle, suspended from the cranium by post-temporal and
supraclavicle, is covered by bony plates. To the small hypercoracoid
and hypocoracoid the pectoral fin is attached. Its basal bones may
be compared to those of the sharks, mesopterygium, propterygium, and
metapterygium, which may with less certainty be again called humerus,
radius, and ulna. These are covered by flesh and by small imbricated
scales. The air-bladder resembles the lungs of terrestrial vertebrates.
It consists of two cylindrical sacs, that on the right the longer, then
uniting in front to form a short tube, which enters the oesophagus from
below with a slit-like glottis. Unlike the lung of the _Dipneusti_,
this air-bladder is not cellular, and it receives only arterial blood.
Its function is to assist the respiration by gills without replacing
it.

=The Polypteridae.=--All the _Polypteridae_ are natives of Africa. Two
genera are known, no species having been found fossil. Of _Polypterus_,
Boulenger, the latest authority, recognizes nine species: six in the
Congo, _Polypterus congicus_, _P. delhezi_, _P. ornatipinnis_, _P.
weeksi_, _P. palmas_, and _P. retropinnis_; one, _P. lapradei_, in the
Niger; and two in the Nile, _Polypterus bichir_ and _P. endlicheri_. Of
these the only one known until very recently was _Polypterus bichir_ of
the Nile.

These fishes in many respects resemble the garpike in habits. They live
close on the mud in the bottom of sluggish waters, moving the pectorals
fan-fashion. If the water is foul, they rise to the surface to gulp
air, a part of which escapes through the gill-openings, after which
they descend like a flash. In the breeding season these fishes are very
active, depositing their eggs in districts flooded in the spring. The
eggs are very numerous, grass-green, and of the size of eggs of millet.
The flesh is excellent as food.

[Illustration: FIG. 380.--_Erpetoichthys calabaricus_ Smith.
Senegambia. (After Dean.)]

The genus _Erpetoichthys_ contains a single species, _Erpetoichthys
calabaricus_,[163] found also in the Senegal and Congo. This species
is very slender, almost eel-like, extremely agile, and, as usual in
wriggling or undulating fishes, it has lost its ventral fin. It lives
in shallow waters among interlaced roots of palms. When disturbed it
swims like a snake.

FOOTNOTES:

[163] This genus was first called _Erpetoichthys_, but the name was
afterwards changed by its author, J. A. Smith, to _Calamoichthys_,
because there is an earlier genus _Erpichthys_ among blennies, and a
_Herpetoichthys_ among eels. But these two names, both wrongly spelled
for _Herpetichthys_, are sufficiently different, and the earlier name
should be retained. "A name in science is a name without necessary
meaning" and without necessarily correct spelling. Furthermore, if
names are spelled differently, they are different, whatever their
meaning. The efforts of ornithologists, notably those of Dr. Coues, to
spell correctly improperly formed generic names have shown that to do
so consistently would throw nomenclature into utter confusion. It is
well that generic names of classic origin should be correctly formed.
It is vastly more important that they should be stable. Stability is
the sole function of the law of priority.




CHAPTER XXXV

SUBCLASS DIPNEUSTI,[164] OR LUNGFISHES


=The Lungfishes.=--The group of Dipneusti, or lung-fishes, is
characterized by the presence of paired fins consisting of a jointed
axis with or without rays. The skull is autostylic, the upper jaw being
made as in the Chimaera of palatal elements joined to the quadrate and
fused with the cranium, without premaxillary or maxillary. The dentary
bones are little developed. The air-bladder is cellular, used as a lung
in all the living species, its duct attached to the ventral side of the
oesophagus. The heart has many valves in the muscular arterial bulb.
The intestine has a spiral valve. The teeth are usually of large plates
of dentine covered with enamel, and are present on the pterygo-palatine
and splenial bones. The nostrils are concealed, when the mouth is
closed, under a fold of the upper lip. The scales are cycloid, mostly
not enameled.

