GUIDE TO THE STUDY OF FISHES

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A GUIDE
TO
THE STUDY OF FISHES

BY
DAVID STARR JORDAN
President of Leland Stanford Junior University

With Colored Frontispieces and 427 Illustrations

IN TWO VOLUMES Vol I.

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

NEW YORK
HENRY HOLT AND COMPANY
1905

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

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

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PREFACE

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

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

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

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

David Starr Jordan.

Palo Alto, Santa Clara County, Cal.,
October, 1904.

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ERRATA[1]
VOL. I

• Frontispiece, for Paramia quinqueviltata read Paramia quinquevittata
• Page xiii, line 10, for Filefish read Tilefish
• 39, " 15, for Science read Sciences
• 52, lines 4 and 5, transpose hypocoracoid and hypercoracoid
• 115, line 24, for Hexagramidæ read Hexagrammidæ
• 162, " 7, The female salmon does as much as the male in covering the eggs.
• 169, last line, for immmediately read immediately
• 189, legend, for Miaki read Misaki
• 313, line 26, for sand-pits read sand-spits
• 322, " 7 and elsewhere, for Wood's Hole read Woods Hole
• 324, " 15, for Roceus read Roccus
• 327, " next to last, for masquinonqy read masquinongy
• 357, " 5, for Filefish read Tilefish
• 361, " 26, for 255 feet read 25 feet
• 368, " 26, for infallibility read fallibility
• 414, " 22, for West Indies read East Indies
• 419, " 23, for-99 read-96
• 420, " 28, for were read are
• 428, " 24, for Geffroy, St. Hilaire read Geoffroy St. Hilaire
• 428, " 25, for William Kitchener Parker read William Kitchen Parker
• 462, " 32, for Enterpneusta read Enteropneusta

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CONTENTS
VOL. I.

LIST OF ILLUSTRATIONS
VOL. I.

Fig. 1.—Long-eared Sunfish, Lepomis megalotis (Rafinesque). (From life by R. W. Shufeldt.)—Page 2.

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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 Cincin[Pg 4]nati, 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.

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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[Pg 6] 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[Pg 7] 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.)

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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,[Pg 9] 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,[Pg 10] 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[Pg 11] 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.

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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[Pg 13] 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.

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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[Pg 15] 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.

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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.

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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.

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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.)

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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[Pg 20] 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.[Pg 21] 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[Pg 22] 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[Pg 23] 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 [Pg 24]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 di[Pg 25]vided 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.

[Pg 26]

CHAPTER III
THE DISSECTION OF THE FISH

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

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

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

[Pg 27]

Fig. 16.—Dissection of the Blue-green Sunfish, Apomotis cyanellus Rafinesque. (After Kellogg.)—27.

[Pg 28] Next it flows into the thick-walled ventricle, whence by the rhythmical contraction of its walls it is forced into an arterial bulb which lies at the base of the ventral aorta, which carries it on to the gills. After passing through the fine gill-filaments, it is returned to the dorsal aorta, a large blood-vessel which extends along the lower surface of the back-bone, giving out branches from time to time.

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

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

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

The lancelets, lampreys, and hagfishes possess no genital ducts. In the former the germ cells are shed into the atrial cavity, and from there find their way to the exterior either through the mouth or the atrial pore; in the latter they are shed directly into the body cavity, from which they escape through the abdominal pores. In the sharks and skates the Wolffian duct in the male, in addition to its function as an excretory duct, serves also as a passage for the sperm, the testes having a direct connection with the kidneys. In these forms there is a pair of Müllerian ducts which serve as oviducts in the females; they extend the length of the body cavity, and at their anterior end have an opening which receives the eggs which have escaped from the ovary into the body cavity. In some bony fishes as the eels and female salmon the germ cells are shed into the body cavity and escape through genital pores, which, however, may[Pg 29] not be homologous with abdominal pores. In most other bony fishes the testes and ovaries are continued directly into ducts which open to the outside.

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

Fig. 17.—Black Swallower, Chiasmodon niger Johnson, containing a fish larger than itself. Le Have Bank.

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

The most usual type of teeth among fishes is that of villiform bands. Villiform teeth are short, slender, even, close-set, making a rough velvety surface. When the teeth are larger and more widely separated, they are called cardiform, like the teeth of a wool-card. Granular teeth are small, blunt, and sand-like. Canine teeth are those projecting above the level of the others, usually sharp, curved, and in some species barbed. Sometimes[Pg 30] the canines are in front. In some families the last tooth in either jaw may be a "posterior canine," serving to hold small animals in place while the anterior teeth crush them. Canine teeth are often depressible, having a hinge at base.

Fig. 18.—Jaws of a Parrot-fish, Sparisoma aurofrenatum (Val.). Cuba.

Teeth very slender and brush-like are called setiform. Teeth with blunt tips are molar. These are usually enlarged and fitted for crushing shells. Flat teeth set in mosaic, as in many rays and in the pharyngeals of parrot-fishes, are said to be paved or tessellated. Knife-like teeth, occasionally with serrated edges, are found in many sharks. Many fishes have incisor-like teeth, some flattened and truncate like human teeth, as in the sheepshead, sometimes with serrated edges. Often these teeth are movable, implanted only in the skin of the lips. In other cases they are set fast in the jaw. Most species with movable teeth or teeth with serrated edges are herbivorous, while strong incisors may indicate the choice of snails and crabs as food. Two or more of these different types may be found in the same fish. The knife-like teeth of the sharks are progressively shed, new ones being constantly formed on the inner margins of the jaw, so that the teeth are marching to be lost over the edge of the jaw as soon as each has fulfilled its function. In general the more distinctly a species is a fish-eater, the sharper are the teeth. Usually fishes show little discrimination in their choice of food; often they devour the young of their own species as readily as any other. The digestive process is rapid, and most fishes rapidly increase in size in the process of development. When food ceases to be abundant the fishes grow more slowly. For this reason the same species will grow to a larger size in large streams than in small ones, in lakes than in brooks. In most cases there is no absolute limit to growth, the species growing as long as it lives. But while some species endure many years, others are certainly very short[Pg 31]lived, and some may be even annual, dying after spawning, perhaps at the end of the first season.

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

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

Fig. 19.—Sheepshead (with incisor teeth), Archosargus probatocephalus (Walbaum). Beaufort, N. C.

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

In the lampreys there is a distinction only of the œsophagus with many longitudinal folds and the intestine with but[Pg 32] one. In the bony fishes the stomach is an enlarged area, either siphon-shaped, with an opening at either end, or else forming a blind sac with the openings for entrance (cardiac) and exit (pyloric) close together at the anterior end. In the various kinds of mullets (Mugil) and in the hickory shad (Dorosoma), fishes which feed on minute vegetation mixed with mud, the stomach becomes enlarged to a muscular gizzard, like that of a fowl. Attached near the pylorus and pouring their secretions into the duodenum or small intestine are the pyloric cæca. These are tubular sacs secreting a pale fluid and often almost as long as the stomach or as wide as the intestine. These may be very numerous as in the salmon, in which case they are likely to become coalescent at base, or they be few or altogether wanting.

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

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

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

The Spiral Valve.—In the sharks or skates the rectum or large intestine is peculiarly modified, being provided with a spiral valve, with sometimes as many as forty gyrations. A spiral valve is also present in the more ancient types of the true fishes as dipnoans, crossopterygians, and ganoids. This valve greatly increases the surface of the intestine, doing away with the necessity for length. In the bowfin (Amia) and the garpike (Lepi[Pg 33]sosteus) the valve is reduced to a rudiment of three or four convolutions near the end of the intestine. In the sharks and skates the intestine opens into a cloaca, which contains also the urogenital openings. In all fishes the latter lie behind the orifice of the intestine. In the bony fishes and the ganoids there is no cloaca.

Fig. 20.—Stone-roller, Campostoma anomalum (Rafinesque). Family Cyprinidæ. Showing nuptial tubercles and intestines coiled about the air-bladder.

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

[Pg 34]

CHAPTER IV
THE SKELETON OF THE FISH

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

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

The assumption that each element in the shoulder-girdle and the pectoral fin of the fish must correspond in detail to the arm of man has led to great confusion in naming the different[Pg 35] bones. Among the many bones of the fish's shoulder-girdle and pectoral fin, three or four different ones have successively borne the names of scapula, clavicle, coracoid, humerus, radius, and ulna. None of these terms, unless it be clavicle, ought by rights apply to the fish, for no bone of the fish is a true homologue of any of these as seen in man. The land vertebrates and the fishes have doubtless sprung from a common stock, but this stock, related to the crossopterygians of the present day, was unspecialized in the details of its skeleton, and from it the fishes and the higher vertebrates have developed the widely diverging lines.

Fig. 21.—Striped Bass, Roccus lineatus (Bloch). Potomac River.

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

In the sharks, dipnoans, crossopterygians, ganoids, and teleosts or bony fishes, jaws are developed as well as a variety of other bones around the mouth and throat. The jaw-bearing forms are sometimes known by the general name of gnathostomes. In the sharks and their relatives (rays, chimæras, etc.) all the skeleton is composed of cartilage. In the more specialized bony fishes, besides these bones we find also series of membrane bones, more or less external to the skull and composed of[Pg 36] ossified dermal tissues. Membrane bones are not found in the sharks and lampreys, but are developed in an elaborate coat of mail in some extinct forms.

Fig. 22.—Roccus lineatus. Lateral view of cranium.

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

[Pg 37]

Fig. 23.—Roccus lineatus. Superior view of cranium.

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

[Pg 38]

Fig. 24.—Roccus lineatus. Inferior view of cranium.

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

[Pg 39]

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

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

Bones of the Cranium.—The vomer (1) is the anterior part of the roof of the mouth, armed with small teeth in the striped bass and in many other fishes, but often toothless. The ethmoid (2) lies behind the vomer on the upper surface of the skull, and the prefrontal (3) projects on either side and behind the ethmoid, the nostrils usually lying over or near it and near the nasal bone (51). Between the eyes above are the two frontal (4) bones joined by a suture. On the side behind the posterior angle of the frontal is the sphenotic (5) above the posterior part of the eye. Behind each frontal is the parietal (6). Behind the parietal and more or less turned inward over the ear-cavity is the epiotic (7). Between the parietals, and in most fishes rising into a thin crest, is the supraoccipital (8), which bounds the cranium above and behind, its posterior margin being usually a vertical knife-like edge. The pterotic (9) forms a sort of wing or free margin behind the epiotic and over the ear-cavity. The opisthotic (10) is a small, hard, irregular bone behind the pterotic. The exoccipital (11) forms a concave joint or condyle on each side of the basioccipital (12), by which the vertebral column is joined to the skull. The parasphenoid (13) forms a narrow ridge of the roof of the mouth, connecting the vomer with the basioccipital. In some fishes of primitive structure (Salmo, Beryx) there is another bone, called orbitosphenoid, on the middle line above and between the eyes. The basisphe[Pg 40]noid (14) is a little bone above the myotome or tube in which runs the rectus muscle of the eye. It descends toward the parasphenoid and is attached to the prootic. The prootic (15) is an irregular bone below the ear region and lying in advance of the opisthotic. The alisphenoid (16) is a small bone in the roof of the mouth before the prootic. These sixteen bones (with a loose bone of specialized form, the otolith, within the ear-cavity) constitute the cranium. All are well developed in the striped bass and in most fishes. In some specialized forms they are much distorted, coossified, or otherwise altered, and their relations to each other may be more or less changed. In the lower forms they are not always fully differentiated, but in nearly all cases their homologies can be readily traced. In the sharks and lampreys the skull constitutes a continuous cartilaginous box without sutures. In the dipnoans and other forms having a bony casque the superficial bones outside the cranium may not correspond to the cartilaginous elements of the soft skull itself.

Fig. 25—Roccus lineatus. Posterior view of cranium.

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

[Pg 41]

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

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

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

[Pg 42]

Fig. 26.—Roccus lineatus. Face-bones, shoulder and pelvic girdles, and hyoid arch.

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

[Pg 43]

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

Fig. 27.—Lower jaw of Amia calva (Linnæus), showing the gular plate.

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

The Suspensorium of the Mandible.—The lower jaw is attached to the cranium by a chain of suspensory bones, which vary a good deal with different groups of fishes. The articular is jointed with the flat quadrate bone (19), which lies behind the pterygoid. A slender bone passes upward (18) under the preopercle and the metapterygoid, forming a connection above[Pg 44] with a large flattish bone, the hyomandibular (17), which in turn joins the cranium. The slender bone which thus keys together the upper and lower elements, hyomandibular and quadrate, forming the suspensorium of the lower jaw, is known as symplectic (18). The hyomandibular is thought to be homologous with the stapes, or stirrup-bone, of the ear in higher animals. In this case the symplectic may be homologous with its small orbicular bone, and the malleus is a transformation of the articular. The incus, or anvil-bone, may be formed from part of Meckel's cartilage. All these homologies are however extremely hypothetical. The core of the lower jaw is formed of a cartilage called Meckel's cartilage, outside which the membrane bones, dentary, etc., are developed. This cartilage forms the lower jaw in sharks, true jaw-bones not being developed in these fishes. In lampreys and lancelets there is no lower jaw.

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

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

The largest uppermost flat bone of the gill-covers is known[Pg 45] as the opercle (25). Below it, joined by a suture, is the subopercle (26). Before it is the prominent ridge of the preopercle (24), which curves forward below and forms a more or less distinct angle, often armed with serrations or spines. In some cases this armature is very highly developed. The interopercle (27) lies below the preopercle and parallel with the lower limb.

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

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

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

The gill-arches are suspended to the cranium from above by the suspensory pharyngeal (44). Each arch contains three parts—the epibranchial (43), above, the ceratobranchial (42), forming the middle part, and the hypobranchial (41), the lower part[Pg 46] articulating with the series of basibranchials (40) which lie behind the epihyal of the tongue. On the three bones forming the first gill-arch are attached numerous appendages called gill-rakers (47). These gill-rakers vary very greatly in number and form. In the striped bass they are few and spear-shaped. In the shad they are very many and almost as fine as hairs. In some fishes they form an effective strainer in separating the food, or perhaps in keeping extraneous matter from the gills. In some fishes they are short and lumpy, in others wanting altogether.

Fig. 28.—Roccus lineatus. Branchial arches. (After Starks.)

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

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

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

[Pg 47]

Each vertebra consists of a double concave body or centrum (66). Above it are two small projections often turned backward, zygapophyses (71), and two larger ones, neurapophyses (67), which join above to form the neural spine (68) and thus form the neural canal, through which passes the spinal cord from end to end of the body.

Fig. 29.—Pharyngeal bone and teeth of European Chub, Leuciscus cephalus (Linnæus). (After Seelye.)

Fig. 30.—Upper pharyngeals of a Parrot-fish, Scarus strongylocephalus.

Fig. 31.—Lower pharyngeals of a Parrot-fish, Scarus strongylocephalus (Bleeker).

Below in the vertebræ of the posterior half of the body the hæmapophyses (69) unite to form the hæmal spine (70), and through the hæmal canal thus formed passes a great artery. The vertebræ having hæmal as well as neural spines are known as caudal vertebræ, and occupy the posterior part of the body, usually that behind the attachment of the anal fin (78).

The anterior vertebræ known as abdominal vertebræ, bounding the body-cavity, possess neural spines similar to those of[Pg 48] the caudal vertebræ. In place, however, of the hæmapophyses are projections known as parapophyses (72), which do not meet below, but extend outward, forming the upper part of the wall of the abdominal cavity.

Fig. 32.—Pharyngeals of Italian Parrot-fish, Sparisoma cretense (L.). a, upper; b, lower.

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

Fig. 33.—Roccus lineatus. Vertebral column and appendages, with a typical vertebra. (After Starks.)

• 64. Abdominal vertebræ.
• 65. Caudal vertebræ.
• 66. Centrum.
• 67. Neurapophysis.
• 68. Neural spine.
• 69. Hæmapophysis.
• 70. Hæmal spine.
• 71. Zygapophysis.
• 72. Parapophysis.
• 73. Ribs.
• 74. Epipleurals.
• 75. Interneural.
• 76. Dorsal fin.
• 77. Interhæmal.
• 78. Anal fin.
• 79. Hypural.
• 80. Caudal fin.

