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Mh in TA : - a 1 ' ‘ ane 7 = : : so ; : poe ci As “" ma Rete. a oOo aye ni cee iy, (Mi vil Peer th Ae i ? : ‘ eae hee Wee f 4 “ ee yy © i i) ’ 7 i a Dy 7 ony ve - : ; in j j ip ri ey he a0 a Fiat 7, oy Ree aa pe JOeie te a ‘ his ey ty ve Ne Be fl i , / i A) yr aN y iy uy) } i Ii ih iyi i Af ! i } Slob" “SMITHSONIAN SCIENTIFIC SERI hoe Editor-in-chief CHARLES GREELEY ABBOT, D.Sc. Secretary of the Smithsonian Institution “INCRE FVSIC,4. oO ea we: A Oa ue m par ae £ os Re, Loa “25 ig te) ° 1 Js plore) ee j INGTO : ah Ree ites oR Te RR ig PO SRIRTT ee Published by SMITHSONIAN INSTITUTION SERIES, Ine. NEW ay ORK . iret in Ney ee he ein Mi ‘ STOMP EH usydaig Ag “TIE M® FL wolf “ysyo[y 1IO “ysylooy COLD-BLOODED VERTEBRATES Part I FISHES By SAMUEL F. HILDEBRAND Director, United States Fisheries Biological Station Beaufort, North Carolina Parts II anp III AMPHIBIANS AND REPTILES By ) CHarRLes W. GIL_mMore Curator, Division of Vertebrate Paleontology United States National Museum and Doris M. Cocuran Assistant Curator, Division of Reptiles and Batrachians United States National Museum VOLUME EIGHT OF THE SMITHSONIAN SCIENTIFIC SERIES 1930 CopyRIGHT 1930, BY SMITHSONIAN INSTITUTION SERIES, Inc. [Printed in the United States of America] All rights reserved Copyright Under the Articles of the Copyright Convention of the Pan-American Republics and the United States, August 11, 1910 IIf. CONTENTS Part I FISHES Tue First BaAcKBONES PEDIGREE AND KINSHIPS . THE STRUCTURE OF A FISH SEX AND REPRODUCTION MIGRATIONS GROWTH AND Foo. SoME ADAPTATIONS GEOGRAPHICAL AND eee Diode oe BIBLIOGRAPHY . PAge LI AMPHIBIANS Fossit AMPHIBIANS ANN SoME EVOLUTIONARY Aree oF Am- PHIBIANS . CAECILIANS AND Sees Frocs AND Toaps . Parr it REPTILES INTRODUCTION . THE Dinosaurs RULERS OF THE ANCIENT Sane ANCIENT FLy1ING REPTILES Fosst_t TRACKS AND TRAILS THE PRESERVATION AND Qonmeerie OF Fosstt VERTEBRATES THe Prace or REptTILEs ake epee. BRATES THE TUATARA . CROCODILIANS TURTLES. LizaRDs . SNAKES . BIBLIOGRAPHY TO eee Il AND IIL INDEX a Ay d We ILLUSTRATIONS LIST OF PLATES Part I Foolfish, or filefish, from Hawaii . Frontispiece 1. Atlantic spadefish ; 8 2. Ostracoderms from the Lower Devonian . 16 3. Chimaeroid from the Philippines 17 4. The naked-skinned paddlefish, Mississippi River 2 5. Porcupine fish, normal and inflated 2 6. Atlantic erunkfish 36 7. Two-winged flying fish from the western Pacific 40 8. Shark suckers, or remoras : 41 g. Male Mexican swordtail minnow . 48 10. Scorpion fish from the western Pacific. 49 11. Male stingray 56 12. Heads of barracuda and of man- 1-eating shark 57 13. South Atlantic parrot fish 60 14. Adult four-spot flounder, showing migration of right eye 72 15. Hammerhead shark . icy a 16. Luminous fishes by night . . 80 17. Goggle-eyed mudskipper of Australia . 88 18. Squirrel fish of Honolulu 100 19. Egg cases of the dogfish shark . 104 20. Ten-spined stickleback, and three- spined stickleback 112 21. Female gaff-topsail catfish, and eggs in mouth of male . 113 22. Colorful fishes of the Hawaiian shore . 116 23. Metamorphosis of eel larva 120 24. Pacific salmon after spawning . neh 25. Atlantic moonfishes and a cowfish 136 26. Whale shark . 140 27. Segments of branchial sieves of three common fishes 141 28. Black swallower, showing undigested fish inside 142 29. Summer flounders, showing protective coloration 143 30. Deep-sea fishes off Nonsuch Island, Bermuda 152 Parr II Fossil Amphibia Eumicrerpeton and Ricnodon Skull of Diceratosaurus . A Permocarboniferous landscape Restoration of Cacops, and skeleton of a fossil frog . Giant salamander of Japan The purple and the red salamander Tiger salamander and axolotl Male crested newt and mudpuppy Female Surinam toad with young . ; The true horned toad of South America . Parr III A Mesozoic scene: Al/osaurus attacking Stegosaurus Upper Cretaceous “bad lands” in Alberta, Canada . Skin impressions of the horned and duck-billed dinosaurs ; Nest of fossil dinosaur eggs Triassic life along the shore. ! Skeleton and restoration of Ceratosaurus n nasicornis . Mounted skeleton of largest carnivorous dinosaur Skeleton and restoration of Diplodocus Nearly complete skeleton of Camarasaurus Restorations of Triceratops and ia a Skull of Styracosaurus sili Restoration of duck-billed dinosaur rf : Skeletons of the duck-billed and crested dinosaurs . Skeleton and restoration of Stegosaurus A fossil fish lizard and mounted skeleton of sea lizard . Restoration of giant sea lizard . Restoration and skeleton of a plesiosaurus Mounted skeleton of fossil turtle, 4rchelon Skeleton of Rhamphorhynchus phyllurus Restoration of giant flying reptile, Preranodon Tracks from the Coconino sandstone, Grand Canyon Trails of invertebrate animals, Grand Canyon Pseudo-fossils: Snake and reptile’s head Dinosaur “prospect” in Canada and Dinosaur Nadonul Monument quarry, Utah Tuatara, a living “fossil” reptile : Salt-water crocodile from southwestern Asia The hawksbill, smallest of marine turtles cage The painted terrapin of the eastern United States . The “flying dragon,” a lizard of the Malay Peninsula . 164 165 168 169 182 184 186 187 200 204 214 215 218 219 222 223 224 225 292 233 236 237 240 241 256 257, 260 261 266 267 276 277 284 285 296 302 R12 314 326 YN Se Se Ss Se eS eS SOS eR OO oo~] An G Yh Ow” on) Auf Oy vw ne YN YW YW YN WY YN Con Ant Ww Frilled lizard of Australia ; Spiny lizard of the southern United States So-called horned toad of southwestern United Bates Land and marine iguanas, Galapagos Islands American Gila monster . East Indian monitor Radiograph of a copperhead Two varieties of king snake The hognose snake ' : The copperhead and the coral Shale : ; The diamond-back rattlesnake and the king cobra . Sea snakes from coasts of southern Asia . LIST OF TEXT. FIGURES Part | African lungfish, aestivating dla B South American lungfish, aestivating bi Lake lampreys and mouth of one . Plaice larva and the lancelet, compared Devonian ostracoderm and artibbed catfish, compared ; Extinct wingfish, Prerichthys One of the extinct Arthrodira . A fringe-fin of the Nile . Two genera of lungfishes and a ie ene aye compared Long-nosed gar pike Lake sturgeon. Common herring of the North Atlantic Yellow perch of North America Examples of fish shapes Further examples of fish shapes Ganoid plates of American gar pike Teeth and dermal denticles of Philippine sl phark compared Progressive stages in larva of goatfish Diagram of a typical fish showing location of fins Pectoral feelers of the sea robin Sucking disk of goby Sucking disk of clingfish ues Atlantic fishing frog with dorsal-spine bait Various shapes of tail fin in fishes . Head of pipefish . Ventral view of sawfish . Mouth of amber fish. cca Crushing teeth of black drum . Skeleton of the perch Tails of gar pike and Hounder compared Progressive stages of larva of plaice Four-eyed fish. Hearing organ of a oh. Barbels and other tactile sense organs Electric ray, showing electric organs : Larval frill shark, showing external gill tufts Left branchial cavity of fresh-water dace Air bladder of fresh-water gar pike : Stomach and intestine of Chirocentrus dorab . Blood-circulation system of dogfish shark Eggs and egg cases of various fishes Common sunfish on nest Eggs attached to female catieh. Male sea horse with breeding pouch Pacific salmon JuEpins falls Bluefish : Sailfish: Adult and young, compared Changes in ventral fin of moonfish during growth Changes in head of male dolphin fish during growth Part II Fossil frog and its larvae A snakelike labyrinthodont Restoration of Diplocaulus and Ly sorophus Development of a caecilian Utes Blind cave salamander . Frog skeleton Metamorphosis of the frog . The frog’s tongue The giant among frogs . A typical tree frog Amazonian nest builder aad nest . So-called flying frog of tropical Asia Parr III Skeleton of Anchisaurus colurus Track thought to have been made by Aen cars Restoration of Struthiomimus . -Sketch restoration of Brachiosaurus Longitudinal section of Triceratops skull . Sketch restoration of Troodon validus skeleton Skeleton of Iguanodon bernissartensis . Skull of fish lizard Paddles of Ophthalmosaurus and Ichthy osaurus Teeth of sea lizard Restored skeleton of Preranodon, a flying reptile Side view of Pteranodon skeleton Sketch restoration of a flying reptile. Slab of Connecticut Valley fossil satiny : Tracks of Iguanodon abe Skeleton of a lizard . Remnant of tuatara’s third eye Crocodylidae skulls . Limbs of land, fresh-water, Aa marine eure Skeleton of turtle The leatherback . American box turtle African chameleon Foot of aboreal and of desert tens compared . New tails grown by so-called glass snakes Snake skulls showing poisonous and nonpoisonous fangs Rattlesnake’s rattle . Recut HG VME FUNNY YL bab Part I BISHES By SAMUEL F. HILDEBRAND Director, United States Fisheries Biological Station Beaufort, North Carolina CHAPTER 1 THE FPERST BACK BONES THE first backboned animal was a fish. Before mammals, before the birds, the reptiles, the amphibians, came the fishes; and paleontologists and zoologists generally agree that all these vertebrate animals, high and low, can trace their ancestry back to the marine creature who first incased his spinal cord in a hard protective covering which gave his body rigidity with suppleness. We may appreciate the importance of these qualities 1f we think of man trying to walk erect without them. Also, without the backbone no skull would probably ever have developed and, consequently, no brains as we now know them. E.very aan embryo passes through a fish phase in its develop. ment, for each at a certain stage possesses several pairs of unmistakable gill slits. As the embryo develops, all these gill slits disappear except one pair, which form the Eustachian tubes, connecting the middle ears with the throat. There exist to this day !iving forms which are believed to closely resemble this ancestral vertebrate who lived so many millions of years ago that we hesitate to count them. These are the lowly lancelets. In the other direction it is not a long stride from fishes to the next higher group of vertebrates, the amphibians. These animals are distinguished from fishes because they generally possess paired limbs provided with toes, instead of having fins. Some of them live in water throughout life and breathe by means of gills, whereas others, such as the frogs and toads, are truly aquatic only during the ba FISHES larval stages, later becoming air-breathing and terrestrial. It is evident, therefore, that the relationship between the more primitive forms of amphibians and fishes is close and it does not require a far fling of the imagination to find evidence that fishes are distant ancestors of the amphibians. Similarly a close relationship links the amphibians with the reptiles. Thus relationships may be pointed out be- tween the successive groups, from the lowest and most primitive vertebrates, namely fishes, to the most recent and highly developed forms, such as mammals. These structural relationships, as already stated, have led to the belief that all land-dwelling vertebrates may have arisen from the lowly fish. In spite of their great antiquity, fishes still form one of the largest and most widely distributed groups of verte- brate animals. In fact there are certainly more of them than of all other classes of vertebrates. This is to be expected, as they represent the most perfect adaptation achieved by any creatures to life in the water. There exist but few bodies of water, such as the Great Salt Lake which is too salt for fish life, that are not inhabited by some members of the class; and as water covers three- fourths of the earth’s surface, the reason for the wide dis- tribution of fishes becomes obvious. We find some forms such as the trout occupying fresh water only, and dying in salt water; others like the mack- erel that can not live in fresh water; and still others like the eel and salmon which pass indiscriminately from one medium to the other. Again, while brook trout can not live in water above a medium temperature, catfish endure water disagreeably warm to the human hand. Some lungfishes (Protopterus and Lepidosiren) live in mud cocoons for months at a time when the pools dry up; certain deep-sea fishes can exist only at depths far below the range of light; while others must keep to the shal- lows or they die. THE FIRST BACKBONES As the environment varies, the animals that inhabit it must vary. The many adaptations necessary to meet the diverse conditions in the waters have given rise to thou- sands upon thousands of fish species. Close to 13,000 species, living and fossil, have been described, and new species come to light continually. Variations in form, structure, and size are exceedingly great. The smallest living fish that has been found is a goby (Mistichthys luzonius) of the Philippine Islands which measures a little less than one-half inch (males 7.5 to 9, and females 10 to Ir millimeters) in length. This pygmy goby also has the distinction of being the smallest living vertebrate. In spite of its size it is so abundant that it has become an important article of food in Luzon. The largest present- day fish known is the whale shark (Rhineodon typicus), of which specimens fifty feet long have been reliably reported, although rumor has it that they reach a length as great as seventy feet (see Plate 26). If a smaller fish than the Philippine goby ever lived, it has not yet been found. However, certain fossilized teeth suggest that some of the ancient sharks greatly exceeded the modern whale shark in size; for it 1s estimated that the possessors of teeth bearing cusps three, four, and five inches long must have attained a length of upward of one hundred and twenty feet. And yet all this variety amounts to little when compared with that attained by land-living animals. Naturalists agree that evolution in fishes has proceeded much less rapidly than among terrestrial vertebrates. They explain this by the fact that conditions in the sea in comparison with the land have remained throughout all the ages essen- tially stable. Geology and paleontology reveal to us that at times in the earth’s history polar conditions have existed well down in the temperate zones while almost tropical conditions have existed at the poles. When changes of such magnitude have occurred, whole groups of land animals have been wiped out. But the sea has known no [3!] BISHES such great modifications; and although conditions through the ages have varied sufficiently to render extinct many species of fish, the number thus extinguished is small compared to the extinct species of land animals. This theory of stabil- ity of marine conditions and slow evolution of fishes gains support from the fact that some of the living fish forms, such as the bullhead sharks and the gar pikes, existed in their present form many ages ago, as their fossil remains prove. But what is a fish? It is customary to call by this name all animals that live in water, especi- ally if they have a com- mercial value. The mere in the water, however, 1s insufficient to make it a fish, just as not all ani- mals that fly are birds. For present purposes a true fish may be defined Fic) 1. \/Affican’ lungfish \Protaprerus:’” astacold=blooded) aiminnal aestivating in its cocoon of mucus. Note with a backbone with funnel by which air passes to mouth. : After Newton