[Illustration: FIG. 381.--Shoulder-girdle of _Neoceratodus forsteri_
Guenther. (After Zittel.)]

The lung-fishes, or _Dipneusti_ (~dis~, two; ~pnein~, to breathe),
arise, with the Crossopterygians, from the vast darkness of Palaeozoic
time, their origin with that or through that of the latter to be traced
to the Ichthyotomi or other primitive sharks. These two groups are
separated from all the more primitive fish-like vertebrates by the
presence of lungs. In its origin the lung or air-bladder arises as a
diverticulum from the alimentary canal, used by the earliest fishes
as a breathing-sac, the respiratory functions lost in the progress of
further divergence. Nothing of the nature of lung or air-bladder is
found in lancelet, lamprey, or shark. In none of the remaining groups
of fishes is it wholly wanting at all stages of development, although
often lost in the adult. Among fishes it is most completely functional
in the _Dipneusti_, and it passes through all stages of degeneration
and atrophy in the more specialized bony fishes.

In the _Dipneusti_, or Dipnoans, as in the Crossopterygians and the
higher vertebrates, the trachea, or air-duct, arises, as above stated,
from the ventral side of the oesophagus. In the more specialized
fishes, yet to be considered, it is transferred to the dorsal side,
thus avoiding a turn in passing around the oesophagus itself. From
the sharks these forms are further distinguished by the presence of
membrane-bones about the head. From the _Actinopteri_ (Ganoids and
Teleosts) Dipnoans and Crossopterygians are again distinguished by
the presence of the fringe-fin, or archipterygium, as the form of
the paired limbs. From the Crossopterygians the Dipnoans are most
readily distinguished by the absence of maxillary and premaxillary,
the characteristic structures of the jaw of the true fish. The upper
jaw in the Dipnoan is formed of palatal elements attached directly to
the skull, and the lower jaw contains no true dentary bones. The skull
in the Dipnoans, as in the _Chimaera_, is autostylic, the mandible
articulating directly with the palatal apparatus, the front of which
forms the upper jaw and of which the pterygoid, hyomandibular and
quadrate elements form an immovable part. The shoulder-girdle, as in
the shark, is a single cartilage, but it supports a pair of superficial
membrane-bones.

In all the Dipnoans the trunk is covered with imbricated cycloid
scales and no bony plates, although sometimes the scales are firm and
enameled. The head has a roof of well-developed bony plates made of
ossified skin and not corresponding with the membrane-bones of higher
fishes. The fish-like membrane-bones, opercles, branchiostegals, etc.,
are not yet differentiated. The teeth have the form of grinding-plates
on the pterygoid areas of the palate, being distinctly shark-like
in structure. The paired fins are developed as archipterygia, often
without rays, and the pelvic arch consists of a single cartilage,
the two sides symmetrical and connected in front. There is but one
external gill-opening leading to the gill-arches, which, as in ordinary
fishes, are fringe-like, attached at one end. In the young, as with
the embryo shark, there is a bushy external gill, which looks not
unlike the archipterygium pectoral fin itself, although its rays are of
different texture. In early forms, as in the Ganoids, the scales were
bony and enameled, but in some recent forms deep sunken in the skin.
The claspers have disappeared, the nostrils, as in the frog, open into
the pharynx, the heart is three-chambered, the arterial bulb with many
valves, and the cellular structure of the skin and of other tissues is
essentially as in the Amphibian.