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

In the garpike the vertebræ are convex anteriorly, concave[Pg 49] behind, being joined by ball-and-socket joints (opisthocœlian). In most other fishes they are double concave (amplicœlian). In sharks the vertebræ are imperfectly ossified, a number of terms, asterospondylous, cyclospondylous, tectospondylous, being applied to the different stages of ossification, these terms referring to the different modes of arrangement of the calcareous material within the vertebra.

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

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

Fig. 34.—Basal bone of dorsal fin, Holoptychius leptopterus (Agassiz). (After Woodward.)

The tail of the striped bass, ending in a broad plate which supports the caudal, is said to be homocercal. In more primitive forms the tail is turned upward more or less, the fin being largely thrown to its lower side. Such a tail as in the sturgeon is said to be heterocercal. In the isocercal tail of the codfish and its relatives the vertebræ are progressively smaller behind and the hypural plate is obsolete or nearly so, the vertebræ remaining in the line of the axis of the body and dividing the caudal fin equally. The simplest form of tail, called diphycercal,[Pg 50] is extended horizontally, tapering backward, the fin equally divided above and below, without hypural plate. In any form of the tail, it may through degeneration be attenuate or whip-like, a form called leptocercal.

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

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

The post-temporal sometimes projects behind through the skin and may bear spines or serrations. In front of the post-temporal and a little to the outside of it is the small supratemporal (52) also usually connecting the shoulder-girdle with the skull. Below the post-temporal, extending downward and backward, is the flattish supraclavicle (posterotemporal) (54). To this is joined the long clavicle (proscapula) (55), which runs forward and downward in the bony fishes, meeting its fellow on the opposite side in a manner suggesting the wishbone of a[Pg 51] fowl. Behind the base of the clavicle, the sword-shaped post-clavicle (56) extends downward through the muscles behind the base of the pectoral fin. In some fishes, as the stickleback and the trumpet-fish, a pair of flattish or elongate bones called interclavicles (infraclavicles) lie between and behind the lower part of the clavicle. These are not found in most fishes and are wanting in the striped bass. They are probably in all cases merely extensions of the hypocoracoid.

Fig. 35.—Inner view of shoulder-girdle of the Buffalo-fish, Ictiobus bubalus Rafinesque, showing the mesocoracoid (59). (After Starks.)

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

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

In the Alaska blackfish (Dallia pectoralis) the two coracoids are represented by a thin, cartilaginous plate, imperfectly divided, and there are no actinosts. In almost all bony fishes, however, these bones are well differentiated and distinct. In most of the soft-rayed fishes an additional V-shaped bone or arch exists on the inner surface of the shoulder-girdle near the insertion of the hypercoracoid. This is known as the meso[Pg 52]coracoid (59). It is not found in the striped bass, but is found in the carp, catfish, salmon, and all their allies.

Fig. 36.—Sargassum-fish, Pterophryne tumida (Osbeck). One of the Anglers. Family Antennariidæ.

Fig. 37.—Shoulder-girdle of Sebastolobus alascanus Gilbert. (After Starks.)

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

[Pg 53]

The Posterior Limbs.—The posterior limb or ventral fin (63) is articulated to a single bone on either side, the pelvic girdle (62).

Fig. 38.—Cranium of Sebastolobus alascanus Gilbert. (After Starks.)

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

In the shark the pelvic girdle is rather largely developed, but in the more specialized fishes it loses its importance. In the less specialized of the bony fishes the pelvis is attached at a distance from the head among the muscles of the side, and free from the shoulder-girdle and other parts of the skeleton. The ventral fins are then said to be abdominal. When very close to the clavicle, but not connected with it, as in the mullet, the fin is still said to be abdominal or subabdominal. In the striped bass the pelvis is joined by ligament between the clavicles, near their tip. The ventral fins thus connected, as seen in most spiny-rayed fishes, are said to be thoracic. In certain forms the pelvis is thrown still farther forward and attached at the throat or even to the chin. When the ventral fins are thus inserted before the shoulder-girdle, they are said to be jugular.[Pg 54] Most of the fishes with spines in the fins have thoracic ventrals. In the fishes with jugular ventrals these fins have begun a process of degeneration by which the spines or soft rays or both are lost or atrophied.

Fig. 39.—Lower jaw and palate of Sebastolobus alascanus. (After Starks.)

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

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

The Skeleton in Primitive Fishes.—To learn the names of bones we can deal most satisfactorily with the higher fishes, those in[Pg 55] which the bony framework has attained completion. But to understand the origin and relation of parts we must begin with the lowest types, tracing the different stages in the development of each part of the system.

Fig. 40.—Maxillary and premaxillary of Sebastolobus alascanus. M, maxillary; PM, premaxillary.

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

Fig. 41.—Part of skeleton of Selene vomer (Linnæus).

In the embryo of the bony fish a similar notochord precedes the segmentation and ossification of the vertebral column. In most of the extinct types of fishes a notochord more or less[Pg 56] modified persisted through life, the vertebræ being strung upon it spool fashion in various stages of development. In the Cyclostomi (lampreys and hagfishes) the limbs and lower jaw are still wanting, but a distinct skull is developed. The notochord is still present, but its anterior pointed end is wedged into the base of a cranial capsule, partly membranous, partly cartilaginous. There is no trace of segmentation in the notochord itself in these or any other fishes, but neutral arches are foreshadowed in a series of cartilages on each side of the spinal chord. The top of the head is protected by broad plates. There are ring-like cartilages supporting the mouth and other cartilages in connection with the tongue and gill structures.

Fig. 42.—Hyostylic skull of Chiloscyllium indicum, a Scyliorhinoid Shark. (After Parker and Haswell.)

Fig. 43.—Skull of Heptranchias indicus (Gmelin), a notidanoid shark. (After Parker and Haswell.)

Fig. 44.—Basal bones of pectoral fin of Monkfish, Squatina. (After Zittel.)

[Pg 57]

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

Fig. 45.—Pectoral fin of Heterodontus philippi. (From nature.)

Fig. 46.—Pectoral fin of Heptranchias indicus (Gmelin). (After Dean.)

The shoulder-girdle is made of a single cartilage, touching[Pg 58] the back-bone at a distance behind the head. To this cartilage three smaller ones are attached, forming the base of the pectoral fin. These are called mesopterygium, propterygium, and metapterygium, the first named being in the middle and more distinctly basal. These three segments are subject to much variation. Sometimes one of them is wanting; sometimes two are grown together. Behind these the fin-rays are attached. In most of the skates the shoulder-girdle is more closely connected with the anterior vertebræ, which are more or less fused together.

Fig. 47.—Shoulder-girdle of a Flounder, Paralichthys californicus (Ayres).

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

[Pg 59]

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

Fig. 48.—Shoulder-girdle of a Toadfish, Batrachoides pacifici (Günther).

Fig. 49.—Shoulder-girdle of a Garfish, Tylosurus fodiator (Jordan and Gilbert).

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

[Pg 60]

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

Fig. 50.—Shoulder-girdle of a Hake, Merluccius productus (Ayres).

The archipterygium of the Dipnoan genus Neoceratodus is thus described by Dr. Günther ("Guide to the Study of Fishes," p. 73): "The pectoral limb is covered with small scales along the middle from the root to the extremity, and is surrounded by a rayed fringe similar to the rays of the vertical fins. A muscle split into numerous fascicles extends all the length of the fin, which is flexible in every part and in every direction. The cartilaginous framework supporting it is joined to the scapular arch by a broad basal cartilage, generally single, sometimes[Pg 61] showing traces of a triple division. Along the middle of the fin runs a jointed axis gradually becoming smaller and thinner towards the extremity. Each joint bears on each side a three-, two-, or one-jointed branch."

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

[Pg 62]

CHAPTER V
MORPHOLOGY OF THE FINS

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

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

The evidence of comparative anatomy is most completely secured, but it is often indecisive as to relative age and primi[Pg 63]tiveness of origin among structures. As to ontogeny, it is, of course, true that through heredity "the life-history of the individual is an epitome of the life-history of the race." "Ontogeny repeats phylogeny," and phylogeny, or line of descent of organisms and structures, is what we are seeking. But here the repetition is never perfect, never nearly so perfect in fact as Hæckel and his followers expected to find it. The demands of natural selection may lead to the lengthening, shortening, or distortion of phases of growth, just as they may modify adult conditions. The interpolation of non-ancestral stages is recognized in several groups. The conditions of the individual development may, therefore, furnish evidence in favor of certain theories of origins, but they cannot alone furnish the absolute proof.

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

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

That the vertical fins, dorsal, anal, and caudal, have their origin in a median fold of the skin admits of no question. In the lowest forms which bear fins these structures are dermal folds, being supported by very feeble rays. Doubtless at first the vertical fins formed a continuous fold, extending around the tail, this fold ultimately broken, by atrophy of parts not needed, into distinct dorsal, anal, and caudal fins. In the lower fishes, as in the earlier sharks, there is an approach to this condition of primitive continuity, and in the embryos[Pg 64] of almost all fishes the same condition occurs. Dr. John A. Ryder points out the fact that there are certain unexplained exceptions to this rule. The sea-horse, pipefish, and other highly modified forms do not show this unbroken fold, and it is wanting in the embryo of the top-minnow, Gambusia affinis. Nevertheless the existence of a continuous vertical fold in the embryo is the rule, almost universal. The codfish with three dorsals, the Spanish mackerel with dorsal and anal finlets, the herring with one dorsal, the stickleback with a highly modified one, all show this character, and we may well regard it as a certain trait of the primitive fish. This fold springs from the ectoblast or external series of cells in the embryo. The fin-rays and bony supports of the fins spring from the mesoblast or middle series of cells, being thrust upward from the skeleton as supports for the fin-fold.

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

Evidence of Paleontology.—If we had representations of all the early forms of fishes arranged in proper sequence, we could decide once for all, by evidence of paleontology, which form of fin appears first and what is the order of appearance. As to this, it is plain that we do not know the most primitive form of fin. Sharks of unknown character must have existed long before the earliest remains accessible to us. Hence the evidence of paleontology seems conflicting and uncertain. On the whole it lends most support to the fin-fold theory. In the later Devonian, a shark, Cladoselache fyleri, is found in which the paired fins are lappet-shaped, so formed and placed as to suggest[Pg 65] their origin from a continuous fold of skin. In this species the dorsal fins show much the same form. Other early sharks, constituting the order of Acanthodei, have fins somewhat similar, but each preceded by a stiff spine, which may be formed from coalescent rays.

Fig. 51.—Cladoselache fyleri (Newberry), restored. Upper Devonian of Ohio. (After Dean.)

Fig. 52.—Fold-like pectoral and ventral fins of Cladoselache fyleri. (After Dean.)

Long after these appears another type of sharks represented by Pleuracanthus and Cladodus, in which the pectoral fin is a jointed organ fringed with rays arranged serially in one or two rows. This form of fin has no resemblance to a fold of skin, but accords better with Gegenbaur's theory that the pectoral limb was at first a modified gill-arch. In the Coal Measures are found also teeth of sharks (Orodontidæ) which bear a strong resemblance to still existing forms of the family of Heterodontidæ, which originates in the Permian. The existing Heterodontidæ have the usual specialized form of shark-fin, with three of the basal segments especially enlarged and placed side[Pg 66] by side, the type seen in modern sharks. Whatever the primitive form of shark-fin, it may well be doubted whether any one of these three (Cladoselache, Pleuracanthus, or Heterodontus) actually represents it. The beginning is therefore unknown, though there is some evidence that Cladoselache is actually more nearly primitive than any of the others. As we shall see, the evidence of comparative anatomy may be consistent with either of the two chief theories, while that of ontogeny or embryology is apparently inconclusive, and that of paleontology is apparently most easily reconciled with the theory of the fin-fold.

Fig. 53.—Pectoral fin of shark, Chiloscyllium. (After Parker and Haswell.)

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

In the codfish (Gadus callarias) the pectoral fin-fold "appears as a slight longitudinal elevation of the skin on either side of the body of the embryo a little way behind the auditory vesicles, and shortly after the tail of the embryo begins to bud out. At the very first it appears to be merely a dermal fold, and in some forms a layer of cells extends out underneath it from the sides of the body, but does not ascend into it. It[Pg 67] begins to develop as a very low fold, hardly noticeable, and, as growth proceeds, its base does not expand antero-posteriorly, but tends rather to become narrowed, so that it has a pedunculated form. With the progress of this process the margin of the fin-fold also becomes thinner at its distal border, and at the basal part mesodermal cells make their appearance more noticeably within the inner contour-line. The free border of the fin-fold grows out laterally and longitudinally, expanding the portion outside of the inner contour-line of the fin into a fan-shape. This distal thinner portion is at first without any evidence of rays; further than that there is a manifest tendency to a radial disposition of the histological elements of the fin."

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

Current Theories as to Origin of Paired Fins.—There are three chief theories as to the morphology and origin of the paired fins. The earliest is that of Dr. Karl Gegenbaur, supported by various workers among his students and colleagues. In his view the pectoral and ventral fins are derived from modifications of primitive gill-arches. According to this theory, the skeletal arrangements of the vertebrate limb are derived from modifications of one primitive form, a structure made up of successive joints, with a series of fin-rays on one or both sides of it. To this structure Gegenbaur gives the name of archipterygium. It is found in the shark, Pleuracanthus, in Cladodus, and in all the Dipnoan and Crossopterygian fishes, its primitive form being still retained in the Australian genus of Dipnoans, Neoceratodus. This biserial archipterygium with its limb-girdle is[Pg 68] derived from a series of gill-rays attached to a branchial arch. The backward position of the ventral fin is due to a succession of migrations in the individual and in the species.

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

Fig. 54.—Skull and shoulder-girdle of Neoceratodus forsteri (Günther), showing the archipterygium.

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

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

"This view is based on the skeletal structures within the fin. It rests upon (1) the assumption that the archipterygium is the primitive type of fin, and (2) the fact that amongst the Selachians is found a tendency for one branchial ray to become larger than the others, and, when this has happened, for the base of attachment of neighboring rays to show a tendency to migrate from the branchial arch on to the base of the larger or, as we may call it, primary ray; a condition coming about which, were the process to continue rather farther than it is known to do in actual fact, would obviously result in a struc[Pg 69]ture practically identical with the archipterygium. Gegenbaur suggests that the archipterygium actually has arisen in this way in phylogeny."

Fig. 55.—Acanthoessus wardi (Egerton). Carboniferous. Family Acanthoessidæ. (After Woodward.)

Fig. 56.—Shoulder-girdle of Acanthoessus. (After Dean.)

Fig. 57.—Pectoral fin of Pleuracanthus. (After Dean.)

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

"This view, in its modern form, was based by Balfour on his observation that in the embryos of certain Elasmobranchs[Pg 70] the rudiments of the pectoral and pelvic fins are at a very early period connected together by a longitudinal ridge of thickened epiblast—of which indeed they are but exaggerations. In Balfour's own words referring to these observations: 'If the account just given of the development of the limb is an accurate record of what really takes place, it is not possible to deny that some light is thrown by it upon the first origin of the vertebrate limbs. The facts can only bear one interpretation, viz., that the limbs are the remnants of continuous lateral fins.'

Fig. 58.—Shoulder-girdle of Polypterus bichir. Specimen from the White Nile.

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

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

We may now take up the evidence in regard to each of the different theories, using in part the language of Kerr, the para[Pg 71]graphs in quotation-marks being taken from his paper. We may first consider Balfour's theory of the lateral fold.

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

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

Fig. 59.—Arm of a frog.

"An apparently powerful support from the side of embryology came in Dohrn and Rabl's discoveries that in Pristiurus all the interpterygial myotomes produce muscle-buds. This, however, was explained away by the Gegenbaur school as being merely evidence of the backward migration of the hind limb—successive myotomes being taken up and left behind again as the limb moved farther back. As either explanation seems an adequate one, I do not think we can lay stress upon this body of facts as supporting either one view or the other. The[Pg 72] facts of the development of the skeleton cannot be said to support the fold view; according to it we should expect to find a series of metameric supporting rays produced which later on become fused at their bases. Instead of this we find a longitudinal bar of cartilage developing quite continuously, the rays forming as projections from its outer side.

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

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

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

[Pg 73]

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

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

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

"The portion of Gegenbaur's view which asserts that the biserial archipterygial fin is of an extremely primitive character is supported by a large body of anatomical facts, and is rendered further probable by the great frequency with which fins apparently of this character occur amongst the oldest known fishes. On the lateral-fold view we should have to[Pg 74] regard these as independently evolved, which would imply that fins of this type are of a very perfect character, and in that case we may be indeed surprised at their so complete disappearance in the more highly developed forms, which followed later on."