Parker arms and legs represented by fins or rudiments of fins, and living in the water in which it breathes by means of gills. Whales and porpoises and their allies often are spoken of as fish. They are excluded, however, because they possess lungs and do not breathe by means of gills. Our definition also excludes the so-called shellfish—oysters [4] aes oes yer wg wee 2 <, > EPR, SRST Ste 7 fact that an animal lives | #4 THE FIRST BACKBONES and clams, crabs and shrimps— for none of these animals possesses a backbone. For the same and many other reasons, starfishes and Jjellyfishes are not true fishes. It would simplify matters greatly if we could distinguish a fish by its shape. But the members of the class vary so much in this re- spect that it is utterly impossible to define them by the form of the body. Some are long and slen- der, resembling worms, others are very deep, short, and narrow, while still others are very flat and broad. The factors that determine form are generally the conditions under which the fish lives and obtains its food, and as these conditions vary, shapes vary. Lampreys or cyclo- stomes, of which exam- ples occur both in salt and fresh water, illus- trate the wormlike type of fish, a form which is readily understood when Fic. 2. South American lungfish, Lepi- their mode of life iS dosiren, aestivating in deep burrow, mouth of which is plugged by a piece of per- known, for they are para- P ieaecd eines : sitic on other fish during at least a part of their existence. In fact the head 1s not even definitely distinct from the body. Nor does the lamprey possess jaws, for the mouth is a sort of funnel- shaped sucking disk, provided with large rasping teeth (Fig. 3). With its sucking disklike mouth this eel attaches Usa FISHES itself to another fish and it obliges the animal of its “choice” to provide free transportation while the rider rasps away at the very flesh and blood of its host with its large teeth. Sometimes the host fish is killed outright. Again the lamprey eel abandons its victim with large gaping sores which are attacked by disease, causing a slow and torturous death. Some of the hosts survive, however, but the deep Lace healed-over scars tell Wo Mnron their own story of hard- ship and suffering. Lam- preys vary in length from several inches to several feet. The spadefishes and their allies offer examples of very short and narrow fishes.’ Whey) often! are quite as deep as long and always strongly com- pressed. Many of these fishes feed chiefly on plants and often on algae and other growths at- tached to plant stems, sticks, and piling. The narrow, compressed body obviously permits them Fic. 3. Left, young lake lampreys attack- to pass with little resist- ing, a fish;yright; mouth) of Jake/lamprey ~ | (ance |betWeen, Closely aser showing rasping teeth. After Gage Sp ea plant stems or pilings and through other narrow spaces, while 1n search of food. The skates and rays typify fishes that are thin, or narrow in the oy direction from the spadefishes, fon they are sinh strongly depressed. Some of the species, as for ex- ample the butterfly rays, are broader than long. These animals generally live on the floor of the waters they [6] THE FIRST BACKBONES occupy, seeking their food on the bottom. Their broad, flat bodies are admirably adapted to the life they live. The examples mentioned are extremes in form. Thou- sands of intermediate shapes exist, each fish being adapted to the life it leads. It is evident, then, that a fish can not be defined by the shape of the body. Neither can a general definition of a fish be based upon the presence of scales, for not all fishes possess scales. More permanent and constant characters than these must be sought. These characters, as we have said, include cold bloodedness and the possession of a Benoa. and of fins, or rudiments of fins, in place of arms or legs. Furthermore, the animal must eG in the water, in which it breathes by means of gills. Unless an seal meets all of these specifications it is not a fish. rut CHAPTER. I PEDIGREE AND KINSHIPS Everysopy is interested in the beginning of things. All the more so when the beginning 1n question 1s not only that of fishes but of all vertebrates... Although interrogated from various angles as to what the first fish was like, na- ture has given no definite answer. The history of extinct animals, if indeed there exists any history, is “written in the rocks.”” But this is inadequate at best. The soft parts of primitive animals are not preserved. The remains that are found generally consist of bones, spines, teeth, and other hard parts. If the earliest or most primitive verte- brate did not possess real bones, and we cannot believe that it did, its remains most probably failed of preservation and man will never know precisely what it was like. When paleontology fails, only one other clue to primi- tive structures and characters remains, and that 1s found in the embryological and larval development of living things. Long-lost characters of the adult often appear in the embryo. It is from the study of embryology that we learn of the several pairs of gill slits possessed by the human embryo. In the early stages of development of the embryo fish we find it a mere string upon the yolk, with no heart or limbs. It 1s apparently a mere worm. The lowest living fish form, the lancelet, 1s just about this sort of creature. If the embryo repeats the history of the race, we have here a striking indication of what the first fish was like. The most primitive living vertebrate, the lancelet (Branchiostoma lanceolatum), generally classed among the [8] slamvpy uaydaig Ag ‘syuvjd azoysut uo Ayasey Surpaay ysy passordwios ‘Mosivu v ‘waqv{ snaadipojavy) “ysyopeds onuepy PEDIGREE AND KINSHIPS fishes although not a true fish, usually reaches a length of only a couple of inches (Fig. 4). It has no jaws, the mouth consisting of a lengthwise slit. True vertebrae are likewise lacking, as the “backbone” is a continuous cartilaginous rod. The median fins, both dorsal and ventral, consist of folds of skin. Of paired fins there are none; hence, no hard parts are present. It seems quite probable that the ancestral vertebrate similarly lacked SS See RRUNS Fic. 6. Extinct wingfish, Pterichthys, showing limblike appendage to the head enter the upper part of the tail fin. This condition of the tail in the larval bony fishes is interpreted by naturalists as indicating the type of tail their ancestors (sharks?) possessed. _ Small fish spines, doubtfully referred to a primitive shark, occur in rocks of a very early period, but it is not till we come to the Carboniferous that we find remains complete enough to be definitely identified as those of a primitive shark. These oldest sharks were of relatively small size possibly only two to six feet in length, yet the shape appears clearly to have been that of a modern shark.’ The skin was covered with hard, bony points, and the teeth bore a large central cusp with smaller cusps on the broad, expanded base, very much as in certain modern sharks. Succeeding the sharklike creatures of Carboniferous times came a host of other sharks, known principally from their teeth and fin spines. Nearly all of these old forms perished, giving place to others better adapted to the changed conditions brought about by time. One family, [13] FISHES the Port Jackson or bullhead sharks (Heterodontidae), however, appears to have survived. The living species, of which there are four, are found only in the Pacific, the Port Jackson shark of Australia being known the longest. These sharks, like other old ones, have a spine in front of each dorsal fin. The lateral teeth are padlike, ridged or rounded, and arranged in many rows, and differ from the ante- rior pointed teeth. The surviving members suggest a bottom-dwelling form with teeth adapted for crushing hard objects. This family, although probably older, has not been traced earlier than the Mesozoic period, some- times estimated at something like fifty million years ago. As the Eocene and Miocene epochs were reached, the warmer oceans seem to have teemed with sharks. Shane were small, but others reached monstrous proportions, apparently much greater than any living animal of today. However, they are known mostly only from their teeth, as their skeletons were cartilaginous and do not appear to have been preserved. It has been estimated from a comparison of the teeth of living sharks with those of extinct forms, which reached three, four, and even five inches in length, that some of the giants of the past attained a length of a hundred and twenty feet, which is longer than any modern whale. They appear to have swarmed everywhere throughout the warmer seas, for their teeth occur in the Tertiary strata in many parts of the world, and dredgings made in the deep sea have brought up their teeth by scores. Eventually, however, these huge wolves of the sea perished. The reason is not known. It is sometimes thought that they may have devoured everything throughout their habitat and then fallen to eating each other. In any case it is certain that they perished while some of the smaller sharks survived and still live, showing that victory does not always belong to the largest. Among the latest sharks to appear are the great man- eating Carcharodon carcharias and its nearest allies, [14] PEDIGREE AND KINSHIPS remains of which have been recognized only in the Terti- ary formations. A second group of important fishes, known as chimae- roids, sharklike in character, appeared early in geological time, certainly as early as the middle Silurian, which greatly antedates the Carboniferous. These animals, having several living representatives, variously known as spookfishes, ratfishes, and elephant fishes, are generally Fic. 7. One of the extinct Arthrodira, with heavy armor hinged at the neck. Ten to eighteen feet long believed to have diverged from the sharks, although the association is not very close. A thick, round head which tapers gradually toward the pointed or filamentous tail usually distinguishes them. The skin is smooth and the paired fins are somewhat sharklike in character. The fin ‘on the back has a long spine and the skeleton is cartilagi- nous, both characteristics suggestive of the sharks. The cranium is a highly compact structure. The upper Jaw is immovably fused with the cranium and the lower one is directly articulated with it, differing sharply in these respects both from the sharks and the bony fishes. The teeth are represented by dental plates, closely fused with the jaws and studded with sharp points. Chimaeroids, like the sharks, are very imperfectly represented in the rocks, probably because their skeletons were cartilaginous and decomposed before they could be- come covered and preserved. Little more than their dental plates and fin spines have been found. The struc- ture of the ancient chimaeroids, however, appears to have [15] FISHES differed little from the living representatives. These modern chimaeroids inhabit the cold and cooler seas and the depths of the ocean. The elephant fish, the best-known species of this group, lives in shallow water, of ten to twenty fathoms, from Sitka, Alaska, to San Diego, Cali- fornia. It is brown in color, with whitish spots, and reaches a length of two and a half feet. It is harmless and worth- less, except for the oil in its. liver, but always of great interest to the naturalist. We come now to the remaining groups of fishes, living and fossil, which are all combined by Dr. David Starr Jordan under one class, Teleostomi, meaning “true mouth.” In these Ashes the lower jaw generally is not attached directly to the skull but is joined with inter- mediate bones. They possess, typically, an air bladder, or a modification thereof; their skeletons are at least partly ossified, and they have membrane bones, such as the opercle and suborbitals, on the head; one or two pairs of limbs are developed; and a single gill opening leads to the gill arches. To this large class belong the vast majority of recent fishes and also a large percentage of the known fossil forms. It may be divided into three major subclasses—the fringe- fins, the lungfishes, and the ray-fins. The ray-fins include the ganoids, represented among living forms by the gar pikes, sturgeons, paddlefish, and bowfin, and the bony fishes or the true teleosts, including from ninety to ninety- five per cent of all living fishes. The fringe-fins generally are regarded as the most primi- tive of the “‘true mouths.” This group once included a large number of important fishes. Representatives first appear in the Devonian formation and continue through the Carboniferous. Only two genera, including about ten known species, all inhabiting African rivers (the Congo, the Niger, and the Nile), occur among living fishes. Of these none have been found fossil, yet they appear to have diverged little in general structure from their Devonian [ 16 | UudyOY TUY “SUPIIPISNIAD YyIM Yury ui2auu09 ev Surusoy soysy petOW se [JBUwS *petojsol ‘uvIuOAdq] TOMO'T ace! wos SUIIIPOIPIISC) = Fu a ae Ao ae ee: -- Ne ET ei ae I / a oF; T 4 rs is wees +02 ay > Xx < >} 2 ¢ ULV Id Buoy soypur usaazy jnoqe ‘saurddiyryg ay3 woz (2uvap vsavuty)) PlorsVUNly € ALVId PEDIGREE AND KINSHIPS kindred. The fringe-fins, living and fossil, resemble the common American gar pike in shape and form and in the body covering which in both groups consists of small, hard, imbricated, rhomboid bony plates. The gar pike, however, has a more elongate snout. But what particularly distinguishes the fringe-fins and stirs our interest in them is the fact that the pectoral fins or fore limbs are jointed. Is this the first step up in the development of the jointed limbs of the higher vertebrates? LL LLOLLAMY Fic. 8. A fringe-fin, Polypterus bichir, of the Nile, most primitive of the true- mouths. After Boulenger Some paleontologists believe so and regard these fishes as the almost immediate ancestors of the amphibians, that is, the salamanders, frogs, and related forms. Further- more, the cranium of the fringe-fins resembles that of amphibians, and the jaws and teeth are similar to those of the higher vertebrates and quite different from ordinary bony fishes. The young of the living forms have external gills which disappear in the adult. These gills, in form and structure, resemble the external gills of amphibians. Finally, the living fringe-fins, at least, possess a somewhat lunglike air bladder. But here the resemblance ends, for the air bladder is not cellular in structure as in the true lungfishes and in the higher, air-breathing vertebrates. Furthermore it lies on the downward or ventral side of the gullet, whereas in other fishes it lies above the gullet, occupying a position similar to the lungs in the higher vertebrates. The position and structure of the air blad- der, therefore, do not support the amphibian ancestor- ship theory. With respect to the development of the air bladder into a lung, it seems more probable that the ay] MUSES lungfishes are nearer ancestors of the amphibians than the fringe-fins. The fringe-fin fishes grow moderately large, the common species from the Nile reaching a length of four feet. The fossil forms apparently were smaller. The recent species are sluggish in habits, occupying mostly quiet waters, living close to the bottom, and occasionally ascending to the surface for a gulp of air. They