The developed lung, fitted for breathing air, which seems the most
important of all these characters, can, of course, be traced only in
the recent forms, although its existence in all others can be safely
predicated. Besides the development of the lung we may notice the
gradual forward movement of the shoulder-girdle, which in most of the
Teleostomous fishes is attached to the head. In bony fishes generally
there is no distinct neck, as the post-temporal, the highest bone of
the shoulder-girdle, is articulated directly with the skull. In some
specialized forms (_Balistes_, _Tetraodon_) it is even immovably fused
with it. In a few groups (_Apodes_, _Opisthomi_, _Heteromi_, etc.)
this connection ancestrally possessed is lost through atrophy and the
slipping backward of the shoulder-girdle leaves again a distinct neck.
In the Amphibians and all higher vertebrates the shoulder-girdle is
distinct from the skull, and the possession of a flexible neck is an
important feature of their structure. In all these higher forms the
posterior limbs remain abdominal, as in the sharks and the primitive
and soft-rayed fishes generally. In these the pelvis or pelvic elements
are attached toward the middle of the body, giving a distinct back as
well as neck. In the spiny-rayed fishes the "back" as well as the neck
disappears, the pelvic elements being attached to the shoulder-girdle,
and in a few extreme forms (as _Ophidion_) the pelvis is fastened at
the chin.

=Classification of Dipnoans.=--By Woodward the _Dipneusti_ are divided
into two classes, the _Sirenoidei_ and the _Arthrodira_. We follow Dean
in regarding the latter as representative of a distinct class, leaving
the _Sirenoidei_, with the _Ctenodipterini_, to constitute the subclass
of _Dipneusti_. The _Sirenoidei_ are divided by Gill into two orders,
the _Monopneumona_, with one lung, and the _Diplopneumona_, with the
lung divided. To the latter order the _Lepidosirenidae_ belong. To the
former the _Ceratodontidae_, and presumably the extinct families also
belong, although nothing is known of their lung structures. Zittel
and Hay adopt the names of _Ctenodipterini_ and _Sirenoidei_ for
these orders, the former being further characterized by the very fine
fin-rays, more numerous than their supports.

=Order Ctenodipterini.=--In this order the cranial roof-bones are small
and numerous, and the rays of the median fins are very slender, much
more numerous than their supports, which are inserted directly on the
vertebral arches.

In the _Uronemidae_ the upper dentition comprises a cluster of small,
blunt, conical denticles on the palatine bones; the lower dentition
consists of similar denticles on the splenial bone. The vertical fins
are continuous and the tail diphycercal. There is a jugular plate, as
in _Amia_. The few species are found in the Carboniferous, _Uronemus
lobatus_ being the best-known species.

In _Dipteridae_ there is a pair of dental plates on the palatines, and
an opposing pair on the splenials below. Jugular plates are present,
and the tail is usually distinctly heterocercal.

In _Phaneropleuron_ there is a distinct anal fin shorter than the very
long dorsal; _Phaneropleuron andersoni_ is known from Scotland, and
_Scaumenacia curta_ is found at Scaumenac Bay in the Upper Devonian of
Canada.

In _Dipterus_ there are no marginal teeth, and the tail is
heterocercal, not diphycercal, as in the other Dipnoans generally.
Numerous species of _Dipterus_ occur in Devonian rocks. In these the
jugular plate is present, as in _Uronemus_. _Dipterus valenciennesi_ is
the best-known European species. _Dipterus nelsoni_ and numerous other
species are found in the Chemung and other groups of Devonian rocks in
America.

[Illustration: FIG. 382.--_Phaneropleuron andersoni_ Huxley; restored;
Devonian. (After Dean.)]

In the _Ctenodontidae_ the tail is diphycercal, and no jugular plates
are present in the known specimens. In _Ctenodus_ and _Sagenodus_
there is no jugular plate and there are no marginal teeth. The
numerous species of _Ctenodus_ and _Sagenodus_ belong chiefly to
the Carboniferous age. _Ctenodus wagneri_ is found in the Cleveland
shale of the Ohio Devonian. _Sagenodus occidentalis_, one of the many
American species, belongs to the coal-measures of Illinois.

As regards the succession of the _Dipneusti_, Dr. Dollo regards
_Dipterus_ as the most primitive, _Scaumenacia_, _Uronemus_,
_Ctenodus_, _Ceratodus_, _Protopterus_, and _Lepidosiren_ following in
order. The last-named genus he thinks marks the terminus of the group,
neither Ganoids nor Amphibians being derived from any Dipnoans.