Fig. 60.—Pleuracanthus decheni (Goldfuss). (After Dean.)

As to Gegenbaur's theory it is urged that no form is known in which a gill-septum develops into a limb during the growth of the individual. The main thesis, according to Professor Kerr, "that the archipterygium was derived from gill-rays, is supported only by evidence of an indirect character. Gegenbaur in his very first suggestion of his theory pointed out, as a great difficulty in the way of its acceptance, the position of the limbs, especially of the pelvic limbs, in a position far removed from that of the branchial arches. This difficulty has been entirely removed by the brilliant work of Gegenbaur's followers, who have shown from the facts of comparative anatomy and embryology that the limbs, and the hind limbs especially, actually have undergone, and in ontogeny do undergo, an extensive backward migration. In some cases Braus has been able to find traces of this migration as far forward as a point just behind the branchial arches. Now, when we consider the numbers, the enthusiasm, and the ability of Gegenbaur's disciples, we cannot help being struck by the fact that the only evidence in favor of this derivation of the limbs has been that which tends to show that a migration of the limbs backwards has taken place from a region somewhere near the last branchial arch, and that they have failed utterly to discover any intermediate steps between gill-rays and archipterygial fin. And if for a moment we apply the test of common sense we cannot but be impressed by the improbability of the evolution of a gill-septum, which in all the lower forms of fishes is fixed[Pg 75] firmly in the body-wall, and beneath its surface, into an organ of locomotion.

Fig. 61.—Embryos of Heterodontus japonicus Maclay and Macleay, a Cestraciont shark, showing the backward migration of the gill-arches and the forward movement of the pectoral fin. a, b, c, representing different stages of growth. (After Dean.)

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

As to the backward migration of the ventral fins, Dr. Bashford Dean has recently brought forward evidence from the embryo of a very ancient type of shark (Heterodontus japonicus) that this does not actually occur in that species. On the other[Pg 76] hand, we have a forward migration of the pectoral fin, which gradually takes its place in advance of the hindmost gill-arches. The accompanying cut is from Dean's paper, "Biometric Evidence in the Problem of the Paired Limbs of the Vertebrates" (American Naturalist for November, 1902). Dean concludes that in Heterodontus "there is no evidence that there has ever been a migration of the fins in the Gegenbaurian sense." "The gill region, at least in its outer part, shows no affinity during proportional growth with the neighboring region of the pectoral fin. In fact from an early stage onward, they are evidently growing in opposite directions."

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

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

"In the Urodela (salamanders) the external gills are well-known structures—serially arranged projections from the body-wall near the upper ends of certain of the branchial arches. When one considers the ontogenetic development of these organs, from knob-like outgrowth from the outer face of the branchial arch, covered with ectoderm and possessing a mesoblastic core, and which frequently if not always appear before the branchial clefts are open, one cannot but conclude that they are morphologically projections of the outer skin and that they have nothing whatever to do with the gill-pouches of the gut-wall. Amongst the Urodela one such gill projects from each of the first three branchial arches. In Lepidosiren there is one on each of the branchial arches I-IV. In Polypterus and Calamoichthys (Erpetoichthys) there is one on the hyoid arch. Finally, in many Urodelan larvæ we have present at the same time as the external gills a pair of curious structures called balancers. At an early stage of my work on Lepidosiren,[Pg 77] while looking over other vertebrate embryos and larvæ for purposes of comparison, my attention was arrested by these structures, and further examinations, by section or otherwise, convinced me that there were serial homologues of the external gills, situated on the mandibular arch. On then looking up the literature, I found that I was by no means first in this view. Rusconi had long ago noticed the resemblance, and in more recent times both Orr and Maurer had been led to the same conclusion as I had been. Three different observers having been independently led to exactly the same conclusions, we may, I think, fairly enough regard the view I have mentioned of the morphological nature of the balancers as probably a correct one.

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

"No doubt these external gills are absent also in a few of the admittedly primitive forms such as, e.g., (Neo-) Ceratodus. But I would ask that in this connection one should bear in mind one of the marked characteristics of external gills—their great regenerative power. This involves their being extremely liable to injury and consequently a source of danger to their possessor. Their absence, therefore, in certain cases may well have been due to natural selection. On the other hand, the presence in so many lowly forms of these organs, the general close similarity in structure that runs through them in different forms, and the exact correspondence in their position and relations to the body can, it seems to me, only be adequately explained by looking on them as being homologous structures[Pg 78] inherited from a common ancestor and consequently of great antiquity in the vertebrate stem."

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

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

Fig. 62.—Polypterus congicus, a Crossopterygian fish from the Congo River. Young, with external gills. (After Boulenger.)

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

"This theory of the vertebrate paired limb seems to me, I confess, to be a more plausible one on the face of it than either of the two which at present hold the field. If untrue, it is so dangerously plausible as to surely deserve more consideration than it appears to have had. One of the main differences between it and the other two hypotheses is that, instead of deriving[Pg 79] the swimming-fin from the walking and supporting limb, it goes the other way about. That this is the safer line to take seems to me to be shown by the consideration that a very small and rudimentary limb could only be of use if provided with a fixed point d'appui. Also on this view, the pentadactyle limb and the swimming-fin would probably be evolved independently from a simple form of limb. This would evade the great difficulties which have beset those who have endeavored to establish the homologies of the elements of the pentadactyle limb with those of any type of fully formed fin."

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

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

[Pg 80]

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

Fig. 63.—Heterocercal tail of Sturgeon, Acipenser sturio (Linnæus). (After Zittel.)

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

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

Dean calls attention to the fact that in fishes which swim,[Pg 81] when adult, by an undulatory motion, the paired fins tend to disappear, as in the eel and in all eel-like fishes, as blennies and eel-pouts.

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

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

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

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

To the same type of tail Mr. Alexander Agassiz gave, in 1877, the name of leptocardial, on the supposition that it represented the adult condition of the lancelet. In this creature,[Pg 82] however, rudimentary basal rays are present, a condition differing from that of the early embryos.

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

Fig. 64.—Heterocercal tail of Bowfin, Amia calva (Linnæus). (After Zittel.)

Fig. 65.—Heterocercal tail of Garpike, Lepisosteus osseus (Linnæus).

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

[Pg 83]

Fig. 66.—Coryphænoides carapinus (Goode and Bean), showing leptocercal tail. Gulf Stream.

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

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

Fig. 67.—Heterocercal tail of Young Trout, Salmo fario (Linnæus). (After Parker and Haswell.)

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

The term gephyrocercal is devised by Ryder for fishes in which the end of the vertebral axis is aborted in the adult, leaving the caudal elements to be inserted on the end of this axis, thus bridging over the interval between the vertical fins,[Pg 84] as the name (γεφύρος, bridge; κέρκος, tail) is intended to indicate. Such a tail has been recognized in four genera only, Mola, Ranzania, Fierasfer, and Echiodon, the head-fishes and the pearl-fishes.

Fig. 68.—Isocercal tail of Hake, Merluccius productus (Ayres).

Fig. 69.—Homocercal tail of a Flounder, Paralichthys californicus.

[Pg 85]

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

Fig. 70.—Gephyrocercal tail of Mola mola (Linnæus). (After Ryder.)

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

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

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

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

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

"1. The forms that are best comparable and that are most nearly related to each other are the Dipnoi, an order of fishes at present represented by Lepidosiren, Protopterus, and Ceratodus, [Pg 86]and the Batrachians as represented by the Ganocephala, Salamanders, and Salamander-like animals.

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

Fig. 71.—Shoulder-girdle of Amia calva (Linnæus).

Fig. 72.—Shoulder-girdle of a Sea Catfish, Selenaspis dowi.

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

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

"The scapula in the Urodele and other Batrachians is entirely or almost wholly excluded from the glenoid foramen, and above[Pg 87] the coracoid. Therefore the corresponding element in Dipnoi must be the scapula.

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

Fig. 73.—Clavicles of a Sea Catfish, Selenaspis dowi (Gill).

"And it is equally impossible to identify the fish's elements with those of the higher reptiles or other vertebrates which have developed from the Batrachians. The elements in the shoulder-girdles of the distantly separated classes may be (to use the terms introduced by Dr. Lankester) homoplastic, but they are not homogenetic. Therefore they must be named accordingly. The element of the Dipnoan's shoulder-girdle, continuous downward from the scapula, and to which the coracoid is closely applied, may be named ectocoracoid.

"Neither the scapula in Batrachians nor the cartilaginous extension thereof, designated suprascapula, is dissevered from the coracoid. Therefore there is an a priori improbability[Pg 88] against the homology with the scapula of any part having a distant and merely ligamentous connection with the humerus-bearing element. Consequently, as an element better representing the scapula exists, the element named scapula (by Owen, Günther, etc.) cannot be the homologue of the scapula of Batrachians. On the other hand, its more intimate relations with the skull and the mode of development indicate that it is rather an element originating and developed in more intimate connection with the skull. It may therefore be considered, with Parker, as a post-temporal.

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

Fig. 74.—Shoulder-girdle of a Batfish, Ogcocephalus radiatus (Mitchill).

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

"With the humerus of the Dipnoans, the element of the Polypterids (single at the base, but immediately divaricating and with its limbs bordering an intervening cartilage which supports the pectoral and its basilar ossicles) must be homologous. But it is evident that the external elements of the[Pg 89] so-called carpus of the teleosteoid Ganoids are homologous with that element in Polypterids. Therefore those elements cannot be carpal, but must represent the humerus.

Fig. 75.—Shoulder-girdle of a Threadfin, Polydactylus approximans (Lay and Bennett).

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

"The two elements of the arch named by Parker, in Lepidosiren, 'supraclavicle' (scapula) and 'clavicle' (ectocoracoid) seem to be comparable together, and as a whole, with the single element carrying the humerus and pectoral fin in the Crossopterygians (Polypterus and Calamoichthys) and other fishes, and therefore not identical respectively with the 'supraclavicle' and 'clavicle' (except in part) recognized by him in other fishes. As this compound bone, composed of the scapula and ectocoracoid fused together, has received no name which is not ambig[Pg 90]uous or deceptive in its homologous allusions, it may be designated as proscapula.

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

[Pg 91]

CHAPTER VI
THE ORGANS OF RESPIRATION

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

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

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

The gills, or branchiæ, are primarily folds of the skin lining the branchial cavity. In most fishes they form fleshy fringes or laminæ throughout which the capillaries are distributed. In the embryos of sharks, skates, chimæras, lung-fishes,[Pg 92] and Crossopterygians external gills are developed, but in the more specialized forms these do not appear outside the gill-cavity. In some of the sharks, and especially the rays, a spiracle or open foramen remains behind the eye. Through this spiracle, leading from the outside into the cavity of the mouth, water is drawn downwards to pass outward over the gills. The presence of this breathing-hole permits these animals to lie on the bottom without danger of inhaling sand.

Fig. 76.—Gill-basket of Lamprey. (After Dean.)

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

The Air-bladder.—The air-bladder, or swim-bladder, must be classed among the organs of respiration, although in the higher fishes its functions in this regard are rudimentary, and in some cases it has taken collateral functions (as a hydrostatic[Pg 93] organ of equilibrium, or perhaps as an organ of hearing) which have no relation to its original purpose.

Fig. 77.—Weberian apparatus and air-bladder of Carp. (From Günther, after Weber.)

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

The homology of the air-bladder with the lung is evident. This is often expressed in the phrase that the lung is a developed air-bladder. This is by no means true. To say that the air-bladder is a modified and degenerate lung is much nearer the[Pg 94] truth, although we should express the fact more exactly to say that both air-bladder and lung are developed from a primitive cellular breathing-sac, originally a diverticulum from the ventral walls of the œsophagus.

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

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

The other functions of the air-bladder have been subject to much question and are still far from understood. The following summary of the various views in this regard we copy[Pg 95] from Professor Tower's paper on "The Gas in the Swim-bladder of Fishes":

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

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

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

"Biot (1807), Provençal and Humboldt (1809), and others made chemical analyses of the gas in the swim-bladder, and[Pg 96] found 1 to 5 per cent of CO₂, 1 to 87 per cent of O₂, and the remainder nitrogen. The most remarkable fact discovered about this mixture was that it frequently consisted almost entirely of oxygen, the per cent of oxygen increasing with the depth of the water inhabited by the fish. The reasons for this phenomenon have never been satisfactorily explained.

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

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

"Hasse, in 1873, published the results of his investigations on the functions of the ossicles of Weber, stating that their action was that of a manometer, acquainting the animal with the degree of pressure that is exerted by the gases in the air-bladder against its walls. This pressure necessarily varies with the different depths of water which the fish occupies. Hasse[Pg 97] did not agree with Weber that the ear is affected by the movements of these ossicles.

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

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

"Again, in 1885, the Weberian mechanism was brought to our attention with a new function attributed to it by Sagemehl who stated that this mechanism exists not for any auditory purposes, nor to tell the fish at what level of the water it is swimming, but to indicate to the fish the variations in the atmospheric pressure. Sörensen tersely contrasts the views of Hasse and Sagemehl by saying that 'Hasse considers the air-bladder with the Weberian mechanism as a manometer; Sagemehl regards it as a barometer.' The theory of Sagemehl has, naturally[Pg 98] enough, met with little favor. Sörensen (1895) held that there is but little evidence for attributing to the air-bladder the function of a lung. It is to be remembered, however, that, according to Sörensen's criterion no matter what exchange of gases takes place between blood and air-bladder, it cannot be considered an organ of respiration, 'unless its air is renewed by mechanical respiration.'

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

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

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

"If now we seek to discover the original purpose of this [Pg 99]organ, there is abundant reason to believe that it had nothing to do with swimming. Certainly the great family of the sharks, which have no bladder, are at no disadvantage in changing their depth or position in the water. Yet if the bladder is necessary to any fish as an aid in swimming, why not to all? And if this were its primary purpose, how shall we explain its remarkable variability? No animal organ with a function of essential importance presents such extraordinary modifications in related species and genera. In the heart, brain, and other organs there is one shape, position, and condition of greatest efficiency, and throughout the lower forms we find a steady advance towards this condition. Great variation, on the other hand, usually indicates that the organ is of little functional importance, or that it has lost its original function. Such we conceive to be the case with the air-bladder. The fact of its absence from some and its presence in other fishes of closely related species goes far to prove that it is a degenerating organ; and the same is shown by the fact that it is useless in some species for the purpose to which it is applied in others. That it had, at some time in the past, a function of essential importance there can be no question. That it exists at all is proof of this. But its modern variations strongly indicate that it has lost this function and is on the road towards extinction. Larval conditions show that it had originally a pneumatic duct as one of its essential parts, but this has in most cases disappeared. The bladder itself has in many cases partly or wholly disappeared. Where preserved, it seems to be through its utility for some secondary purpose, such as an aid in swimming or in hearing. That its evolution began very long ago there can be no question; and the indications are that it began long ago to degenerate, through the loss of its primitive function.

"What was this primitive function? In attempting to answer this question we must first consider the air-bladder in relation to the fish tribe as a whole. No shark or ray possesses the air-bladder. In some few sharks, indeed, there is a diverticulum of the pharynx which may be a rudimentary approach to the air-bladder; but this is very questionable. The conditions of its occurrence in the main body of modern fishes, the Teleostean, we have already considered. But in the most ancient living orders[Pg 100] of fishes it exists in an interesting condition. In every modern Dipnoan, Crossopterygian, and Ganoid the air-bladder has an effective pneumatic duct. This in the Ganoids opens into the dorsal side of the œsophagus, but in the Dipnoans and Crossopterygians, like the windpipe of lung-breathers, it opens into the ventral side. In the Dipnoans, also survivors from the remote past, the duct not only opens ventrally into the œsophagus, but the air-bladder does duty as a lung. Externally it differs in no particular from an air-bladder; but internally it presents a cellular structure which nearly approaches that of the lung of the batrachians. There are three existing representatives of the Dipnoans. One of these, the Australian lung-fish (Neoceratodus) has a single bladder, which, however, is provided with breathing-pouches having a symmetrical lateral arrangement. It has no pulmonary artery, but receives branches from the arteria cœliaca. In the other two forms, Lepidosiren and Protopterus, the kindred 'mudfishes' of the Amazon basin and tropical Africa, the bladder or lung is divided into two lateral chambers, as in the land animals, and is provided with a separate pulmonary artery.