are used as food and their flesh is said to be of excellent flavor. The lungfishes, or dipnoans, have long been looked upon as a connecting link between amphibians and fishes. They first appear in the vast darkness of Devonian time and apparently became rather numerous in the Lower Carboniferous. Thereafter they diminished in number until today only three distinct genera are left. One of these, Neoceratodus, is found in Australia, another, Lepidosiren, in South America, and the third, Protopterus, in Africa. All the species inhabit fresh water, generally sluggish, stagnant, or even muddy pools or streams. Lungfishes, recent and fossil, have the two sets of paired fins corresponding to limbs, either with or without rays. The brain case is cartilaginous but is roofed by dermal bones, somewhat similar to that of the bony fishes. The lower jaw, as in the chimaeroids, is articulated directly with the skull. Two pairs of nostrils are present, the anterior pair opening under the upper lip and the posterior pair within the mouth. All the species have a row of small gill arches, whose single external opening is protected by a membrane ‘bone, the operculum. The living species ior South America and Africa, espe- cially, resemble the salamanders, one of the more primitive orders of amphibians (Fig. 9). The body 1s thick and spindle shaped, but unlike the salamander’s in that the skin, exclusive of the head, 1s covered with round, bony scales. The head bears a distinct resemblance to that of the salamander in shape and is covered with a slimy skin precisely as is the salamander’s. The slender paired fins [18 ] PEDIGREE AND KINSHIPS suggest the legs of the salamanders, although less leglike in structure than the paired fins of the fringe-fins. Young lungfishes, like embryo sharks and salamanders, possess external gills. These almost wholly disappear with age, and the small covered gills that remain are unable to supply the respiratory needs, so that the animal must rise to the surface at intervals for air, like a frog. The cellular air bladder serves as a lung. The South American species, at least, is reported to breathe as con- tinuously and as rythmically as a mammal. During the rainy season the South American lungfish lives in swamps a few feet deep. When the dry season comes it burrows in the mud, closing its burrow at the top with a lid of nears ; as Hep Cae e (AM © AY \ Vi @ my sess Fic. 9. Two lungfishes, A, Neoceratodus of Australia, and B, Lepidosiren of South America. C, a salamander, Amphiuma, showing close external resem- blance, in limbs, etc., to lungfish mud. The fish surrounds itself with a mucous secretion and during its aestivation closes its gill openings and breathes through the mouth, the air reaching it through the perforated clay lid of its burrow. When the rainy [ 19 ] ISEIES season returns the lid is pushed off the burrow and the animal comes out as soon as the water 1s deep enough. The African genus from the White Nile and the Congo which has long been handled by dealers has very similar habits. It encases itself in a cocoon of mud in which it may be transported long distances. When the cocoon 1s placed in tepid water the mud 1s dissolved and the fish soon revives. The Australian genus has better developed gills than the South American and African genera and, so far as known, it always lives in water, although doubtless coming to the surface at intervals for air. No fossil remains of the South American or of the Afri- can genera have been found. However, fossilized parts, especially teeth, of near relatives of the Australian form, known in Queensland as Barramunda, have come to light in abundance. The characteristic triangular, platelike teeth of these fishes occur in fossil form in England, Ger- many, India, South Africa, and also in the United States (in Colorado and Wyoming), indicating a widespread distribution in contrast with the very limited range of the single living species. The barramunda is a food fish of importance 1n Queens- land, having been compared in size and taste with a salmon. Also, its meat is red. In form it is a heavier-bodied fish than its living relatives, though resembling them in the rather sharp head and pointed tail. It reaches a length of from five to six feet. We can not know, of course, how far back in geological times the ancestors of the lungfishes developed lungs and began to breathe air, as the air bladders obviously are not preserved in the rocks. Doubtless this took place a very long time ago. We may suppose, to quote J. T. Cunningham, that . the original fishes, more or less similar to sharks and dog-fishes, were inhabitants of the sea, where the water being saturated with oxygen there was no need of any atmospheric respiration to supplement [ 20 ] PEDIGREE AND KINSHIPS the action of the gills. Some of these original fishes ascended the rivers and became inhabitants of fresh water, as some selachians and rays ascend the Amazon at the present day. Some of these forms made their way into streams or lakes where, from the hot climate and the decomposition of vegetable matter, oxygen was deficient and they began to swallow air at the surface to compensate for the failure of aquatic respiration. This appears from the evidence to have been the origin of the air bladder, and at the same time, of the teleostome type. From these air-breathing teleostomes at a very early stage arose the earliest Amphibia, and on the other hand they multiplied in all the fresh waters, until some of them again reached the sea, where the air bladder lost entirely its respiratory function and became a swim blad- der. The fishes we know best are ray-fins, which include all fishes not grouped under the sharks and rays, chimaeroids, fringe-fins, and lungfishes. This subclass of the Teleo- stomi generally is subdivided into two time-honored groups, the ganoids and teleosts, but they have representa- tives which are extremely closely related and can scarcely be separated. In general the ganoids include the “‘old- fashioned” fishes, possessing rhombic bony plates and a cartilaginous skeleton, while the teleosts constitute the “modern” bony fishes, covered with round scales and having a well calcified skeleton. To these definitions there are several exceptions, as the one group merges almost imperceptibly into the other. The gar pike probably is the most typical living repre- sentative of the one-time numerous group of ganoids. Every midwestern boy knows this very elongate fish, with its long jaws well provided with teeth, two pairs oF limbs, and complete body covering of a series of hard, enameled, rhombic plates. The skeleton 1s cartilaginous, the heart (arterial cone) contains several valves, and the air bladder 1s somewhat cellular, as in the lungfishes, and -has a respiratory function. The alimentary canal is short and possesses a spiral valve, as in the sharks and the chimaeroids. The gar pike occurs commonly in the fresh waters of North America and is especially abundant in the Missis- [21] HISHES SIppl, Great Lakes, and the rivers of the Southern States. It is represented by three species, the largest of which, the alligator gar (Lepisosteus tristoechus), is reported to reach a length of twenty feet. The gar pike prefers quiet water and is rather sluggish in its habits, yet at times aggressive. The complete armor of dermal plates saves 1t from easy destruction by fish or other predatory animals, and the enamel scales of the alligator gar may even resist shot. When taken from the water it exhibits a remarkable te- nacity to life, often living for hours. Fishermen consider the fish a general nuisance, for it “‘steals’’ bait, tears nets, and its flesh is almost worthless. Of all living