=Order Sirenoidei.=--The living families of _Dipneusti_ differ from
these extinct types in having the cranial roof-bones reduced in number.
There are no jugular plates and no marginal teeth in the jaws. The tail
is diphycercal in all, ending in a long point, and the body is covered
with cycloid scales. To these forms the name _Sirenoidei_ was applied
by Johannes Mueller.

=Family Ceratodontidae.=--The _Ceratodontidae_ have the teeth above and
below developed as triangular plates, set obliquely each with several
cusps on the outer margin. Nearly all the species, representing the
genera _Ceratodus_, _Gosfordia_, and _Conchopoma_, are now extinct,
the single genus _Neoceratodus_ still existing in Australian rivers.
Numerous fragments of _Ceratodus_ are found in Mesozoic rocks in
Europe, Colorado, and India, _Ceratodus latissimus_, figured by
Agassiz in 1838, being the best-known species.

The abundance of the fossil teeth of _Ceratodus_ renders the discovery
of a living representative of the same type a matter of great interest.

[Illustration: FIG. 383.--Teeth of _Ceratodus runcinatus_ Plieninger.
Carboniferous. (After Zittel.)]

[Illustration: FIG. 384.--_Neoceratodus forsteri_ (Guenther). Australia.
Family _Ceratodontidae_. (After Dean.)]

[Illustration: FIG. 385.--Archipterygium of _Neoceratodus forsteri_
Guenther.]

In 1870 the Barramunda of the rivers of Queensland was described by
Krefft, who recognized its relationship to _Ceratodus_ and gave it the
name of _Ceratodus forsteri_. Later, generic differences were noticed,
and it was separated as a distinct group by Castelnau in 1876, under
the name of _Neoceratodus_ (later called _Epiceratodus_ by Teller).
_Neoceratodus forsteri_ and a second species, _Neoceratodus miolepis_,
have been since very fully discussed by Dr. Guenther and Dr. Krefft.
They are known in Queensland as _Barramunda_. They inhabit the rivers
known as Burnett, Dawson, and Mary, reaching a length of six feet, and
being locally much valued as food. From the salmon- flesh, they
are known to the settlers in Queensland as "salmon." According to Dr.
Guenther, "the Barramunda is said to be in the habit of going on land,
or at least on mud-flats; and this assertion appears to be borne out
by the fact that it is provided with a lung. However, it is much more
probable that it rises now and then to the surface of the water in
order to fill its lung with air, and then descends again until the air
is so much deoxygenized as to render a renewal of it necessary. It is
also said to make a grunting noise which may be heard at night for some
distance. This noise is probably produced by the passage of the air
through the oesophagus when it is expelled for the purpose of renewal.
As the Barramunda has perfectly developed gills besides the lung,
we can hardly doubt that, when it is in water of normal composition
and sufficiently pure to yield the necessary supply of oxygen, these
organs are sufficient for the purpose of breathing, and that the
respiratory function rests with them alone. But when the fish is
compelled to sojourn in thick muddy water charged with gases, which are
the products of decomposing organic matter (and this must be the case
very frequently during the droughts which annually exhaust the creeks
of tropical Australia), it commences to breathe air with its lung in
the way indicated above. If the medium in which it happens to be is
perfectly unfit for breathing, the gills cease to have any function;
if only in a less degree, the gills may still continue to assist in
respiration. The Barramunda, in fact, can breathe by either gills or
lung alone or by both simultaneously. It is not probable that it lives
freely out of water, its limbs being much too flexible for supporting
the heavy and unwieldy body and too feeble generally to be of much
use in locomotion on land. However, it is quite possible that it is
occasionally compelled to leave the water, although we cannot believe
that it can exist without it in a lively condition for any length of
time.

[Illustration: FIG. 386.--Upper jaw of _Neoceratodus forsteri_ Guenther.
(After Zittel.)]

"Of its propagation or development we know nothing except that it
deposits a great number of eggs of the size of those of a newt, and
enveloped in a gelatinous case. We may infer that the young are
provided with external gills, as in _Protopterus_ and _Polypterus_.