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

"The facts of embryology lend strong support to this hypothesis. For the air-bladder is found to arise in a manner very similar to the development of the lung. They each begin as an[Pg 101] outgrowth from the fore part of the alimentary tract, the only difference being that the air-bladder usually rises dorsally and the lung ventrally. The fact already cited, that the pneumatic duct is always present in the larval form in fishes that possess a bladder, is equally significant. All the facts go to show that the introduction of external air into the body was a former function of the air-bladder, and that the atrophy of the duct in many cases, and the disappearance of the bladder in others, are results of the loss of this function.

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

"It is probable that the Ganoid, as well as the Dipnoan, air-bladder is to some extent still used in breathing. The Dipnoans have both lungs and gills, and probably breathe with the latter in ordinary cases, but use their lungs when the inland waters in which they live become thick and muddy, or are charged with gases from decomposing organic matter. The Ganoid fishes to some extent breathe the air. In Polypterus the air-bladder resembles the Dipnoan lung in having lateral divisions and a ventral connection with the œsophagus, while in Lepisosteus (the American garpike) it is cellular and lung-like. This fish keeps near the surface, and may be seen to emit air-bubbles, probably taking in a fresh supply of air. The American bowfin, or mudfish (Amia), has a bladder of the same lung-like character, and has been seen to come to the surface, open its jaws widely, and apparently swallow a large quantity of air. He[Pg 102] considers that both Lepisosteus and Amia inhale and exhale air at somewhat regular intervals, resembling in this the salamanders and tadpoles, 'which, as the gills shrink and the lungs increase, come more frequently to the surface for air.'

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

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

"We seem, then, to perceive the lung-bearing fishes, driven by their foes into bays and estuaries, and the waters of shallow coasts, ascending streams and dwelling in inland waters. Here two influences probably acted on them. The waters they dwelt in were often thick with sediment, and were doubtless in many instances poorly aerated, rendering gill-breathing difficult. And the land presented conditions likely to serve as a strong inducement to fishes to venture on shore. Its plant-life was abundant, while its only animal inhabitants seem to have been insects, worms, and snails. There can be little doubt that the active fish forms of that period, having no enemies to fear on the land, and much to gain, made active efforts to obtain a share of this vegetable and animal food. Even to-day, when they have numerous foes to fear, many fishes seek food on the shore, and[Pg 103] some even climb trees for this purpose. Under the conditions of the period mentioned there was a powerful inducement for them to assume this habit.

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

"It is not difficult to draw a hypothetical plan of the development of the air-bladder as a breathing organ. In the two families of fishes just mentioned, whose air-bladders indicate that they once possessed the air-breathing function and have lost it, we perceive the process of formation of an air-breathing organ beginning over again under stress of similar circumstances. The larval development of the air-bladder points significantly in the same direction. In fact we have strong reason to believe that[Pg 104] air-breathing in fishes was originally performed, as it probably often is now, by the unchanged walls of the œsophagus. Then these walls expanded inwardly, forming a simple cavity, partly closed by a fold of membrane, like that of the Ophiocephalidæ. A step further reduced this membranous fold to a narrow opening, leading to an inner pouch. As the air-breathing function developed, the opening became a tube, and the pouch a simple lung, with compressing muscles and capillary vessels. By a continuation of the process the smooth-walled pouch became sacculated, its surface being increased by folding into breathing cells. Finally, a longitudinal constriction divided it into two lateral pouches, such as we find in the lung of the Dipnoans. This brings us to the verge of the lung of the amphibians, which is but a step in advance, and from that the line of progress is unbroken to the more intricate lung of the higher land animals.

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

"When the carboniferous era opened there may have been many lung- and gill-breathing fishes which spent much of their time on land, and some of which, by a gradual improvement of their organs of locomotion, changed into batrachians. But with the appearance of the latter, and of their successors, the reptiles, the relations of the fish to the land radically changed. The fin, or the simple locomotor organ, of the Dipnoans could not compete with the leg and foot as organs of land locomotion, and the fish tribe ceased to be lords of the land, where, instead of feeble prey, they now found powerful foes, and were driven back to[Pg 105] their native habitat, the water. Nor did the change end here. In time the waters were invaded by the reptiles, numerous swimming forms appearing, which it is likely were abundant in the shallower shore-line of the ocean, while they sent many representatives far out to sea. These were actively carnivorous, making the fish their prey, the great mass of whom were doubtless driven into the deeper waters, beyond the reach of their air-breathing foes.

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

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

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

[Pg 106]

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

The Heart of the Fish.—The heart of the fish is simple in structure, small in size, and usually placed far forward, just behind the branchial cavity, and separated from the abdominal[Pg 107] cavity by a sort of "diaphragm" formed of thickened peritoneum. In certain eels the heart is remote from the head.

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

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

The Flow of Blood.—The blood in fishes is thin and pale red (colorless in the lancelet) and with elliptical blood-corpuscles. It enters the sinus venosus from the head through the jugular vein, from the kidney and body walls through the cardinal vein, and from the liver through the hepatic veins. Hence it passes to the auricle and ventricle, and from the ventricle through the arterial bulb, or conus arteriosus to the ventral aorta. Thence it flows to the gills, where it is purified. After passing through the capillaries of the gill-filaments it is collected in paired arteries from each pair of gills. These vessels unite to form the dorsal aorta, which extends the length of the body just below the back-bone. From the dorsal aorta the subclavian arteries branch off toward the pectoral fins. From a point farther back arise the mesenteric arteries carrying blood to the stomach, intestine, liver, and spleen. In the tail the caudal vein carries blood to the kidneys. These secrete impurities arising from[Pg 108] waste of tissues, after which the blood again passes to the heart through the cardinal vein. From the intestine the blood, charged with nutritive materials in solution, is carried by the portal vein to the liver. Here it again passes by the hepatic sinus to the sinus venosus and the heart.

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

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

[Pg 109]

CHAPTER VII
THE NERVOUS SYSTEM

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

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

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

In front of the cerebellum lies the largest pair of lobes, each of them hollow, the optic nerves being attached to the lower surface. These are known as the optic lobes, or mesencephalon. In front of these lie the two lobes of the cerebrum, also called the hemispheres, or prosencephalon. These lobes are usually smaller than the optic lobes and solid. In some fishes they are crossed by a furrow, but are never corrugated as in the brain[Pg 110] of the higher animals. In front of the cerebrum lie the two small olfactory lobes, which receive the large olfactory nerve from the nostrils. From its lower surface is suspended the hypophysis or pituitary gland.

Fig. 78.—Brain of a Shark (Squatina squatina L.). (After Dean.)

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

Fig. 79.—Brain of Chimæra monstrosa. (After Wilder per Dean.)

Fig. 80.—Brain of Protopterus annectens. (After Burckhardt per Dean.)

In most of the bony fishes the structure of the brain does not differ materially from that seen in the perch. In the sturgeon, however, the parts are more widely separated. In the Dipnoans the cerebral hemispheres are united, while the optic lobe and cerebellum are very small. In the sharks and rays the large cerebral hemispheres are usually coalescent into one, and the olfactory nerves dilate into large ganglia below the nostrils. The optic lobes are smaller than the hemispheres and also coalescent. The cerebellum is very large, and the surface of the[Pg 111] medulla oblongata is more or less modified or specialized. The brain of the shark is relatively more highly developed than that of the bony fishes, although in most other regards the latter are more distinctly specialized.

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

Fig. 81.—Brain of a Perch, Perca flavescens. (After Dean.)

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

Fig. 82.—Petromyzon marinus unicolor (Dekay). Head of Lake Lamprey, showing pineal body. (After Gage.)

It has been supposed that this eye, once possessed by all vertebrate forms, has been gradually lost with the better de[Pg 112]velopment of the paired eyes, being best preserved in reptiles as "an outcome of the life-habit which concealed the animal in sand or mud, and allowed the forehead surface alone to protrude, the median eye thus preserving its ancestral value in enabling the animal to look directly upward and backward." This theory receives no support from the structures seen in the fishes.

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

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

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

The Spinal Cord.—The spinal cord extends from the brain to the tail, passing through the neural arches of the different vertebræ when these are developed. In the higher fishes it is cylindrical [Pg 113]and inelastic. In a few fishes (headfish, trunkfish) in which the posterior part of the body is shortened or degenerate, the spinal cord is much shortened, and replaced behind by a structure called cauda equina. In the headfish it has shrunk into "a short and conical appendage to the brain." In the Cyclostomes and chimæra the spinal cord is elastic and more or less flattened or band-like, at least posteriorly.

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

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

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

The eighth pair, nervus acousticus, leads to the ear. The ninth pair, glosso-pharyngeal, passes to the tongue and pharynx, and forms a ganglion connected with the sympathetic system. The tenth pair, nervus vagus, or pneumogastric nerve, arises from strong roots in the corpus restiforme and the lower part of the medulla oblongata. Its nerves, motor and sensory, reach the muscles of the gill-cavity, heart, stomach, and air-bladder, as well as the muscular system and the skin. In fishes covered with bony plates the skin may be nearly or quite without sen[Pg 114]sory nerves. The eleventh pair, nervus accessorius, and twelfth pair, nervus hypoglossus, are wanting in fishes.

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

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

[Pg 115]

CHAPTER VIII
THE ORGANS OF SENSE

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

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

In the lancelet there is a single median organ supposed to[Pg 116] be a nostril, a small depression at the front of the head, covered by ciliated membrane. In the hagfish the single median nostril pierces the roof of the mouth, and is strengthened by cartilaginous rings, like those of the windpipe. In the lamprey the single median nostril leads to a blind sac. In the Barramunda (Neoceratodus) there are both external and internal nares, the former being situated just within the upper lip. In all other fishes there is a nasal sac on either side of the head. This has usually, but not always, two openings.

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

Fig. 83.—Dismal Swamp Fish, Chologaster cornutus Agassiz. Supposed ancestor of Typhlichthys. Virginia.

Fig. 84.—Blind Cavefish, Typhlichthys subterraneus Girard. Mammoth Cave, Kentucky.

The Organs of Sight.—The eyes of fishes differ from those of the higher vertebrates mainly in the spherical form of the crystalline lens. This extreme convexity is necessary because the lens itself is not very much denser than the fluid in which the fishes live. The eyes vary very much in size and somewhat in form and position. They are larger in fishes living at a moderate depth than in shore fishes or river fishes. At great depths,[Pg 117] as a mile or more, where all light is lost, they may become aborted or rudimentary, and may be covered by the skin. Often species with very large eyes, making the most of a little light or of light from their own luminous spots, will inhabit the same depths with fishes having very small eyes or eyes apparently useless for seeing, retained as vestigial structures through heredity. Fishes which live in caves become also blind, the structures showing every possible phase of degradation. The details of this gradual loss of eyes, whether through reversed selection or hypothetically through inheritance of atrophy produced by disuse, have been given in a number of memoirs on the blind fishes of the Mississippi Valley by Dr. Carl H. Eigenmann.

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

Fig. 85.—Four-eyed Fish, Anableps dovii Gill. Tehuantepec, Mexico.

In the eyes of the fishes there is no lachrymal gland. True eyelids no fishes possess; the integuments of the head pass over the eye, becoming transparent as they cross the orbit. In some fishes part of this integument is thickened, covering the eye fully although still transparent. This forms the adipose eyelid characteristic of the mullet, mackerel, and ladyfish. Many of the sharks possess a distinct nictitating membrane or special eyelid, moved by a set of muscles. The iris in most fishes surrounds a[Pg 118] round pupil without much power of contraction. It is frequently brightly colored, red, orange, black, blue, or green. In fishes, like rays or flounders, which lie on the bottom, a dark lobe covers the upper part of the pupil—a curtain to shut out light from above. The cornea is little convex, leaving small space for aqueous humor. In two genera of fishes, Anableps, Dialommus, the cornea is divided by a horizontal partition into two parts. This arrangement permits these fishes, which swim at the surface of the water, to see both in and out of the medium. Anableps, the four-eyed fish, is a fresh-water fish of tropical America, which swims at the surface like a top-minnow, feeding on insects. Dialommus is a marine blenny from the Panama region, apparently of similar habit.

Fig. 86.—Ipnops murrayi Günther.

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

Fig. 87.—Pond-skipper, Boleophthalmus chinensis (Osbeck). Bay of Tokyo, Japan; from nature. K. Morita. (Eye-stalks shrunken in preservation.)

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

In ordinary fishes there is one eye on each side of the head, but in the flounders, by a distortion of the cranium, both ap[Pg 119]pear on the same side. This side is turned uppermost as the fish swims in the water or when it lies on the bottom. This distortion is a matter of development. The very young flounder swims with its broad axis vertical in the water, and it has one eye on either side. As soon as it rests on the bottom it begins to lean to one side. The lower eye changes its axis and by degrees travels across the face of the fish, part of the bony interorbital moving with it across to the other side. In some soles it is said to pass through the substance of the head, reappearing on the other side. In all species which the writer has examined the cranium is twisted, the eye moving with the bones; and the frontal bone is divided, a new orbit being formed by this division. In most northern flounders the eyes are on the right side in the adult, in tropical forms more frequently on the left, these distinctions corresponding with others in the structure of the fish.

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

The Organs of Hearing.—The ear of the typical fish consists of the labyrinth only, including the vestibule and usually three semicircular canals, these dilating into sacs which contain one or more large, loose bones, the ear-stones or otoliths. In the lampreys there are two semicircular canals, in the hagfish but one. There is no external ear, no tympanum, and no Eustachian tube. The ear-sac on each side is lodged in the skull or at the base of the cranial cavity. It is externally surrounded by bone or cartilage, but sometimes it lies near a fontanelle or opening in the skull above. In some fishes it is brought into very close connection with the anterior end of the air-bladder. The latter organ it is thought may form part of the apparatus for hearing. The arrangement for this purpose is especially elaborate in the carp and the catfish families. In these fishes and their relatives[Pg 120] (called Ostariophysi) the two vestibules are joined in a median sac (sinus impar) in the substance of the basioccipital. This communicates with two cavities in the atlas, which again are supported by two small bones, these resting on a larger one in connection with the front of the air-bladder. The system of bones is analogous to that found in the higher vertebrates, but it connects with the air-bladder, not with an external tympanum. The bones are not homologous with those of the ear of higher animals, being processes of the anterior vertebræ. The tympanic chain of higher vertebrates has been thought homologous with the suspensory of the mandible.

Fig. 88.—Brook Lamprey, Lampetra wilderi Jordan and Evermann. (After Gage.) Cayuga Lake.

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

Fig. 89.—European Lancelet, Branchiostoma lanceolatum (Pallas). (After Parker and Haswell.)

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

[Pg 121]

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

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

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

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

The Sense of Touch.—The sense of touch is better developed among fishes. Most of them flee from contact with actively[Pg 122] moving objects. Many fishes use sensitive structures as a means of exploring the bottom or of feeling their way to their food. The barbel or fleshy filament wherever developed is an organ of touch. In some fishes, barbels are outgrowths from the nostrils. In the catfish the principal barbel grows from the rudimentary maxillary bone. In the horned dace and gudgeon the little barbel is attached to the maxillary. In other fishes barbels grow from the skin of the chin or snout. In the goatfish and surmullet the two chin barbels are highly specialized. In Polymixia the chin barbels are modified branchiostegals. In the codfish the single beard is little developed. In the gurnards and related forms the lower rays of the pectoral are separate and barbel-like. Detached rays of this sort are found in the thread-fins (Polynemidæ), the gurnards (Triglidæ), and in various other fishes. Barbels or fleshy flaps are often developed over the eyes and sometimes on the scales or the fins.

Fig. 90.—Goatfish, Pseudupeneus maculatus (Bloch). Woods Hole.

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

[Pg 123]

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

[Pg 124]

CHAPTER IX
THE ORGANS OF REPRODUCTION

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

Fig. 91.—Sword-tail Minnow, male, Xiphophorus helleri Heckel. The anal fin modified as an intromittent organ. Vera Cruz.

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

[Pg 125]

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

Fig. 92.—White Surf-fish, viviparous, with young, Cymatogaster aggregatus Gibbons. San Francisco.