ganoids the gar pike resembles most per- fectly the structural characters of the abundant Paleozoic and Mesozoic forms, although its genus occurs first in the Fic. 10. The long-nosed gar pike, Lepisosteus osseus, of the Great Lakes and Mississippi Valley. A typical ganoid with rhomboid plates instead of true scales Eocene. Many of the fossil ganoids are not elongate in body, nor do they possess long jaws, yet they resemble the gar pike in the structure of the rhombic bony plates cover- ing the body, in the fins, the teeth, and in the partially calcified skeleton. The sturgeons, of which about thirty species are known, and the paddlefishes also belong to the ganoid group. The dermal plates, completely covering the body in the gar pike, have been reduced in the sturgeons to five longitudinal rows of large shields, leaving most of the body naked, while in the paddlefishes dermal plates are entirely wanting. Very young sturgeons have conical teeth which they lose early in life, leaving the mouth [22] PEDIGREE AND KINSHIPS toothless. ° The tail of this fish resembles closely that of the shark, the upper lobe being much the larger and sup- ported in part by the upward curved spinal column, a character indicating a relationship to the sharks. The sturgeon also has a long projecting snout, underneath which is placed the comparatively small, protractile mouth and one or more pairs of barbels or feelers. They are large animals, varying in length from three to thirty feet when adult, and inhabit streams, lakes, and the coasts of the Northern Hemisphere. They are used everywhere as food, and the roe, known as “caviar” after it is salted and dried, brings more than the fish on the market. The sturgeons apparently are the most recent of the ganoids. Although fossils of an extinct sturgeonlike fish belonging to an early period have been found, a modern genus does not appear until the Lower Eocene. The paddlefishes, close relatives of the sturgeon, include only two living species, one, Polyodon spathula, inhabiting the Mississipp1 Valley, and the other Psephurus gladius, the great rivers of China. The American species reaches a length of about four feet, and is very sharklike in appear- ance. Instead of the complete plating of the gar pike, or the five rows of dermal plates possessed by the sturgeons, the paddlefishes have the skin quite naked. A long snout projects far beyond the mouth and accounts for the several names—paddlefish, spoon-billed cat, duck-billed cat, and shovelfish—bestowed on the American species. No representatives of the living genera appear to have been found fossilized, although a relative and probably an ancestor of the living species has come to light in Eocene shales. This primitive paddlefish had a shorter snout, and small, thin, quadrate scales covered the body, a fact which appears to indicate that the paddlefishes may have sprung from a primitive type of fish having rhombic plates, like the gar pike. The bowfin (Amia calva), also known as dogfish, mud- 23:] FISHES fish, lawyer, and John A. Grindle, completes the ganoids. The single species of this order abounds in lakes and swamps of the Mississippi Valley, the Great Lakes region, and southward to Georgia. It has a calcified skeleton and is covered with round scales like most modern fishes. The tail is round and the backbone does not enter into it e tie “! 3 A E aR.S 7 - ; er “ Ye came d PASE Sins Nobu ets i ey 2 — : J -@ Bt Nad aa 2) VY UR ate SS “reblek) rhea hah 7 fe VASA D eehe\ Geen aa ou eae — i SEE te REDS oe ae S ae <9 Fic. 11. Lake sturgeon, Acipenser rubicundus, from the Great Lakes. Compare dermal plates with those of gar pike. Courtesy Bureau of Fisheries prominently. For these reasons it very closely resembles the true bony fishes or teleosts and forms a sort of connect- ing link between them and the ganoids. It retains a cellu- lar air bladder which is of respiratory value, as in the gar pike. The bowfin probably is the most tenacious of life after removal from water, of any American species of fish, as it is able to breath air by means of its lunglike air bladder. Bowfins reach a length of about two and a half feet. The bad flavor of the flesh limits their use for food, though in recent years it has been found that smoking much im- proves the taste, so that in certain localities the species is being eaten somewhat more extensively than formerly. Several fossil forms from the Miocene and Eocene, although differing from the only living one as to species, have been referred to the modern genus. Evidently this group attained much greater numbers in prehistoric times and also enjoyed much wider distribution, for related fossil forms have been found in Europe as well as in America. We have in the ganoids as a whole, a case of decline so far inexplicable. Beyond a doubt they were once much more numerous than at present. Yet they appear ex- [ 24 ] salJaysiy Jo nvaing yi jo Asaqinod ‘iddississtpyy ayy wo. ‘vynyjvds uopodjzog ‘ysyatpped pouurys-payeu ay]. QAI or ‘ ¢ ULV Id PEDIGREE AND KINSHIPS tremely well adapted for carrying on the processes of life, at least with conditions such as we now know them. Why, then, should they have diminished so greatly in numbers? While the ganoids proper were on the decline a side branch appears to have been developing which grew out of all proportion to the size. of the original trunk. This branch constitutes the now overwhelmingly predominat- ing subclass of fishes, namely the teleosts or bony fishes, which naturalists generally agree are descendants, many times removed, from the ancient ganoid stock. As fossils the teleosts, or ‘‘true bones,” first appear in the Jurassic, whereas the first fossil ganoids date back to the ancient Devonian period. These earlier fishes made their appearance contemporaneously with air- breathing, land-living vertebrates, which were well estab- lished long before the first teleost came on the scene. The other fishes—sharks, chimaeroids, fringe-fins, and lung- fishes—equal or exceed the ganoids in antiquity, so that the teleosts are by far the youngest of the great class of fishes. Some of the species have diverged far from their ancestral group, but others have retained many of the characters possessed by the recent ganoids, leaving certain species that merge almost imperceptibly from one genus into the other. And even those species that have got farthest away from the ancestral stock still often display ganoid characters in the embryonic and larval stages. In general, teleosts have a thoroughly calcified skeleton; the backbone seldom enters the upper lobe of the tail fin; the air bladder no longer is lunglike and does not assist in respiration; and the spiral valve of the intestine disappears. They differ from ganoids also in having fewer valves in the heart (or arterial bulb), and in the arrangement of the optic nerves, which are not interfused but remain separate, one running to each eye without crossing. The dermal bones of the head, which in the ancestral ganoids are at the surface and enamel-coated, are here deep-seated in the head and not infrequently covered with skin and scales. [ 25 ] FISHES Scales disclose great variety and sometimes are missing altogether, but when present they generally are round, with free posterior margins which overlap succeeding rows of scales. Teleosts vary widely in the detail of almost every structural character. Great differences occur among the species with respect to the gills, teeth, scales, the diges- tive tract, circulatory system, nervous system, and sensory structures. T’hese wide variations are interpreted as indicating that the species are competing keenly in their struggle for existence, undergoing whatever