"The discovery of _Ceratodus_ does not date farther back than the year
1870, and proved to be of the greatest interest, not only on account
of the relation of this creature to the other living _Dipneusti_ and
_Ganoidei_, but also because it threw fresh light on those singular
fossil teeth which are found in strata of Triassic and Jurassic
formations in various parts of Europe, India, and America. These teeth,
of which there is a great variety with regard to general shape and
size, are sometimes two inches long, much longer than broad, depressed,
with a flat or slightly undulated, always punctated, crown, with one
margin convex, and with from three to seven prongs projecting on the
opposite margin."

[Illustration: FIG. 387.--Lower jaw of _Neoceratodus forsteri_ Guenther.
(After Guenther.)]

=Development of Neoceratodus.=--From DEAN'S "Fishes, Recent and
Fossil," pp. 218-221, we condense the following account (after the
observations of Dr. F. Semon) of the larval history of the Barramunda,
_Neoceratodus forsteri_:

It offers characters of exceptional interest, uniting features of
Ganoids with those of Cyclostomes and Amphibians.

The newly hatched _Neoceratodus_ does not strikingly resemble the early
larva of shark. No yolk-sac occurs, and the distribution of the yolk
material in the ventral and especially the hinder ventral region is
suggestive rather of lamprey or amphibian; it is, in fact, as though
the quantum of yolk material had been so reduced that the body form had
not been constricted off from it. The caudal tip in this stage appears,
however, to resemble that of the shark, and, as far as can be inferred
from surface views, a neurenteric canal persists. Like the shark there
then exists no unpaired fin; the gill-slits (five?) are well separated
and there is an abrupt cephalic flexure. In this stage pronephros
(primitive kidney) and primitive segments are well marked, and are
outwardly similar to those structures in Ganoid; the mouth is on the
point of forming its connection with the digestive cavity; the anus
is the persistent blastophore; the heart, well established, takes a
position, as in Cyclostomes, immediately in front of the yolk material.

In a later stage the unpaired fin has become perfectly established, the
tail increasing in length; the gill-slits have now been almost entirely
concealed by a surrounding dermal outgrowth, the embryonic operculum; a
trace of the pectoral fin appears; the lateral line is seen proceeding
down the side of the body; near the anal region the intestine[165]
becomes narrower, and the beginnings of the spiral valve appear. In a
larva of two weeks a number of developmental advances are noticed; the
fish has become opaque; the primitive segments are no longer seen; the
size of the yolk mass is reduced; the anal fin-fold appears; sensory
canals are prominent in the head region; lateral line is completely
established; the rectum becomes narrowed; and the cycloidal body-scales
are already outlined. Gill-filaments may still be seen beyond the rim
of the outgrowing operculum. In the ventral view of a somewhat later
larva the following structures are to be noted: the pectoral fins,
which have now suddenly budded out,[166] reminding one in their late
appearance of the mode of origin of the anterior extremity of urodele;
the greatly enlarged size of the opercular flap; external gills,
still prominent; the internal nares, becoming constricted off into
the mouth-cavity by the dermal fold of the anterior lip (as in some
sharks); and finally (as in _Protopterus_ and some batrachian larvae)
the one-sided position of the anus.

The larva of six weeks suggests the outline of the mature fish; head
and sides show the various openings of the tubules of the insunken
sensory canals; and the archipterygium of the pectoral fin is well
defined. The oldest larva figured is ten weeks old; its operculum and
pectoral fin show an increased size; the tubular mucous openings,
becoming finely subdivided, are no longer noticeable; and although the
basal supports of the remaining fins are coming to be established,
there is as yet little more than a trace of the ventrals.