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

[Pg 126]

Fig. 93.—Goodea luitpoldi (Steindachner). A viviparous fish from Lake Patzcuaro, Mexico. Family Pæciliidæ. (After Meek.)

[Pg 127] In others, as the surf-fishes (Embiotocidæ), they are relatively large and few in number. In the viviparous sharks, which constitute the majority of the species of living sharks, the young are large at birth and prepared to take care of themselves.

Fig. 94.—Egg of Callorhynchus antarcticus, the Bottle-nosed Chimæra. (After Parker and Haswell.)

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

Fig. 95.—Egg of the Hagfish, Myxine limosa Girard, showing threads for attachment. (After Dean.)

In the higher fishes the eggs are spherical, large or small according to the species, and varying in the firmness of their[Pg 128] outer walls. All contain food-yolk from which the embryo in its earlier stages is fed. The eggs of the eel (Anguilla) are microscopic. According to Günther 25,000 eggs have been counted in the herring, 155,000 in the lumpfish, 3,500,000 in the halibut, 635,200 in the sturgeon, and 9,344,000 in the cod. Smaller numbers are found in fishes with large ova. The red salmon has about 3500 eggs, the king salmon about 5200. Where an oviduct is present the eggs are often poured out in glutinous masses, as in the bass. When, as in the salmon, there is no oviduct, the eggs lie separate and do not cohere together. It is only with the latter class of fishes, those in which the eggs remain distinct, that artificial impregnation and hatching is practicable. In this regard the value of the salmon and trout is predominant. In some fishes, especially those of elongate form, as the needle-fish (Tylosurus), the ovary of but one side is developed.

Fig. 96.—Egg of Port Jackson Shark, Heterodontus philippi (Lacépède). (After Parker and Haswell.)

Protection of the Young.—In most fishes the parents take no care of their eggs or young. In some catfishes (Platystacus) the eggs adhere to the under surface of the female. In a kind of pipefish (Solenostomus), a large pouch for retention of the eggs is formed on the belly of the female. In the sea-horses and pipefishes a pouch is formed in the skin, usually underneath the tail of the male. Into this the eggs are thrust, and here the young fishes hatch out, remaining until large enough to take care of themselves. In certain sea catfishes (Galeichthys, Conorhynchos) the male carries the eggs in his mouth, thus protecting them from the attacks of other fishes. In numerous cases the male constructs a rough nest, which he defends against all intruders, against the female as well as against outside enemies. The nest-building habit is especially developed in the stickle[Pg 129]backs (Gasterosteidæ), a group in which the male fish, though a pygmy in size, is very fierce in disposition.

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

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

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

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

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

There is no evidence that the bright colors or nuptial adornments of the males are enhanced by sexual selection. In most species the males deposit the sperm-cells in spawning-grounds[Pg 130] without much reference to the preference of the females. In general the brightest colors are not found among viviparous fishes. None of the groups in which the males are showily colored, while the females are plain, belong to this class. The brightest colors are found on the individuals most mature or having greatest vitality.

[Pg 131]

CHAPTER X
EMBRYOLOGY AND GROWTH OF FISHES

Segmentation of the Egg.—The egg of the fish develops only after fertilization (amphimixis). This process is the union of its nuclear substance with that of the sperm-cell from the male, each cell carrying its equal share in the function of heredity. When this process takes place the egg is ready to begin its segmentation. The eggs of all fishes are single cells containing more or less food-yolk. The presence of this food-yolk affects the manner of segmentation in general, those eggs having the least amount of food-yolk developing most typically. The simplest of all fish like vertebrates, the lancelet (Branchiostoma) has very small eggs, and in their early development it passes through stages that are typical for all many-celled animals. The first stage in development is the simple splitting of the egg into two halves. These two daughter cells next divide so that there are four cells; each of these divides, and this division is repeated until a great number of cells is produced. The phenomenon of repeated division of the germ-cell is called cleavage, and this cleavage is the first stage of development in the case of all many-celled animals. Instead of forming a solid mass the cells arrange themselves in such a way as to form a hollow ball, the wall being a layer one cell thick. The included cavity is called the segmentation cavity, and the whole structure is known as a blastula. This stage also is common to all the many-celled animals. The next stage is the conversion of the blastula into a double-walled cup, known as a gastrula by the pushing in of one side. All the cells of the blastula are very small, but those on one side are somewhat larger than those of the other, and here the wall first flattens and then bends in until finally the larger cells come into contact with the smaller and the segmentation cavity is entirely obliterated. There is now[Pg 132] an inner layer of cells and an outer layer, the inner layer being known as the endoblast and the outer as the ectoblast. The cavity of the cup thus formed is the archenteron and gives rise primarily to the alimentary canal. This third well-marked stage is called the gastrula stage; and it is thought to occur either typically or in some modified form in the development of all metazoa, or many-celled animals. In the lampreys, the Ganoids, and the Dipnoans the eggs contain a much greater quantity of yolk than those of the lancelet, but the segmentation resembles that of the lancelet in that it is complete; that is, the whole mass of the egg divides into cells. There is a great difference, however, in the size of the cells, those at the upper pole being much smaller than those at the lower. In Petromyzon and the Dipnoans blastula and gastrula stages result, which, though differing in some particulars from the corresponding stages of the lancelet, may yet readily be compared with them. In the hagfishes, sharks, rays, chimæras, and most bony fishes there is a large quantity of yolk, and the protoplasm, instead of being distributed evenly throughout the egg, is for the most part accumulated upon one side, the nucleus being within this mass of protoplasm. When the food substance or yolk is consumed and the little fish is able to shift for itself, it leaves the egg-envelopes and is said to be hatched. The figures on page 135 show some of the stages by which cells are multiplied and ultimately grouped together to form the little fish.

Post-embryonic Development.—In all the fishes the development of the embryo goes on within the egg long after the gastrula stage is passed, and until the embryo becomes a complex body, composed of many differing tissues and organs. Almost all the development may take place within the egg, so that when the young animal hatches there is necessary little more than a rapid growth and increase of size to make it a fully developed mature animal. This is the case with most fishes: a little fish just hatched has most of the tissues and organs of a full-grown fish, and is simply a small fish. But in the case of some fishes the young hatches from the egg before it has reached such an advanced state of development, and the young looks very different from its parent. It must yet undergo considerable change before it reaches the structural condition of a fully developed[Pg 133] and fully grown fish. Thus the development of most fishes is almost wholly embryonic development—that is, development within the egg or in the body of the mother—while the development of some of them is to a considerable degree post-embryonic or larval development. There is no important difference between embryonic and post-embryonic development. The development is continuous from egg-cell to mature animal and, whether inside or outside of an egg, it goes on with a degree of regularity. While certain fishes are subject to a sort of metamorphosis, the nature of this change is in no way to be compared with the change in insects which undergo a complete metamorphosis. In the insects all the organs of the body are broken down and rebuilt in the process of change. In all fishes a structure once formed maintains a more nearly continuous integrity although often considerably altered in form.

General Laws of Development.—The general law of development may be briefly stated as follows: All many-celled animals begin life as a single cell, the fertilized egg-cell; each animal goes through a certain orderly series of developmental changes which, accompanied by growth, leads the animal to change from single-cell to many-celled, complex form characteristic of the species to which the animal belongs; this development is from simple to complex structural condition; the development is the same for all individuals of one species. While all animals begin development similarly, the course of development in the different groups soon diverges, the divergence being of the nature of a branching, like that shown in the growth of a tree. In the free tips of the smallest branches we have represented the various species of animals in their fully developed condition, all standing clearly apart from each other. But in tracing back the development of any kind of animal we soon come to a point where it very much resembles or becomes apparently identical with some other kind of animal, and going farther back we find it resembling other animals in their young condition, and so on until we come to that first stage of development, that trunk stage where all animals are structurally alike. Any animal at any stage in its existence differs absolutely from any other kind of animal, in this respect: it can develop into only its own kind. There is something inherent in each develop[Pg 134]ing animal that gives it an identity of its own. Although in its young stages it may be indistinguishable from some other species of animal in its young stages, it is sure to come out, when fully developed, an individual of the same kind as its parents were or are. The young fish and the young salamander may be alike to all appearance, but one embryo is sure to develop into a fish, and the other into a salamander. This certainty of an embryo to become an individual of a certain kind is called the law of heredity. Viewed in the light of development, there must be as great a difference between one egg and another as between one animal and another, for the greater difference is included in the less.

The Significance of Facts of Development.—The significance of the process of development in any species is yet far from completely understood. It is believed that many of the various stages in the development of an animal correspond to or repeat the structural condition of the animal's ancestors. Naturalists believe that all animals having a notochord at any stage in their existence are related to each other through being descended from a common ancestor, the first or oldest chordate or back-boned animal. In fact it is because all these chordate animals—the lancelets, lampreys, fishes, batrachians, the reptiles, the birds, and the mammals—have descended from a common ancestor that they all develop a notochord, and those most highly organized replace this by a complete back-bone. It is believed that the descendants of the first back-boned animal have, in the course of many generations, branched off little by little from the original type until there came to exist very real and obvious differences among the back-boned animals—differences which among the living back-boned animals are familiar to all of us. The course of development of an individual animal is believed to be a very rapid and evidently much condensed and changed recapitulation of the history which the species or kind of animal to which the developing individual belongs has passed through in the course of its descent through a long series of gradually changing ancestors. If this is true, then we can readily understand why the fish and the salamander and the tortoise and bird and rabbit are all alike in their earlier stages of development, and gradually[Pg 135] come to differ more and more as they pass through later and later developmental stages.

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

Fig. 97.—Development of Sea-bass, Centropristes striatus (Linnæus). a, egg prior to germination; b, germ-disk after first cleavage; c, germ-disk after third cleavage; d, embryo just before hatching. (After H. V. Wilson.)

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

In stages of late segmentation the segmentation cavity is [Pg 136]greatly flattened, but extends to the marginal cells of the germ-disk; its roof consists of two tiers of blastomeres, its floor of a thin film of the unsegmented substance of the germ; the marginal blastomeres are continuous with both roof and floor of the cavity, and are produced into a thin film which passes downward, around the sides of the yolk. Later the segmentation cavity is still further flattened; its roof is now a dome-shaped mass of blastomeres; the marginal cells have multiplied, and their nuclei are seen in the layer of the germ, below the plane of the segmentation cavity. These are seen in the surface view of the marginal cells of this stage; they are separated by cell boundaries only at the sides; below they are continuous in the superficial down-reaching layer of the germ. The marginal cells shortly lose all traces of having been separate; their nuclei, by continued division, spread into the layer of germ flooring the segmentation cavity, and into the delicate film of germ which now surrounds the entire yolk. Thus is formed the periblast of the Teleost development, which from this point onward is to separate the embryo from the yolk; it is clearly the specialized inner part of the germ, which, becoming fluid-like, loses its cell-walls, although retaining and multiplying its nuclei. Later the periblast comes into intimate relations with the growing embryo; it lies directly against it, and appears to receive cell increments from it at various regions; on the other hand, the nuclei of the periblast, from their intimate relations with the yolk, are supposed to subserve some function in its assimilation.

Aside from the question of periblast, the growth of the blastoderm appears not unlike that of the sturgeon. From the blastula stage to that of the early gastrula, the changes have been but slight; the blastoderm has greatly flattened out as its margins grow downward, leaving the segmentation cavity apparent. The rim of the blastoderm has become thickened as the 'germ-ring'; and immediately in front of the dorsal lip of the blastopore its thickening marks the appearance of the embryo. The germ-ring continues to grow downward, and shows more prominently the outline of the embryo; this now terminates at the head region; while on either side of this point [Pg 137]spreads out tail-ward on either side the indefinite layer of outgrowing
mesoderm. In the next stage the closure of the blastopore
is rapidly becoming completed; in front of it stretches the widened and elongated form of the embryo. The yolk-plug is next replaced by periblast, the dorsal lip by the tail-mass, or more accurately the dorsal section of the germ-rim; the cœlenteron under the dorsal lip has here disappeared, on account of the close approximation of the embryo to the periblast; its last remnant, the Kupffer's vesicle, is shortly to disappear. The germ-layers become confluent, but, unlike the sturgeon, the flattening of the dorsal germ-ring does not permit the formation of a neurenteric canal.

Fig. 98. Sea-bass, Centropristes striatus, natural size. (From life, by R. W. Shufeldt.)—Page 137.

[Pg 138]

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

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

The mode of formation of the gill-slit of the Teleost does not differ from that in other groups; an evagination of the entoderm coming in contact with an invaginated tract of[Pg 139] ectoderm fuses, and at this point an opening is later established.

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

Fig. 99.—Young Sword-fish, Xiphias gladius (Linnæus). (After Lütken.)

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

Fig. 100.—Sword-fish, Xiphias gladius (Linnæus). (After Day.)

The fish may, in general, be said to begin its existence as [Pg 140]a larva as soon as it emerges from its egg-membranes. In some instances, however, it is difficult to decide at what point the larval stage is actually initiated: thus in sharks the excessive amount of yolk material which has been provided for the growth of the larva renders unnecessary the emerging from the egg at an early stage; and the larval period is accordingly to be traced back to stages that are still enclosed in the egg-membranes. In all cases the larval life may be said to begin when the following conditions have been fulfilled: the outward form of the larva must be well defined, separating it from the mass of yolk, its motions must be active, it must possess a continuous vertical fin-fold passing dorsally from the head region to the body terminal, and thence ventrally as far as the yolk region; and the following structures, characteristic in outward appearance, must also be established: the sense-organs—eye, ear, and nose—mouth and anus, and one or more gill-clefts.

Fig. 101.—Larva of the Sail-fish, Istiophorus, very young. (After Lütken.)

Fig. 102.—Larva of Brook Lamprey, Lampetra wilderi, before transformation, being as large as the adult, toothless, and more distinctly segmented.

Fig. 103.—Common Eel. Anguilla chrisypa Rafinesque. Family Anguillidæ.

[Pg 141]

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

Fig. 104.—Larva of Common Eel, Anguilla chrisypa (Rafinesque), called Leptocephalus grassii. (After Eigenmann.)

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

Fig. 105.—Larva of Sturgeon, Acipenser sturio (Linnæus). (After Kupffer, per Dean.)

[Pg 142]

Fig. 106.—Larva (called Tholichthys) of Chætodon sedentarius (Poey). Cuba. (After Lütken,)

Fig. 107.—Butterfly-fish, Chætodon capistratus Linnæus. Jamaica.

Peculiar Larval Forms.—The young fish usually differs from the adult mainly in size and proportions. The head is larger in the young, the fins are lower, the appendages less developed, and the body more slender in the young than in the adult. But to most of these distinctions there are numerous exceptions, and in some fish there is a change so marked as to be fairly called a metamorphosis. In such cases the young fish in its first condition is properly called a larva. The larva of the lamprey (Petromyzon) is nearly blind and toothless, with slender head, and was long supposed to belong to a different genus (Ammocœtes) from the adult. The larva of sharks and rays, and also of Dipnoans and Crossopterygians, are provided with bushy external gills, which disappear in the process of development. In most soft-rayed fishes the embryonic fringe which precedes the[Pg 143] development of the vertical fins persists for a considerable time. In many young fishes, especially the Chætodontidæ and their allies (butterfly-fishes), the young fish has the head armed with broad plates formed by the backward extension of certain membrane-bones. In other forms the bones of the head are in the young provided with long spines or with serrations, which vanish totally with age. Such a change is noticeable in the swordfish. In this species the production of the bones of the snout and upper jaw into a long bony sword, or weapon of offense, takes place only with age. The young fish have jaws more normally formed, and armed with ordinary teeth. In the headfish (Mola mola) large changes take place in the course of growth, and the young have been taken for a different type of fishes. Among certain soft-rayed fishes and eels the young is often developed in a peculiar way, being very soft, translucent, or band-like, and formed of large or loosely aggregated cells. These peculiar organisms, long known as leptocephali, have been shown to be the normal young of fishes when mature very different. In the ladyfish (Albula) Dr. Gilbert has shown, by a full series of specimens, that in their further growth these pellucid fishes shrink in size, acquiring greater compactness of body, until finally reaching about half their maximum length as larvæ. After this, acquiring essentially the form of the adult fish, they begin a process of regular growth. This leptocephalous condition is thought by Günther to be due to arrest of growth in abnormal individuals, but this is not the case in Albula, and it is probably fully normal in the conger and other eels. In the surf-fishes the larvæ have their vertical fins greatly elevated, much higher than in the adult, while the body is much more closely compressed. In the deal-fish (Trachypterus) the form of the body and fins changes greatly with age, the body becoming more elongate and the fins lower. The differences between different stages of the same fish seem greater than the differences between distinct species. In fact with this and with other forms which change with age, almost the only test of species is found in the count of the fin-rays. So far as known the numbers of these structures do not change. In the moonfishes (Carangidæ) the changes with age are often very considerable. We copy Lütken's figure of the changes in the genus Selene (fig. 113). Similar changes take place in Alectis, Vomer, and other genera.

