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]
Coryphænoides 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 Chimæra 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 Chimæra. [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 Chætodon sedentarius.[142]
Chætodon capistratus, Butterfly-fish.[142]
Mola mola, Very Early Larval Stage of Headfish, called Centaurus boöps.[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]
Æthoprora 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 mæandricus, 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 matsushimæ, 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 spelæus, 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 jessiæ.[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 enosimæ, a Fish of the Rock-pools of the Sacred Island of Enoshima, Japan.[294]
Halichœres 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]
Chætodipterus 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'Évêque".[361]
Regalecus russelli, Oarfish.[362]
Regalecus glesne, Glesnæs 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 Lacépède.[399]
Georges Dagobert Cuvier.[399]
Louis Agassiz.[399]
Johannes Müller.[399]
Albert Günther.[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]
Léon 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]
Lutianidæ, 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 Chimæra.[449]
Dipterus valenciennesi, Extinct Dipnoan.[449]
Holoptychius giganteus, Extinct Crossopterygian.[451]
Platysomus gibbosus, Ancient Ganoid fish.[452]
Lepisosteus platystomus, Short-nosed Gar.[452]
Palæoniscum macropomum, Primitive Ganoid fish.[453]
Diplomystus humilis, Fossil Herring.[453]
Holcolepis lewesiensis.[454]
Elops saurus, Ten-pounder.[454]
Apogon semilineatus, Cardinal-fish.[455]
Pomolobus æstivalis, 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 adhærens.[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 Larvæ 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 Vertebræ 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 zygæna, 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 linguæformis.[554]
Polyrhizodus radicans.[555]
Dasyatis sabina, Sting-ray.[556]
Aëtobatis narinari, Eagle-ray.[558]
Manta birostris, Devil-ray or Sea-devil.[559]
Skeleton of Chimæra monstrosa.[564]
Chimæra 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]
Palæospondylus 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]
Cœlacanthus 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]

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.

Fig. 2.—Long-eared Sunfish, Lepomis megalotis (Rafinesque). (From Clear Creek, Bloomington, Indiana.) Family Centrarchidæ.

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.

Fig. 3.—Common sunfish, Eupomotis gibbosus (Linnæus). 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-colored 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 colored 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 striæ, 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.

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 vertebræ. 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 cæca, 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.

Fig. 5.—Common Sunfish, Eupomotis gibbosus (Linnæus). 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 vertebræ 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.

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.

Fig. 7.—Porcupine-fish, Diodon hystrix (Linnæus). Tortugas Islands.

Fig. 8.—Thread-eel, Nemichthys avocetta Jordan and Gilbert. Vancouver Island.

Fig. 9.—Sea-horse, Hippocampus hudsonius Dekay. Virginia.

Fig. 10.—Harvest-fish, Peprilus paru (Linnæus). Virginia.

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.

Fig. 12.—Rock Hind or Cabra Mora of the West Indies, Epinephelus adscensionis (Osbeck). Family Serranidæ.

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.

Fig. 13.—Scales of Acanthoessus bronni (Agassiz). (After Dean.)

Placoid Scales.—Placoid scales are ossified papillæ, 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.

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 (Gobiidæ, Pæciliidæ) 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 chimæras, 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.

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 vertebræ. 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.