The early development of a lung-fish has thus far been described
(Semon) only from the outward appearance of the embryo. The egg of
_Neoceratodus_ has its upper pole distinguished by its fine covering of
pigment. From the first fine planes of cleavage it will be seen that
the yolk material of the lower pole is not sufficient to prevent the
egg's total segmentation. The first plane of cleavage is a vertical
one, passing down the side of the egg as a shallow surface furrow,
not appearing to entirely separate the substance of the blastomeres,
although traversing completely the lower hemisphere. A second vertical
furrow at right angles to the first is seen from the upper pole. The
third cleavage is again a vertical one (as in all other fishes, but
unlike _Petromyzon_), approximately meridional; its furrows appear
less clearly marked than those of earlier cleavages, and seem somewhat
irregular in occurrence. The fourth cleavage is horizontal above the
plane of the equator. Judging from Semon's figure, at this stage the
furrows of the lower pole seem to have become fainter, if not entirely
lost. In a blastula showing complete segmentation the blastomeres of
the upper hemisphere are the more finely subdivided. In the earlier
stage the dorsal lip of the blastopore is crescent-like; in the later
the blastopore acquires its oblong outline, through which the yolk
material is apparent; its conditions may later be compared to those of
a Ganoid.

The next change of the embryo is strikingly amphibian-like; the
medullary folds rise above the egg's surface, and, arching over, fuse
their edges in the median dorsal line. The medullary folds are seen
closely apposed in the median line; hindward, however, they are still
separate, and through this opening the blastopore may yet be seen.
At this stage primitive segments are shown; in the brain region the
medullary folds are still slightly separated.

In an older embryo the fish-like form may be recognized. The medullary
folds have completely fused in the median line, and the embryo is
coming to acquire a ridge-like prominence; optic vesicles and primitive
segments are apparent, and the blastopore appears to persist as the
anus. The continued growth of the embryo above the yolk mass is
apparent; the head end has, however, grown the more rapidly, showing
gill-slits, auditory, optic, and nasal vesicles, at a time when the
tail mass has hardly emerged from the surface. Pronephros has here
appeared. It is not until the stage of the late embryo that the hinder
trunk region and tail come to be prominent. The embryo's axis elongates
and becomes straighter; the yolk mass is now much reduced, acquiring a
more and more oblong form, lying in front of the tail in the region of
the posterior gut. The head and even the region of the _pronephros_ are
clearly separate from the yolk-sac; the mouth is coming to be formed.

According to Eastman (Ed. Zittel), the skeleton of _Neoceratodus_ is
less developed and less ossified than that of its supposed Triassic
ancestors. A similar rule holds with regard to the sturgeons and some
Amphibians.

[Illustration: FIG. 388.--Adult male of _Lepidosiren paradoxa_
Fitzinger. (After Kerr.)]

=Lepidosirenidae.=--The family _Lepidosirenidae_, representing the
suborder _Diplopneumona_, is represented by two genera of mudfishes
found in streams of Africa and South America. _Lepidosiren paradoxa_
was discovered by Natterer in 1837 in tributaries of the Amazon. It
was long of great rarity in collections, but quite recently large
numbers have been obtained, and Dr. J. Graham Kerr of the University
of Cambridge has given a very useful account of its structure and
development. From his memoir we condense the following record of its
habits as seen in the swamps in a region known as Gran Chaco, which
lies under the Tropic of Capricorn. These swamps in the rainy season
have a depth of from two to four feet, becoming entirely dry in the
southern winter (June, July).

[Illustration: FIG. 389.--Embryo (3 days before hatching) and larva (13
days after hatching) of _Lepidosiren paradoxa_ Fitzinger. (After Kerr.)]

=Kerr on the Habits of Lepidosiren.=--The loalach, as the _Lepidosiren_
is locally called, is normally sluggish, wriggling slowly about at the
bottom of the swamp, using its hind limbs in irregular alternation
as it clambers through the dense vegetation. More rapid movement is
brought about by lateral strokes of the large and powerful posterior
end of the body. It burrows with great facility, gliding through the
mud, for which form of movement the shape of the head, with the upper
lip overlapping the lower and the external nostril placed within the
lower lip, is admirably adapted. It feeds on plants, algae, and leaves
of flower-plants. The gills are small and quite unable to supply
its respiratory needs, and the animal must rise to the surface at
intervals, like a frog. It breathes with its lungs as continuously and
rhythmically as a mammal, the air being inhaled through the mouth. The
animal makes no vocal sound, the older observation that it utters a
cry like that of a cat being doubtless erroneous. Its strongest sense
is that of smell. In darkness it grows paler in color, the black
chromatophores shrinking in absence of light and enlarging in the
sunshine. In injured animals this reaction becomes much less, as they
remain pale even in daylight.