Fig. 108.—Mola mola (Linnæus). Very early larval stage of the Headfish, called Centaurus boöps. (After Richardson.)

[Pg 144]

Fig. 109.—Mola mola (Linnæus). Early larval stage, called Molacanthus nummularis. (After Ryder.)

Fig. 110.—Mola mola (Linnæus). Advanced larval stage. (After Ryder.)

[Pg 145]

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

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

The Age of Fishes.—The age of fishes is seldom measured by a definite period of years. Most of them grow as long as they live, and apparently live until they fall victims to some stronger species. It is reputed that carp and pike have lived for a century, but the evidence needs verification. Some fishes, as the salmon of the Pacific (Oncorhynchus), have a definite period[Pg 146] of growth (usually four years) before spawning. After this act all the individuals die so far as known. In Japan and China the Ice-fish (Salanx), a very long, slender, transparent fish allied to the trout, may possibly be annual in habit, all the individuals perhaps dying in the fall to be reproduced from eggs in the spring. But this alleged habit needs verification.

Fig. 111.—Headfish (adult), Mola mola (Linnæus). Virginia.

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

[Pg 147]

Fig. 112.—Albula vulpes (Linnæus). Transformation of the Ladyfish, from the translucent, loosely compacted larva to the smaller, firm-bodied young. Gulf of California. (After Gilbert.)

[Pg 148]

Fig. 113.—Development of the Horsehead-fish, Selene vomer (Linnæus). Family Carangidæ. (After Lütken.)

[Pg 149] Some fresh-water fishes are very fragile, dying soon in the air, often with injured air-bladder or blood-vessels. They will die even sooner in foul water. Other fishes are extremely tenacious of life. The mud-minnow (Umbra) is sometimes ploughed up in the half-dried mud of Wisconsin prairies. The related Alaskan blackfish (Dallia) has been fed frozen to dogs, escaping alive from their stomachs after being thawed out. Many of the catfishes (Siluridæ) will live after lying half-dried in the dust for hours. The Dipnoan, Lepidosiren, lives in a ball of half-dried mud during the arid season, and certain fishes, mostly Asiatic, belonging to the group Labyrinthici, with accessory breathing organ can long maintain themselves out of water. Among these is the China-fish (Ophiocephalus), often kept alive in the Chinese settlements in California and Hawaii. Some fishes can readily endure prolonged hunger, while others succumb as readily as a bird or a mammal.

Fig. 114.—Ice-fish, Salanx hyalocranius Abbott. Family Salangidæ. Tientsin, China.

Fig. 115.—Alaska Blackfish, Dallia pectoralis (Bean). St. Michaels, Alaska.

The Effects of Temperature on Fish.—The limits of distribution of many fishes are marked by changes in temperature. Few marine fishes can endure any sudden or great change in this regard, although fresh-water fishes adapt themselves to the seasons. I have seen the cutlass-fish (Trichiurus) benumbed with cold off the coast of Florida while the temperature was still above the frost-line. Those fishes which are tenacious of life and little sensitive to changes in climate and food are most successfully acclimatized or domesticated. The Chinese carp[Pg 150] (Cyprinus carpio) and the Japanese goldfish (Carassius auratus) have been naturalized in almost all temperate and tropical river basins. Within the limits of clear, cold waters most of the salmon and trout are readily transplanted. But some similar fishes (as the grayling) are very sensitive to the least change in conditions. Most of the catfish (Siluridæ) will thrive in almost any fresh waters except those which are very cold.

Fig. 116.—Snake-headed China-fish, Ophiocephalus barca. India. (After Day.)

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

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

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

[Pg 151]

Fig. 117.—Monstrous Goldfish (bred in Japan), Carassius auratus (Linnæus). (After Günther.)

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

[Pg 152]

CHAPTER XI
INSTINCTS, HABITS, AND ADAPTATIONS

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

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

Nerve-cells and Fibres.—In the development of the individual animal certain cells from the primitive external layer or ectoblast of the embryo are set apart to preside over the relations of the creature to its environment. These cells are highly specialized, and while some of them are highly sensitive, others are adapted for carrying or transmitting the stimuli received by the sensitive cells, and still others have the function of receiv[Pg 153]ing sense-impressions and of translating them into impulses of motion. The nerve-cells are receivers of impressions. These are gathered together in nerve-masses or ganglia, the largest of these being known as the brain, the ganglia in general being known as nerve-centres. The nerves are of two classes. The one class, called sensory nerves, extends from the skin or other organ of sensation to the nerve-centre. The nerves of the other class, motor nerves, carry impulses to motion.

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

Reflex Action.—These impressions brought to the brain by the sensory nerves represent in some degree the facts in the animal's environment. They teach something as to its food or its safety. The power of locomotion is characteristic of animals. If they move, their actions must depend on the indications carried to the nerve-centre from the outside; if they feed on living organisms, they must seek their food; if, as in many cases, other living organisms prey on them, they must bestir themselves to escape. The impulse of hunger on the one hand and of fear on the other are elemental. The sensorium receives an impression that food exists in a certain direction. At once an impulse to motion is sent out from it to the muscles necessary to move the body in that direction. In the higher animals these movements are more rapid and more exact. This is because organs of sense, muscles, nerve-fibres, and the nerve-cells are all alike highly specialized. In the fish the sensation is slow, the muscular response sluggish, but the method remains the same. This is simple reflex action, an impulse from the[Pg 154] environment carried to the brain and then unconsciously reflected back as motion. The impulse of fear is of the same nature. Reflex action is in general unconscious, but with animals, as with man, it shades by degrees into conscious action, and into volition or action "done on purpose."

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

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

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

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

Egoistic instincts are those which concern chiefly the individual animal itself. To this class belong the instincts of feed[Pg 155]ing, those of self-defense and of strife, the instincts of play, the climatic instincts, and environmental instincts, those which direct the animal's mode of life.

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

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

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

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

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

The movements of many migratory animals are mainly con[Pg 156]trolled by the impulse to reproduce. Some pelagic fishes, especially flying fishes and fishes allied to the mackerel, swim long distances to a region favorable for a deposition of spawn. Some species are known only in the waters they make their breeding homes, the individuals being scattered through the wide seas at other times. Many fresh-water fishes, as trout, suckers, etc., forsake the large streams in the spring, ascending the small brooks where they can rear their young in greater safety. Still others, known as anadromous fishes, feed and mature in the sea, but ascend the rivers as the impulse of reproduction grows strong. An account of these is given in a subsequent paragraph.

Fig. 118.—Jaws of Nemichthys avocetta. Jordan and Gilbert.

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

Adaptation to Environment.—In general food-securing structures are connected with the mouth, or, as in the anglers, are hung as lures above it; spines of offense and defense, electric organs, poison-glands, and the like are used in self-protection; the bright nuptial colors and adornments of the breeding season are doubtfully classed as useful in rivalry; the egg-sacs, nests, and other structures or habits may serve to defend the young, while skinny flaps, sand or weed-like markings, and[Pg 157] many other features of mimicry serve as concessions to the environment.

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

Fig. 119.—Catalina Flying Fish, Cypsilurus californicus (Cooper). Santa Barbara.

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

The flight of one of the largest flying fishes (Cypsilurus californicus) has been carefully studied by Dr. Charles H. Gilbert and the writer. The movements of the fish in the water are extremely rapid. The sole motive power is the action under the water of the strong tail. No force can be acquired while the fish is in the air. On rising from the water the movements[Pg 158] of the tail are continued until the whole body is out of the water. When the tail is in motion the pectorals seem in a state of rapid vibration. This is not produced by muscular action on the fins themselves. It is the body of the fish which vibrates, the pectorals projecting farthest having the greatest amplitude of movement. While the tail is in the water the ventral fins are folded. When the action of the tail ceases the pectorals and ventrals are spread out wide and held at rest. They are not used as true wings, but are held out firmly, acting as parachutes, enabling the body to skim through the air. When the fish begins to fall the tail touches the water. As soon as it is in the water it begins its motion, and the body with the pectorals again begins to vibrate. The fish may, by skimming the water, regain motion once or twice, but it finally falls into the water with a splash. While in the air it suggests a large dragon-fly. The motion is very swift, at first in a straight line, but is later deflected in a curve, the direction bearing little or no relation to that of the wind. When a vessel passes through a school of these fishes, they spring up before it, moving in all directions, as grasshoppers in a meadow.

Fig. 120.—Sand-darter, Ammocrypta clara (Jordan and Meek). Des Moines River.

Quiescent Fishes.—Some fishes, as the lancelet, lie buried in the sand all their lives. Others, as the sand-darter (Ammocrypta pellucida) and the hinalea (Julis gaimard), bury themselves in the sand at intervals or to escape from their enemies. Some live in the cavities of tunicates or sponges or holothurians or corals or oysters, often passing their whole lives inside the cavity of one animal. Many others hide themselves in the interstices of kelp or seaweeds. Some eels coil themselves in the crevices of rocks or coral masses, striking at their prey like snakes. Some sea-horses cling by their tails to gulfweed or sea-wrack. Many[Pg 159] little fishes (Gobiomorus, Carangus, Psenes) cluster under the stinging tentacles of the Portuguese man-of-war or under ordinary jellyfishes. In the tide-pools, whether rock, coral, or mud, in all regions multitudes of little fishes abound. As these localities are neglected by most collectors, they have proved of late years a most prolific source of new species. The tide-pools of Cuba, Key West, Cape Flattery, Sitka, Unalaska, Monterey, San Diego, Mazatlan, Hilo, Kailua and Waiahæ in Hawaii, Apia and Pago-Pago in Samoa, the present writer has found peculiarly rich in rock-loving forms. Even richer are the pools of the promontories of Japan, Hakodate Head, Misaki, Awa, Izu, Waka, and Kagoshima, where a whole new fish fauna unknown to collectors in markets and sandy bays has been brought to light. Some of these rockfishes are left buried in the rock weeds as the tide flows, lying quietly until it returns. Others cling to the rocks by ventral suckers, while still others depend for their safety on their powers of leaping or on their quickness of their movements in the water. Those of the latter class are often brilliantly colored, but the others mimic closely the algæ or the rocks. Some fishes live in the sea only, some prefer brackish-water. Some are found only in the rivers, and a few pass more or less indiscriminately from one kind of water to another.

Fig. 121.—Pearl-fish, Fierasfer acus (Linnæus), issuing from a Holothurian. Coast of Italy. (After Emery.)

[Pg 160]

Fig. 122.—Portuguese Man-of-war Fish, Gobiomorus gronovii. Family Stromateidæ.

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

Anadromous Fishes.—Many fresh-water fishes, as trout and suckers, forsake the large streams in the spring, ascending the small brooks where their young can be reared in greater safety. Still others, known as anadromous fishes, feed and mature in the sea, but ascend the rivers as the impulse of reproduction grows strong. Among such fishes are the salmon, shad, alewife, sturgeon, and striped bass in American waters. The most remarkable case of the anadromous instinct is found in the king salmon or quinnat (Oncorhynchus tschawytscha) of the Pacific Coast. This great fish spawns in November, at the age of four years and an average weight of twenty-two pounds. In the Columbia River it begins running with the spring freshets in March and April. It spends the whole summer, without feeding, in the ascent of the river. By autumn the individuals have reached the mountain streams of Idaho, greatly changed in appearance,[Pg 161] discolored, worn, and distorted. The male is humpbacked, with sunken scales, and greatly enlarged, hooked, bent, or twisted jaws, with enlarged dog-like teeth. On reaching the spawning beds, which may be a thousand miles from the sea in the Columbia, over two thousand in the Yukon, the female deposits her eggs in the gravel of some shallow brook. The male covers them and scrapes the gravel over them. The female salmon does as much as the male in covering the eggs. Then both male and female drift tail foremost helplessly down the stream; none, so far as certainly known, ever survive the reproductive act. The same habits are found in the five other species of salmon in the Pacific, but in most cases the individuals do not start so early nor run so far. The blue-back salmon or redfish, however, does not fall far short in these regards. The salmon of the Atlantic has a similar habit, but the distance traveled is everywhere much less, and most of the hook-jawed males drop down to the sea and survive to repeat the acts of reproduction.

Fig. 123.—Tide-pools of Misaki. The Misaki Biological Station, from the north side.

[Pg 162]

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

Fig. 124.—Squaw-fish, Ptychocheilus oregonensis (Richardson). Columbia River.

Pugnacity of Fishes.—Some fishes are very pugnacious, always ready for a quarrel with their own kind. The sticklebacks show this disposition, especially the males. In Hawaii the natives take advantage of this trait to catch the Uu (Myripristis murdjan), a bright crimson-colored fish found in those waters. The species lives in crevices in lava rocks. Catching a live one, the fishermen suspend it by a string in front of the rocks. It remains there with spread fins and flashing scales, and the others come out to fight it, when all are drawn to the surface by a[Pg 163] concealed net. Another decoy is substituted and the trick is repeated until the showy and quarrelsome fishes are all secured.

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

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

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

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

[Pg 164]

Fig. 125.—Squaw-fish, Ptychocheilus grandis Agassiz. Running up a stream to spawn, the high water, after a rain, falling, leaves the fishes stranded. Kelsey Creek, Clear Lake, California, April 29, 1899. (Photograph by O. E. Meddaugh.)—Page 164.

[Pg 165]

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

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

Fear is shown by fish in many ways. There is not an angler unacquainted with the natural timidity of fishes, nor a keeper in [Pg 166]charge of a salmon-pass, who does not know how easy it is for poachers to deter the salmon from venturing along the path raised expressly for his use.

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

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

Pennant tells us that the river bullhead (Cottus gobio) "deposits its spawn in a hole it forms in the gravel, and quits it with great reluctance." General Hardwicke tells how the [Pg 167]gouramy (Osphromenus gouramy), in the Mauritius, forms a nest amongst the herbage growing in the shallow water in the sides of tanks. Here the parent continues to watch the place with the greatest vigilance, driving away any interloping fish. The amphibious walking-fish of Mysore (Ophiocephalus striatus) appears to make a nest very similar to that of the gouramy, and over it the male keeps guard; but should he be killed or captured, the vacant post is filled by his partner. (Colonel Puckle.) When very young the fishes keep with and are defended by their parents, but so soon as they are sufficiently strong to capture prey for themselves they are driven away to seek their own subsistence. (See Fishes of India, p. 362.) But it is not only these monogamous amphibious fishes which show an affection for their eggs and also for their fry, but even the little Etroplus maculatus has been observed to be equally fond of its ova. "The eggs are not very numerous and are deposited in the mud at the bottom of the stream, and, when hatched, both parents guard the young for many days, vigorously attacking any large fish that passes near them."[17]

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

Calling the Fishes.—At many temples in India fishes are called to receive food by means of ringing bells or musical sounds. Carew, in Cornwall, is said to have called the gray mullet together by making a noise like chopping with a cleaver. Lacépède relates that some fishes, which had been kept in the basins out of the Tuileries for more than a century, would come when called by their names, and that in many parts of Germany trout, carp, and tench are summoned to their food by the sound of a bell. These instances are mostly due to the [Pg 168]fishes having learned by experience that on the hearing certain sounds they may expect food. But Lacépède mentions that some were able to distinguish their individual names; and the same occurs in India. Lieutenant Connolly[18] remarked upon seeing numerous fishes coming to the ghaut at Sidhnath to be fed when called; and on "expressing our admiration of the size of the fish, 'Wait,' said a bystander, 'until you have seen Raghu.' The Brahmin called out his name in a peculiar tone of voice; but he would not hear. I threw in handful after handful of ottah (flour) with the same success, and was just leaving the ghaut, despairing and doubting, when a loud plunge startled me. I thought somebody had jumped off the bastion of the ghaut into the river, but was soon undeceived by the general shout of 'Raghu, raghu,' and by the fishes, large and small, darting away in every direction. Raghu made two or three plunges, but was so quick in his motions that I was unable to guess at his species." [It may be said in relation to these stories quoted by Dr. Day, that they probably belong to the mythology of fishes. It is very doubtful if fishes are able to make any such discrimination among sounds in the air.]