modifica- tions are necessary to meet the conditions confronting them. But we can not deal with large numbers of anything without classification, and the true bony fishes have to be classified in spite of their wide divergencies in detail. The basis of grouping is inevitably technical, but we shall try to avoid technicality here as much as possible by dis- cussing only the major divisions. Two general super- orders of teleosts, based on the position of the ventral fins, are recognized. To each of those, however, several exceptions exist. The first large division consists of those fishes that have the ventral fins (hind limbs) attached to the abdomen, that is, inserted back of the pectoral fins (fore limbs) and with the supporting arches of bones (clavicle and pelvis) separate and attached to different parts of the skeleton. These fishes sometimes are referred to as abdominal fishes. Most of the Abdominales have only soft rays in the fin on the median line of the back (dorsal fin) and in the one on the median ventral line (anal fin), or at least these fins do not possess a series of spines. The other superorder includes those species that gener- ally have the ventral fins (hind limbs) attached forward and the supporting bones united with the shoulder girdle. These fishes usually have several spines in the dorsal and anal fins and, as a rule, the ventral fins are composed of [ 26 |] PEDIGREE AND KINSHIPS one spine and five soft rays. The animals thus defined are referred to as spiny-rayed fishes. The common herring of the North Atlantic and a near relative in the North Pacific are probably the most typical of all abdominal or soft-rayed fishes (Fig. 12). In shape they are elongate, tapering both toward the head avs) ia NE va Fic. 12. The common herring of the North Atlantic, C/upea harengus, the typical abdominal fish. Courtesy Bureau of Fisheries and the tail, and are rather compressed. They havea single fin on the back and one of like size below the body, each composed of soft rays only. The tail fin is forked and the paired fins, representing the limbs, are far apart. The ventral fins (hind limbs) occur far back and are attached to the abdomen, while the pectoral fins are inserted low under the margin of the gill covers. Mod- erately large, smooth scales cover the body. Many species are allied to the herrings and more than a few have been found fossilized. Their relationship with the ganoids on the one hand is very close, and they merge by various stages through several families almost imperceptibly into the spiny-rayed group, on the other. In fact, there are those who believe that a primitive her- ring is the ancestor of all modern bony fishes. The Abdominales include, besides the commercially important herrings, the salmons, trouts, smelts, and cat- fishes. They include also numerous minnows and a con- siderable number of deep-sea species. The yellow perch of the fresh waters of North America, yey: FISHES which has a near relative in European waters, is often referred to as the most perfect example of a fish, and it serves well as a typical example of a spiny-rayed fish (Fig. 13). The yellow perch is rather round or spindle- shaped, yet somewhat deeper than broad. It has a point- ed head and a somewhat raised back, upon which are Fic. 13. Yellow perch of North America, Perca flavescens. Most perfect example of a fish and typical spiny-rayed fish. Courtesy Bureau of Fisheries inserted two separate fins, the first one consisting of a series of spines and the second one of two spines and several soft rays. The tail fin possesses a straight to slightly concave margin; the fin below the body, known as the anal fin, situated just behind the vent, has two spines and several soft rays. The paired fins are close together, the ventral fins, representing the hind limbs, being inserted on the chest, only slightly behind the pectorals, or front limbs. The latter, instead of being attached low under the margin of the gill covers, as in the Abdominales, are inserted higher on the body and only a little below the median line of the side. The body is covered with moderately small, firm, rough scales. The reader must understand that even though the perch may be a typical spiny-rayed fish, the variation [ 28 ] PEDIGREE AND KINSHIPS in shape and form and with respect to nearly every detail of structure 1s exceedingly great within the group. In fact, no other group shows a wider divergence of its mem- bers from the typical form. The spiny-rayed fishes are very numerous and include the majority of common food fishes, among which, in addition to the typical perches, may be mentioned the mackerels, pompanos, sunfishes, sea basses, butterfishes, the bluefish, and a host of others. We begin now, perhaps, to have some idea of the antiq- uity of fishes, their geological history and relationships, and their many kinds. Great as are the numbers of species now living, we know whole groups of fossilized fishes which have no living representatives. We can only assume that changes in the environment to which they were unable to adapt themselves led to their extinc- tion. The infinite variety of shape, structure, and habits of the living species will engage the attention of the remainder of this book. } [ 29 ] CHAPTER: TEE HE ST RUC IRE (OB, x0 FISet Tue shape of the fish is generally a key to its habits, past or present. It would be unreasonable to suppose other- wise. The aquatic mammals illustrate the manner in which life suits its form to the mechanical needs of its environment, for so completely have the seals, whales, dolphins and other mammals adapted themselves to water-living that early naturalists were deceived into including them among the fishes. The typical fish shape may be likened to a boat or, still more appropriately, to a cigar. The fresh-water perch and salt-water mackerel are common examples. It is a shape which offers little resistance in the water, and those fishes possessing It generally are capable of swimming. at great speed. As such species habitually live by preying on other fishes and animals, and have often to swim long distances in search of food speed is essential to them. It is said that a mackerel, shark, or pike can swim twenty or twenty-five miles per hour, including stoppages, for weeks at a time. As examples of extreme variation from this cigar-shaped norm, I have already mentioned the wormlike lampreys, the narrow and deep spadefishes, and the horizontally flat skates and rays. Another oddity is the globular type, well represented by the common porcupine fish, which is short and plump and can inflate its body either with air or water, making itself nearly as round as a ball. Many intermediate forms exist, of course, too numerous for description here, but we ought to call attention to two, so odd in outline that they have attracted special notice [ 30 ] THE STRUCTURE OF A FISH ever since their discovery hundreds of years ago. I refer to the ocean sunfish and the sea horse. The ocean sunfish, known also as headfish, found occasionally in all tropical and temperate seas, may cer- tainly claim the distinction of being one of the most unnatural looking creatures in all nature. It is an enormous head, moving bodiless and tailless through the water, about as extraordinary as the disembodied grin of the Cheshire cat which Alice saw in Wonderland. No suggestion of a tail or even a tail fin relieves its seeming incompleteness, and great size exaggerates the freak—a specimen more than eight feet long, or better, across, and weighing 1,200 pounds having been reported. Early naturalists saw in it a resemblance to an immense mill wheel, wherefore they named it Mo/a mola. It has no eee but a tough leathery skin. One