[Illustration: FIG. 390.--Larva of _Lepidosiren paradoxa_ 30 days after
hatching. (After Kerr.)]

[Illustration: FIG. 391.--Larva of _Lepidosiren paradoxa_ 40 days after
hatching. (After Kerr.)]

[Illustration: FIG. 392.--Larva of _Lepidosiren paradoxa_ 3 months
after hatching. (After Kerr.)]

In the rainy season when food is abundant the Lepidosiren eats
voraciously and stores great quantities of orange- fat in the
tissues between the muscles. In the dry season it ceases to feed, or,
as the Indians put it, it feeds on water. When the water disappears the
Lepidosiren burrows down into the mud, closing its gill-openings, but
breathing through the mouth. As the mud stiffens it retreats to the
lower part of its burrow, where it lies with its tail folded over its
face, the body surrounded by a mucous secretion. In its burrow there
remains an opening which is closed by a lid of mud. At the end of the
dry season this lid is pushed aside, and the animal comes out when the
water is deep enough. When the waters rise the presence of Lepidosirens
can be found only by a faint quivering movement of the grass in the
bottom of the swamp. When taken the body is found to be as slippery as
an eel and as muscular. The eggs are laid in underground burrows in the
black peat. Their galleries run horizontally and are usually two feet
long by eight inches wide. After the eggs are laid the male remains
curled up in the nest with them. In the spawning season an elaborate
brush is developed in connection with the ventral fins.

_Protopterus_, a second genus, is found in the rivers of Africa, where
three species, _P. annectens_, _P. dolloi_, and _P. aethiopicus_, are
now known.

The genus has five gill-clefts, instead of four as in _Lepidosiren_. It
retains its external gills rather longer than the latter, and its limbs
are better developed. The habits of _Protopterus_ are essentially like
those of _Lepidosiren_, and the two types have developed along parallel
lines doubtless from a common ancestry. No fossil _Lepidosirenidae_ are
known.

[Illustration: FIG. 393.--_Protopterus dolloi_ Boulenger. Congo River.
Family _Lepidosirenidae_. (After Boulenger.)]

Just as the last page of this volume passes through the press, there
has appeared a bold and striking memoir on the "Phylogeny of the
Teleostomi," by Mr. C. Tate Regan of the British Museum of Natural
History. In this paper Mr. Regan takes the view that the Chondrostean
Ganoids (_Palaeoniscum_, _Chondrosteus_, _Polyodon_, _Psephurus_,
etc.) are the most primitive of the Teleostomous fishes; that
the _Crossopterygii_, the _Dipneusti_, the _Placodermi_, and the
_Teleostei_ (as well as the higher vertebrates) are descended from
these; that the _Coccosteidae_ (Arthrodires) are the most generalized
of the Placoderms, the _Osteostraci_ and most of the other forms
called Ostracophores (_Antiarcha_, _Anaspida_) being allied to the
Arthrodires, and to be included with them among the _Placodermi_;
that the cephalic appendage of _Pterichthyodes_, etc., is really a
pectoral fin; that the _Heterostraci_ (_Lanarkia_, _Pteraspis_, etc.)
are not Ostracophores or Placoderms at all, but mailed primitive
sharks, derived from the early sharks as the Chimaeras are, and that
the Holostean Ganoids (_Lepisosteus_, _Amia_, etc.) should be separated
from the _Chondrostei_ and referred to the _Teleostei_, of which they
are the primitive representatives.