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

Some years since, at Madras, I (Dr. Day) obtained several specimens of a fresh-water Siluroid fish (Macrones vittatus) which is termed the "fiddler" in Mysore. I touched one which was on the wet ground, at which it appeared to become very irate, erecting its dorsal fin, making a noise resembling the buzzing of [Pg 169]a bee. Having put some small carp into an aquarium containing one of these fishes, it rushed at a small example, seized it by the middle of its back, and shook it like a dog killing a rat; at this time its barbels were stiffened out laterally like a cat's whiskers.

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

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

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

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

Lurking Fishes.—Mr. Whitmee supposes that most carnivorous fish capture their prey by outswimming them; but to this there are numerous exceptions; the angler or fishing-frog (Lophis piscatorius), "while crouching close to the ground, by the action of its ventral and pectoral fins stirs up the sand and mud; hidden by the obscurity thus produced, it elevates its anterior dorsal spines, moves them in various directions by way of attraction as a bait, and the small fishes, approaching either to examine or to seize them, immediately become the[Pg 170] prey of the fisher." (Yarrell.) In India we find a fresh-water Siluroid (Chaca lophioides) which "conceals itself among the mud, from which, by its lurid appearance and a number of loose filamentous substances on its skin, it is scarcely distinguishable; and with an immense open mouth it is ready to seize any small prey that is passing along." (Ham. Buchanan.) In March, 1868, I obtained a fine example of Ichthyscopus lebeck (Fishes of India, p. 261), which I placed in water having a bed of mud; into this it rapidly worked itself, first depressing one side and then another, until only the top of its head and mouth remained above the mud, whilst a constant current was kept up through its gills. It made a noise, half snapping and half croaking, when removed from its native element.

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

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

"Among the Siluroid fishes of Guiana there are several species which, at certain seasons of the year, have their mouths and branchial cavities filled either with eggs or young, and, as is believed, for the purpose of incubation. My attention was first called to this singular habit by the late Dr. Francis W.[Pg 171] Cragin, formerly United States consul at Paramaribo, Surinam. In a letter dated August, 1854, he says:

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

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

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

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

"The eggs in all instances were carried in the mouths of the males. This protection, or gestation of the eggs by the males, corresponds with what has been long noticed with regard to other fishes, as, for example, Syngnathus, where the mar[Pg 172]supial pouch for the eggs or young is found in the males only, and Gasterosteus, where the male constructs the nest and protects the eggs during incubation from the voracity of the females.

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

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

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

"The question will be very naturally asked, how under such circumstances these fishes are able to secure and swallow their food. I have made no observations bearing upon such a question. Unless the food consists of very minute particles it would seem necessary that during the time of feeding the eggs should be disgorged. If this supposition be correct, it would give a very probable explanation of the only fact which might be considered at variance with the conclusion stated above, viz., that[Pg 173] we have in these fishes a mouth gestation. In the mass of eggs with which the mouth is filled I have occasionally found the eggs, rarely more than one or two, of another species. The only way in which their presence may be accounted for, it seems to me, is by the supposition that while feeding the eggs are disgorged, and as these fishes are gregarious in their habits, when the ova are recovered the stray eggs of another species may be introduced into the mouth among those which naturally belong there."

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

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

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

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

It is well known that in the very young flounder the body rests upright in the water. After a little there is a tendency to turn to one side and the lower eye begins its migration to the other side, the interorbital bones or part of them moving before[Pg 174] it. In most flounders the eye seems to move over the surface of the head, before the dorsal fin, or across the axil of its first ray. In the tropical genus Platophrys the movement of the eye is most easily followed, as the species reach a larger size than do most flounders before the change takes place. The larva, while symmetrical, is in all cases transparent.

Fig. 126.

Fig. 127.

Figs. 126, 127.—Larval stages of Platophrys podas, a flounder of the Mediterranean, showing the migration of the eye. (After Emery.)

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

1. The young of Limanda ferruginea (the rusty dab) are probably in the larval stage at the same time as those of Pseudopleuronectes americanus (the winter flounder).

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

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

4. Correlated with this is an increase in distance between[Pg 175] the eyes and the brain, caused by the growth of the facial cartilages.

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

Fig. 128.—Platophrys lunatus (Linnæus), the Wide-eyed Flounder. Family Pleuronectidæ. Cuba. (From nature by Mrs. H. C. Nash.)

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

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

Fig. 129.—Young Flounder, just hatched, with symmetrical eyes. (After S. R. Williams.)

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

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

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

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

[Pg 176]

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

13. There is a perfect chiasma.

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

Fig. 130.—Larval Flounder, Pseudopleuronectes americanus. (After S. R. Williams.)

Fig. 131.—Larval Flounder, Pseudopleuronectes americanus. (After S. R. Williams.)

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

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

[Pg 177]

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

Fig. 132.—Face view of recently hatched Flounder. (After S. R. Williams.)

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

"This deviation from the bilateral condition must have come about either as a 'sport' or by gradual modification of the adults. If by the latter method—the change proving to be advantageous—selection favored its appearing earlier and earlier in ontogeny, until it occurred in the stages of planktonic life. Metamorphosis at a stage earlier than this would be a distinct disadvantage, because of the lack of the customary planktonic food at the sea-bottom. At present some forms of selection are probably continually at work fixing the limit of the period of metamorphosis by the removal of those individuals which attempt the transformation at unsuitable epochs; for instance, at the time of hatching. That there are such individuals is shown by Fullarton, who figures a fish just hatched 'anticipating the twisting and subsequent unequal development exhibited by the head of Pleuronectids.' Those larvæ which remain pelagic until better able to compete at the sea-bottom[Pg 178] become the adults which fix the time of metamorphosis on their progeny." (S. R. Williams.)

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

[Pg 179]

CHAPTER XII
ADAPTATIONS OF FISHES

Fig. 133.—Mad-tom, Schilbeodes furiosus Jordan and Meek. Showing the poisoned pectoral spine. Family Siluridæ. Neuse River.

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

[Pg 180]

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

Fig. 134.—Black Nohu, or Poison-fish, Emmydrichthys vulcanus Jordan. A species with stinging spines, showing resemblance to lumps of lava among which it lives. Family Scorpænidæ. From Tahiti.

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

In Thalassophryne, a genus of toad-fishes of tropical America, is found the most perfect system of poison organs known among fishes. The spinous armature of the opercle and the two spines of the first dorsal fin constitute the weapons. The details are known from the dissections of Dr. Günther. According to his[19] observations, the opercle in Thalassophryne "is very narrow, [Pg 181]vertically styliform and very mobile. It is armed behind with a spine eight lines long and of the same form as the hollow venom-fang of a snake, being perforated at its base and at its extremity. A sac covering the base of the spine discharges its contents through the apertures and the canal in the interior of the spine. The structure of the dorsal spines is similar. There are no secretory glands imbedded in the membranes of the sacs and the fluid must be secreted by their mucous membrane. The sacs are without an external muscular layer and situated immediately below the thick, loose skin which envelops the spines at their extremity. The ejection of the poison into a living animal, therefore, can only be effected as in Synanceia, by the pressure to which the sac is subjected the moment the spine enters another body."

Fig. 135.—Brown Tang, Teuthis bahianus (Ranzani). Tortugas, Florida.

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

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

[Pg 182]

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

Fig. 136.—Common Filefish, Stephanolepis hispidus (Linnæus). Virginia.

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

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

The flesh of the filefishes (Stephanolepis tomentosus), which[Pg 183] the writer has tested, is very meager and bitter, having a decidedly offensive taste. It is suspected, probably justly, of being poisonous. In the globefishes the flesh is always more or less poisonous, that of Tetraodon hispidus, called muki-muki, or death-fish, in Hawaii, is reputed as excessively so. The poisonous fishes have been lately studied in detail by Dr. Jacques Pellegrin, of the Museum d'Histoire Naturelle at Paris. He shows that any species of fish may be poisonous under certain circumstances, that under certain conditions certain species are poisonous, and that certain kinds are poisonous more or less at all times. The following account is condensed from Dr. Pellegrin's observations.

Fig. 137.—Tetraodon meleagris (Lacépède). Riu Kiu Islands.

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

[Pg 184]

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

Fig. 138.—The Trigger-fish, Balistes carolinensis Gmelin. New York.

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

[Pg 185]

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

Fig. 139.—Numbfish, Narcine brasiliensis Henle, showing electric cells. Pensacola, Florida.

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

It is claimed in tropical regions that the germs of the bubonic plague may be carried through the mediation of fishes which feed on sewage. It is suggested by Dr. Charles B. Ash[Pg 186]mead that leprosy may be so carried. It is further suggested that the custom of eating the flesh of fishes raw almost universal in Japan, Hawaii, and other regions may be responsible for the spread of certain contagious diseases, in which the fish acts as an intermediate host, much as certain mosquitoes spread the germ of malaria and yellow fever.

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

Fig. 140.—Electric Catfish, Torpedo electricus (Gmelin). Congo River. (Alter Boulenger.)

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

The electric eel (Electrophorus electricus), the most powerful[Pg 187] of electric fishes, is not an eel, but allied rather to the sucker or carp. It is, however, eel-like in form and lives in rivers of Brazil and Guiana. The electric organs are in two pairs, one on the back of the tail, the other on the anal fin. These are made up of an enormous number of minute cells. In the electric eel, as in the other electric fishes, the nerves supplying these organs are much larger than those passing from the spinal cord for any other purpose. In all these cases closely related species show a no trace of the electric powers.

Fig. 141.—Star-gazer (Astroscopus guttatus) settling in the sand. (From life by R. W. Shufeldt.)

Dr. Gilbert has described the electric powers of species of star-gazer (Astroscopus y-græcum and A. zephyreus), the electric cells lying under the naked skin of the top of the head. Electric power is ascribed to a species of cusk (Urophycis regius), but this perhaps needs verification.

Photophores or Luminous Organs.—Many fishes, chiefly of the deep seas, develop organs for producing light. These are known as luminous organs, phosphorescent organs, or photophores. These are independently developed in four entirely unrelated groups of fishes. This difference in origin is accompanied by corresponding difference in structure. The best-known type is found in the Iniomi, including the lantern-fishes and their many relatives. These may have luminous spots, differ[Pg 188]entiated areas round or oblong which shine star-like in the dark. These are usually symmetrically placed on the sides of the body. They may have also luminous glands or diffuse areas which are luminous, but which do not show the specialized structure of the phosphorescent spots. These glands of similar nature to the spots are mostly on the head or tail. In one genus, Æthoprora, the luminous snout is compared to the headlight of an engine.

Fig. 142.—Headlight Fish, Æthoprora lucida Goode and Bean. Gulf Stream.

Fig. 143.—Corynolophus reinhardti (Lütken), showing luminous bulb (modified after Lütken). Family Ceratiidæ. Deep sea off Greenland.

[Pg 189]

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

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

Fig. 144.—Etmopterus lucifer Jordan and Snyder. Misaki, Japan.

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

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

In all these structures, according to Dr. von Lendenfeld, the whole or part of the organ is glandular. The glandular part is at the base and the other structures are added distally. The primitive organ was a gland which produced luminous slime.[Pg 190] To this in the process of specialization greater complexity has been added.

Fig. 145.—Argyropelecus olfersi Cuvier. Gulf Stream.

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

Photophores of Porichthys.—Entirely distinct in their origin are the luminous spots in the midshipman (Porichthys notatus), a shore fish of California. These have been described in detail by Dr. Charles Wilson Greene (late of Stanford University, now of the University of Missouri) in the Journal of Morphology, xv., p. 667. These are found on various parts of the body in connection with the mucous pores of the lateral lines and about the mucous pores of the head. The skin in Porichthys is naked, and the photophores arise from a modification of its epidermis. Each is spherical, shining white, and consists of four parts—the[Pg 191] lens, the gland, the reflector, and the pigment. As to its function Prof. Greene observes:

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

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

"Electrical stimulation of the live fish was also tried with good success. The interrupted current from an induction coil was used, one electrode being fixed on the head over the brain or on the exposed spinal cord near the brain, and the other moved around on different parts of the body. No results followed relatively weak stimulation of the fish, although such currents produced violent contractions of the muscular system[Pg 192] of the body. But when a current strong enough to be quite painful to the hands while handling the electrodes was used then stimulation of the fish called forth a brilliant glow of light apparently from every well-developed photophore. All the lines on the ventral and lateral surfaces of the body glowed with a beautiful light, and continued to do so while the stimulation lasted. The single well-developed organ just back of and below the eye was especially prominent. No luminosity was observed in the region of the dorsal organs previously described as rudimentary in structure. I was also able to produce the same effect by galvanic stimulation, rapidly making and breaking the current by hand.

Fig. 146.—Luminous organs and lateral line of Midshipman, Porichthys notatus Girard. Family Batrachoididæ. Monterey, California. (After Greene.)

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

"An important fact brought out by the above experiment is that an electrical stimulation strong enough to most violently stimulate the nervous system, as shown by the violent contractions of the muscular system, may still be too weak to produce phosphorescence. This fact gives a physiological con[Pg 193]firmation of the morphological result stated above that no specific nerves are distributed to the phosphorescent organs.

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

Fig. 147.—Cross-section of a ventral phosphorescent organ of the Midshipman, Porichthys notatus Girard. l, lens; gl, gland; r, reflector; bl, blood; p, pigment. (After Greene.)

"The experiments just related were all tried on specimens of the fish taken from under the rocks where they were guarding the young brood. Two specimens, however, taken by hooks from the deeper water of Monterey Bay, could not be made to show phosphorescence either by electrical stimulation or by treatment with ammonia. These specimens did net have the high development of the system of mucous cells of the skin exhibited by the nesting fish. My observations were, how[Pg 194]ever, not numerous enough to more than suggest the possibility of a seasonal high development of the phosphorescent organs.

Fig. 148.—Section of the deeper portion of phosphorescent organ of Porichthys notatus, highly magnified. (After Greene.)

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

"The literature on the histological structure of known phosphorescent organs of fishes is rather meager and unsatisfactory. Von Lendenfeld describes twelve classes of phosphorescent organs from deep-sea fishes collected by the Challenger expe[Pg 195]dition. All of these, however, are greater or less modifications of one type. This type includes, according to von Lendenfeld's views, three essential parts, i.e., a gland, phosphorescent cells, and a local ganglion. These parts may have added a reflector, a pigment layer, or both; and all these may be simple or compounded in various ways, giving rise to the twelve classes. Blood-vessels and nerves are distributed to the glandular portion. Of the twelve classes direct ocular proof is given for one, i.e., ocellar organs of Myctophum which were observed by Willemoes-Suhm at night to shine 'like a star in the net.' Von Lendenfeld says that the gland produces a secretion, and he supposes the light or phosphorescence to be produced either by the 'burning or consuming' of this secretion by the phosphorescent cells, or else by some substance produced by the phosphorescent cells. Furthermore, he says that the phosphorescent cells act at the 'will of the fish' and are excited to action by the local ganglion.

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

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

[Pg 196]

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

"In the phosphorescent organs of Porichthys the only part the function of which cannot be explained on physical grounds is the group of cells called the gland. If the large granular cells of this portion of the structure produce a secretion, as seems probable from the character of the cells and their behavior toward reagents, and this substance be oxidizable and luminous in the presence of free oxygen, i.e., photogenic, then we have the conditions necessary for a light-producing organ. The numerous capillaries distributed to the gland will supply free oxygen sufficient to meet the needs of the case. Light produced in the gland is ultimately all projected to the exterior, either directly from the luminous points in the gland or reflected outward by the reflector, the lens condensing all the rays into a definite pencil or slightly diverging cone. This explanation of the light-producing process rests on the assumption of a secretion product with certain specific characters. But comparing the organ with structures known to produce such a substance, i.e., the glands of the firefly or the photospheres of Eu[Pg 197]phausia, it seems to me the assumption is not less certain than the assumption that twelve structures resembling each other in certain particulars have a common function to that proved for one only of the twelve.

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

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

Fig. 149.—Sucking-fish, or Pegador, Leptecheneis naucrates (Linnæus). Virginia.