wonders how this odd fish® without a tail and much too heavy forward of the thee high fins (one on the back and the other on the ventral edge near the posterior end of the body) 1s able to orient itself in its watery home. However, not only does it hold itself erect, but swims lazily and with apparent ease near the surface, with its high dorsal fin often projecting above water. he sea horse, Hippocampus, is complete enough, but it looks nothing like a fish. The head, horselike in appear- ance, 1s attached at right angles to the body and can not be straightened without injury to the “‘neck.”” A long, coiled tail serves the fish as a prehensile appendage by which it clings to plants, some species attaching them- selves to floating seaweeds which carry them great distances; others, to rooted plants which hold them close to the shore. In contrast to the naked skin of the head- fish, the sea horse’s body is encased in bony rings, which frequently bear spines to which fleshy flaps are attached. These curious fishes, represented by numerous species, inhabit nearly all warm seas, and vary in length from five to ten inches. [31] FISHES 3yi “1aMO] Sasroy vas adueIIS 9y3 soysy jo sazis 03 uoisodosd ut jou ase sBuimesq, ‘ed A} ay!I-]29 QySia tfarayovw SurMUms-3JiMs Jo Mata apis pue pua ‘aaddy ‘sadeys ysy jo sajdwexy ry oo” pees TIT DY oo 20 GBD P o (32 ae DS220° ODD ° 33593 PF 52 2.7,.°4) AD Sp 222 DO ‘V1 Oly [324 PLATE 5 <- ja a Porcupine fish, Diodon hystrix, and the same fish inflated with water or air for self-protection. Courtesy of Dr. Myron Gordon THE STRUCTURE! OF A: FISH seysy jo sazis 03 uonsodoid ut jou ase sduimeiq §=*Japunoy QYSts tAes 31439373 passaidap ‘3j2] :49Mo] Sysy [adue ydeI[q passasdwos Qysis Sysypeay uvado ssajjiei jay :4addq ‘sadeys ysy jo sajdwexe s9yaing COUT LEO: feast wut I oS) I- [33 FISHES In recognition of their oddity, people often dry them for preservation as curios. SKIN AND SCALES Not all fishes have scales, in spite of the vague popular belief to that effect. In general scales seem to be a per- quisite of the higher or bony fishes, though by no means of all of these. Their chief purpose, no doubt, is protective; and they are only one of several means which nature, with her unlimited capacity for invention, has found for achieving that purpose for different fishes. Bony plates, spines, prickles, and shagreen are all made use of, and when all these are wanting the usually tender skin becomes hard and leathery in compensation. The evidence leads us to believe that all true fishes have had an outer cover- ing of some sort and that those which appear naked have lost it. Whichever one of these extra protective devices nature employs, they all grow out of the skin itself. As in other vertebrates, the skin of the fish consists of two layers. On the outside 1s the epidermis, made up of several layers of cells without blood vessels, and on the inside the thicker dermis, composed of fibers and supplied with blood vessels and nerves. The epidermis is almost transparent and so soft that friction will easily remove it. ‘In the dermis, incidentally, we find the explanation of the dis- tinctive beauty of fishes—their silvery iridescence. This results from the presence of elements with a remarkable power of reflecting light, called iridocytes. When they occur in a thick and dense layer called the argenteum, on the inner surface of the skin, they give to the fish its silvery appearance. When they are scattered singly they cause an iridescence, or play of colors. Besides the iridocytes, the dermis contains the fish’s pigment cells, some of which are black and others colored. Scales grow out of the dermis beneath the epidermis, but the posterior edge may project to some extent through [ 34] THE, STRUCTURE: OF A “‘PISH the latter. They are thin calcified plates, being more hornlike in composition than bones and comparable to human finger nails. True scales in an adult fish usually overlap one another like shingles on a roof, the outer edges always directed toward the tail of the fish, and the scale in front covering about three-fourths of the one behind it. Once a race of fishes have developed a complete body covering of scales, they may for some reason lose it wholly or in part; or they may retain it only in a de- generate form as oblong plates partly embedded in the skin, or as small spines or prickles. The carp (Cyprinus carpio), ofters an example of a single species some members of which may be completely scaled, others, such as the mirror carp, only partly scaled, and still others, such as the leather carp, wholly naked. The fresh-water eels have degenerated scales consisting of oblong plates arranged in groups set at right angles to each other and partly buried in the skin. Only very close examination will disclose these structures in the eel. The filefishes, some of the puffers, and certain sculpins all show degener- ate or modified scales in the form of small spines or prickles (see Plate 5). To return now to the protective body coverings other than scales, we call attention again to the bony rings which inclose the sea horses. They form a sort of exterior or exoskeleton and an effective armor; yet they do not make the body rigid, as each ring is at least partly free from its neighbor, thereby permitting movement and con- siderable flexibility. The trunkfishes, on the other hand, are completely inclosed, exclusive of the tail, in a rigid bony case composed of thoroughly united plates, forming a continuous armor and leaving flexibility to the tail only. This bony case generally is provided with several strong - spines. One of the species, Lactophrys tricornis, for example, has a spine above and in front of each eye, which is directed forward and resembles a horn, earning the crea- ture the popular name “‘cowfish.”’ [35] FISHES The word ganoid, used to describe one of the two time- honored groups of the subclass ray-fins, refers to the sub- stance which covers the outer surface of the bony plates protecting members of this great group of fishes. This substance resembles dentine and is called ganoin. The ganoid plates on the American gar pike (Fig. 16), which may be taken as the most typical example of this kind of covering, are large, bony, and rectangular, arranged in rows and placed edge to edge, in contrast to the over- lapping scales already described. These plates are very hard and form an excellent armor. Although interlocked for additional strength, they do not make the body rigid,» as they may separate or partly slide over each other when the fish moves its body from side to side. The free edges of the plates are so sharp that a large live fish, when held in the hands, can cut deep wounds by throwing its body from side to side in its effort to escape, thereby pinching the hands and fingers between the margins of the plates. In the sturgeons—among the largest fishes of our sea- shores, rivers, and lakes—the ganoid plates only partly cov- er the body (see Fig. 11). Generally two rows of plates lie on the back, one along the side, and another along the a edge of the abdomen. Those Fic. 16. Ganoid plates of the | sections unprotected by bony American gay pikes cipiateas ” shields\are covered with komen leathery skin. Finally, in the paddlefishes the ganoid plates have disappeared entirely, leaving the skin almost smooth (see Plate 4). The porcupine fishes, already referred to because of their spherical or globular body, derive their name from the sharp spines with which they are covered. The spines often are so broad at the base that, together with the roots they bear, a continuous coat of mail is formed. [ 36 ] UUP TAY WUPETTTT AA “Iq jo Asaqno7 “STOMP LT uaydais Ag “OHUR AV ey WUOd]