Mr. Regan especially calls attention to the very close similarity in
structure of pectoral and ventral fins in the Chondrostean Ganoids,
_Psephurus_ and _Polyodon_, with that of the anal fin in the same
fishes. From this he derives additional evidence in favor of the origin
of paired fins from a lateral fold. In his view, the _Chondrostei_
have sprung directly, through ancestors of the _Lysopteri_ and
_Selachostomi_, from pleuropterygian sharks (_Cladoselache_) of
the Lower Silurian, and the true fishes on the one hand and the
Crossopterygian-Dipneustan-Placoderm series on the other are descended
from these. The absence of the lower jaw in fossil remains of
Ostracophores may be due to its cartilaginous structure. "There is no
justification for regarding the _Crossopterygii_ as less specialized
than the _Chondrostei_ because they were the earlier dominant group."

These views are very suggestive and contain at least some elements
of taxonomic advance, although few naturalists of to-day will regard
the Chondrostean Ganoids as more primitive than the fishes called
_Crossopterygii_ and _Placoderms_.

These conclusions are summarized by Mr. Regan as follows:

  (1) The _Chondrostei_ are the most generalized _Teleostomi_.

  (2) The _Crossopterygii_ differ from them

      (_a_) in the lobate pectoral fin;

      (_b_) in the larger paired gular plates.

  (3) The _Placodermi_ (_Coccosteidae_, _Asterolepidae_,
      _Cephalaspidae_) are a natural group, not related to the
      _Heterostraci_, which are _Chondropterygii_. They may probably
      be regarded as armored _primitive Crossopterygii_, this view
      being most in accordance with

      (_a_) the arrangement of the cranial roof-bones in _Coccosteus_;

      (_b_) the structure of the ventral fin in _Coccosteus_;

      (_c_) the structure of the pectoral limb of the _Asterolepidae_.

  (4) The _Dipneusti_ probably originated from more specialized
      _Crossopterygii_, e.g., from the neighborhood of the
      _Holoptychiidae_.

  (5) The Teleostei differ in so many respects from the _Chondrostei_
      that they should rank as an order, in which the _Holostei_ are
      included.

FOOTNOTES:

[164] This group has been usually known as _Dipnoi_, a name chosen
by Johannes Mueller in 1845. But the latter term was first taken by
Leuckart in 1821 as a name for Amphibians before any of the living
_Dipneusti_ were known. We therefore follow Boulenger in the use of the
name _Dipneusti_, suggested by Haeckel in 1866. The name Dipnoan may,
however, be retained as a vernacular equivalent of _Dipneusti_.

[165] The yolk appears to be contained in the digestive cavity, as in
_Ichthyophis_ and lamprey.

[166] The abbreviated mode of development of the fins is most
interesting; from the earliest stage they assume outwardly the
archipterygial form; the retarded development of the limbs seems
curiously amphibian-like; the pectorals do not properly appear until
about the third week, the ventrals not until after the tenth.




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  Transcriber's Notes:


  Simple spelling, grammar, and typographical errors were corrected.

  Punctuation normalized.

  Anachronistic and non-standard spellings retained as printed.

  The ERRATA on p. ix has been corrected in the text.

  P. xxiii corrected "_Salmo gairdneri_, the Steelhead Trout. 326" to
  "_Salmo irideus_, the Rainbow Trout. 326" to agree with the actual
  illustration caption.

  P. xxiii corrected "_Salmo rivularis_, the Steelhead Trout. 327"
  to "_Salmo gairdneri_, the Steelhead Trout. 327" to agree with the
  actual illustration caption.

  Italics markup is enclosed in _underscores_.

  Bold markup is enclosed in =equals=.

  Greek text is transliterated and enclosed in ~tildes~.

              Proofreading Symbols for Diacritical Marks
        (In the table below, the "x" represents a letter with a
                          diacritical mark.)
  diacritical mark             sample      above        below
  macron (straight line)         -         [=x]         [x=]





End of the Project Gutenberg EBook of A Guide to the Study of Fishes, Volume
1 (of 2), by David Starr Jordan

*** 