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

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

Remoras.—The different species of Remora, or shark-suckers, fasten themselves to the surface of sharks or other fishes and are carried about by them often to great distances. These[Pg 198] fishes attach themselves by a large sucking-disk on the top of the head, which is a modified spinous dorsal fin. They do not harm the shark, except possibly to retard its motion. If the shark is caught and drawn out of the water, these fishes often instantly let go and plunge into the sea, swimming away with great celerity.

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

Fig. 150.—Clingfish, Caularchus mæandricus (Girard). Monterey, California.

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

The hagfishes (Myxine, Eptatretus) alone among fishes are truly parasitic. These fishes, worm-like in form, have round mouths, armed with strong hooked teeth. They fasten themselves at the throats of large fishes, work their way into the muscle without tearing the skin, and finally once inside devour all the muscles of the fish, leaving the skin unbroken and the viscera undisturbed. These fishes become living hulks before[Pg 199] they die. If lifted out of the water, the slimy hagfish at once slips out and swims quickly away. In gill-nets in Monterey Bay great mischief is done by hagfish (Polistotrema stouti). It is a curious fact that large numbers of hagfish eggs are taken from the stomachs of the male hagfish, which seems to be almost the only enemy of his own species, keeping the numbers in check.

Fig. 151.—Hagfish, Polistotrema stouti (Lockington).

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

The Paddle-fishes.—In the paddle-fishes (Polyodon spatula and Psephurus gladius) the snout is spread out forming a broad paddle or spatula. This the animal uses to stir up the mud on the bottoms of rivers, the small organisms contained in mud constituting food. Similar paddle-like projections are developed in certain deep-water Chimæras (Harriottia, Rhinochimæra), and in the deep-sea shark, Mitsukurina.

[Pg 200]

Fig. 152.—Indian Sawfish, Pristis zysron Latham. River mouths of Hindustan. (After Day.)

[Pg 201]

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

Fig. 153.—Saw-shark, Pristiophorus japonicus Günther. Specimen from Nagasaki.

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

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

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

In four different and wholly unrelated groups of fishes the teeth are grown fast together, forming a horny beak like that of the parrot. These are the Chimæras, the globefishes (Tetraodon), and their relatives, the parrot-fishes (Scarus, etc.), and the stone-wall perch (Oplegnathus). The structure of the beak varies considerably in these four cases, in accord with the difference in the origin of its structures. In the globefishes the[Pg 202] jaw-bones are fused together, and in the Chimæras they are solidly joined to the cranium itself.

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

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

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

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

It has been known for some years that in several groups of [Pg 203]fishes (wrasse-fishes, flounders, and "rock-cod," for example) those species which inhabit northern waters have more vertebræ than those living in the tropics. Certain arctic flounders, for example, have sixty vertebræ; tropical flounders have, on the average, thirty. The significance of this fact is the problem at issue. In science it is assumed that all facts have significance, else they would not exist. It becomes necessary, then, to find out first just what the facts are in this regard.

Fig. 154.—Skeleton of Pike, Esox lucius Linnæus, a river fish with many vertebræ.

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

Number of Vertebræ.—The numbers of vertebræ in different groups may be summarized as follows:

Lancelets.—Among the lancelets the numbers of segments range from 50 to 80, there being no vertebræ.

Lampreys.—In this group the number of segments ranges from 100 to 150.

[Pg 204]

Elasmobranchs.—Among sharks and skates the usual number of segments is from 100 to 150 and upwards. In the extinct species as far as known the numbers are not materially different. The Carboniferous genus, Pleuracanthus, has about 115 vertebræ. The Chimæras have similar numbers; Chimæra monstrosa has about 100 in the body and more than as many more in the filamentous tail.

Cycliæ.—Palæospondylus has about 85 vertebræ.

Arthrodires.—There are about 100 vertebræ in Coccosteus.

Dipnoans.—In Protopterus there are upwards of 100 vertebræ, the last much reduced in size. Figures of Neoceratodus show about 80.

Crossopterygians.—Polypterus has 67 vertebræ; Erpetichthys, 110; Undina, about 85.

Ganoids.—In this group the numbers are also large—95 in Amia, about 55 in the short-bodied Microdon. The Sturgeons all have more than 100 vertebræ.

Soft-rayed Fishes.—Among the Teleostei, or bony fishes, those which first appear in geological history are the Isospondyli, the allies of the salmon and herring. These have all numerous vertebræ, small in size, and none of them in any notable degree modified or specialized. They abound in the depths of the ocean, but there are comparatively few of them in the tropics. The Salmonidæ which inhabit the rivers and lakes of the northern zones have from 60 to 65 vertebræ. The Myctophidæ, Stomiatidæ, and other deep-sea forms have from 40 upwards in the few species in which the number has been counted. The group of Clupeidæ is nearer the primitive stock of Isospondyli than the salmon are. This group is essentially northern in its distribution, but a considerable number of its members are found within the tropics. The common herring (Clupea harangus) ranges farther into the arctic regions than any other. Its vertebræ are 56 in number. In the shad (Alosa sapidissima), a northern species which ascends the rivers, the same number is recorded. The sprat (Clupea sprattus) and sardine (Sardinia pilchardus), ranging farther south, have from 48 to 50, while in certain small herrings (Sardinella) which are strictly confined to tropical shores the number is but 40. Allied to the herring are the anchovies, mostly tropical. The northern[Pg 205]most species, the common anchovy of Europe (Engraulis enchrasicolus), has 46 vertebræ. A tropical species (Anchovia browni) has 41.

There are, however, a few soft-rayed fishes confined to the tropical seas in which the numbers of vertebræ are still large, an exception to the general rule. Among these are Albula vulpes, the bonefish, with 70 vertebræ, Elops saurus, the ten-pounder, with 72, the tarpon (Tarpon atlanticus), with about 50, and the milkfish, Chanos chanos, with 72.

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

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

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

The Flounders.—In the flounders, the halibut and its relatives, arctic genera (Hippoglossus and Atheresthes), have from 49 to 50 vertebræ. The northern genera (Hippoglossoides, Lyopsetta, and Eopsetta) have from 43 to 45; the members of[Pg 206] a large semi-tropical genus (Paralichthys) of wide range have from 35 to 41; while the tropical forms have from 35 to 37.

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

Spiny-rayed Fishes.—Among the spiny-rayed fishes the facts are more striking. Of these, numerous families are chiefly or wholly confined to the tropics, and in the great majority of all the species the number of vertebræ is constantly 24,—10 in the body and 14 in the tail (10+14). This is true of all or nearly all the Berycidæ, Serranidæ, Sparidæ, Sciænidæ, Chætodontidæ, Hæmulidæ, Gerridæ, Gobiidæ, Acanthuridæ, Mugilidæ, Sphyrænidæ, Mullidæ, Pomacentridæ, etc.

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

Thus Balistes, the trigger-fish, has 17 vertebræ; Monacanthus and Alutera, foolfishes, about 20; the trunkfish, Ostracion, 14; the puffers, Tetraodon and Spheroides, 18; Canthigaster, 17; and the headfish, Mola, 17. Among the Pediculates, Malthe and Antennarius have 17 to 19 vertebræ, while in their near relatives, the anglers, Lophiidæ, the number varies with the latitude. Thus, in the northern angler, Lophius piscatorius, which is never found south of Cape Hatteras, there are 30 vertebræ. In a similar species, inhabiting the north of Japan (Lophius litulon), there are 27. In another Japanese species, ranging[Pg 207] farther south, Lophiomus setigerus, the vertebræ are but 19. Yet in external appearance these two fishes are almost identical. It is, however, a notable fact that some of the deep-water Pediculates, or angling fishes, have the body very short and the number of vertebræ correspondingly reduced. Dibranchus atlanticus, from a depth of 3600 fathoms, or more than 4 miles, has but 18 vertebræ, and others of its relatives in deep waters show also small numbers. These soft-bodied fishes are simply animated mouths, with a feeble osseous structure, and they are perhaps recent offshoots from some stock which has extended its range from muddy bottom or from floating seaweed to the depths of the sea.

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

The facts of variation with latitude were first noticed among the Labridæ. In the northern genera (Labrus, Tautoga, etc.) there are 38 to 41 vertebræ; in the semi-tropical genera (Crenilabrus, Bodianus, etc.), 30 to 33; in the tropical genera (Halichœres, Xyrichthys, Thalassoma, etc.), usually 24.

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

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

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

Allied to the Scorpænidæ, but confined to the tropical or semi-tropical seas, are the Platycephalidæ, with 27 vertebræ, and[Pg 208] the Cephalacanthidæ (flying gurnards), with but 22. In the deeper waters of the tropics are the Peristediidæ, with 33 vertebræ, and extending farther north, belonging as much to the temperate as to the torrid zone, is the large family of the Triglidæ (gurnards) in which the vertebræ range from 25 to 38.

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

Of the true Blennidæ, which are all tropical or semi-tropical, Blennius has 28 to 35 vertebræ; Salarias, 35 to 38; Lepisoma, 34; Clinus, 49; Cristiceps, 40. A fresh-water species of Cristiceps found in Australia has 46. Blennioid fishes in the arctic seas are Anarrhichas, with 76 vertebræ; Anarrhichthys, with[Pg 209] 100 or more; Lumpenus, 79; Pholis, 85; Lycodes, 112; Gymnelis, 93. Lycodes and Gymnelis have lost all the dorsal spines.

In the cod family (Gadidæ) the number of vertebræ is usually about 50. The number is 51 in the codfish (Gadus callarias), 58 in the Siberian cod (Eleginus navaga), 54 in the haddock (Melanogrammus æglifinus), 54 in the whiting (Merlangus merlangus), 54 in the coalfish (Pollachius virens), 52 in the Alaskan coalfish (Theragra chalcogramma), 51 in the hake (Merluccius merluccius). In the burbot (Lota lota), the only fresh-water codfish, 59; in the deep-water ling (Molva molva), 64; in the rocklings (Gaidropsarus), 47 to 49. Those few species found in the Mediterranean and the Gulf of Mexico have fewer fin-rays and probably fewer vertebræ than the others, but none of the family enter warm water, the southern species living at greater depths.

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

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

Pelagic Fishes.—Among the free-swimming or migratory pelagic fishes, the number of vertebræ is usually greater than among their relatives of local habits. This fact is most evident among the scombriform fishes, the allies of the mackerel and tunny. All of these belong properly to the warm seas, and the reduction of the vertebræ in certain forms has no evident rela[Pg 210]tion to the temperature, though it seems to be related in some degree to the habits of the species. Perhaps the retention of many segments is connected with that strength and swiftness in the water for which the mackerels are preeminent.

The variations in the number of vertebræ in this group led Dr. Günther to divide it into two families, the Carangidæ and Scombridæ.

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

The mackerel (Scomber scombrus) has 31 vertebræ; the chub mackerel (Scomber japonicus), 31; the tunny (Thunnus thynnus), 39; the long-finned albacore (Germo alalonga), 40; the bonito (Sarda sarda), 50; the Spanish mackerel (Scomberomorus maculatus), 45.

Other mackerel-like fishes are the cutlass-fishes (Trichiuridæ), which approach the eels in form and in the reduction of the fins. In these the vertebræ are correspondingly numerous, the numbers ranging from 100 to 160. Aphanopus has 101 vertebræ; Lepidopus, 112; Trichurus, 159.

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

The Eels.—The eels constitute a peculiar group of soft-rayed ancestry, in which everything else has been subordinated to muscularity and flexibility of body. The fins, girdles, gill-arches, scales, and membrane bones are all imperfectly developed or wanting. The eel is perhaps as far from the primitive stock as the most highly "ichthyized" fishes, but its progress has been of another character. The eel would be regarded in the ordinary sense as a degenerate type, for its bony structure is greatly simplified as compared with its ancestral forms, but in its eel-like qualities it is, however, greatly specialized. All the eels have vertebræ in great numbers. As the great majority of the species are tropical, and as the vertebræ in very few of[Pg 211] the deep-sea forms have been counted, no conclusions can be drawn as to the relation of their vertebræ to the temperature.

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

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

Among the morays, Muræna helena has 140; Gymnothorax meleagris, 120; G. undulatus, 130; G. moringa, 145; G. concolor, 136; Echidna catenata, 116; E. nebulosa, 142; E. zebra, 135. In other families the true eel, Anguilla anguilla, has 115; the conger-eel, Leptocephalus conger, 156; and Murænesox cinereus, 154.

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

Thus in the Scorpænidæ, Sebastes, and Sebastolobus arctic genera have the dorsal rays xv, 13, the vertebræ 12+19. The tropical genus Scorpæna has the dorsal rays xii, 10, the vertebræ 10+14, while the genus Sebastodes of temperate waters has the intermediate numbers of dorsal rays xii, 12, and vertebræ 12+15.

[Pg 212]

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

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

All these effects should be referable to the same group of causes. They may, in fact, be combined in one statement. All other fishes now extant, as well as all fishes existing prior to Cretaceous times, have a larger number of vertebræ than the marine shore fishes of the tropics of the present period. There is good reason to believe that in most groups of spiny-rayed fishes, those with the smaller number of segments are at once the most highly organized and the most primitive. This is true among the blennies, the sculpins, the flounders, the perches, and probably the labroid fishes as well. The present writer once[Pg 213] held the contrary view, that the forms with the higher numbers were primitive, but the evidence both from comparative anatomy and from palæontology seems to indicate that among spiny-rayed fishes the forms most ancient, most generalized, and most synthetic are those with about 24 vertebræ. The soft-rayed fishes without exception show larger numbers, and these are still more primitive. This apparent contradiction is perhaps explained by Dr. Boulenger's suggestion that the prevalence of the same number, 24, in the vertebræ of various families of spiny-rayed fishes is due to common descent, probably from Cretaceous berycoids having this number. In this theory, perches, sparoids, carangoids, chætodonts, labroids, parrot-fishes, gobies, flounders, and sculpins must be regarded as having a common origin from which all have diverged since Jurassic times. This view is not at all unlikely and is not inconsistent with the facts of palæontology. If this be the case, the members of these and related families which have larger numbers of vertebræ must have diverged from the primitive stock. The change has been one of degeneration, the individual vertebræ being reduced in size and complexity, with a vegetative increase in their number. At the same time, the body having the greater number of segments is the more flexible though the segments themselves are less specialized.

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

It is certain that the possession of fewer vertebræ indicates the higher rank, the greater specialization of parts, even though the many vertebræ be a feature less primitive. The evolution of fishes is rarely a movement of progress toward complexity. The time movement in some groups is accompanied by degradation and loss of parts, by vegetative repetition of structures, and often by a movement from the fish-form toward the [Pg 214]eel-form. Water life is less exacting than land life, having less variation of conditions. It is, therefore, less effective in pushing forward the differentiation of parts. When vertebræ are few in number each one is relatively larger, its structure is more complicated, its appendages larger and more useful, and the fins with which it is connected are better developed. In other words, the tropical fish is more intensely and compactly a fish, with a better fish equipment, and in all ways better fitted for the business of a fish, especially for that of a fish that stays at home.

Fig. 155.—Skeleton of Red Rockfish, Sebastodes miniatus Jordan and Gilbert. California.

Fig. 156.—Skeleton of a spiny-rayed fish of the tropics, Holacanthus ciliaris (Linnæus).

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

The conditions most favorable to fish life are among the rocks[Pg 215] and reefs of the tropical seas. About the coral reefs is the center of fish competition. A coral archipelago is the Paris of fishes. In such regions is found the greatest variety of surroundings, and therefore the greatest number of possible adjustments. The struggle is between fish and fish, not between fishes and hard conditions of life. No form is excluded from the competition. Cold, darkness, and foul water do not shut out competitors, nor does any evil influence sap the strength. The heat of the tropics does not make the sea-water hot. It is never sultry or laden with malaria.

Fig. 157.—Skeleton of the Cowfish, Lactophrys tricornis (Linnæus).

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

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

Conversely, as the causes of these changes are still in opera[Pg 216]tion, we should find that in cold waters, deep waters, dark waters, fresh waters, and inclosed waters the strain would be less, the relapses to less complex organization more frequent, the numbers of vertebræ would be larger, while the individual vertebræ would become smaller, less complete, and less perfectly ossified.

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

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

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

An exception to the general rule in regard to the number of vertebræ is found in the case of the eel. Eels inhabit nearly all seas, and everywhere they have many vertebræ. The eels of the tropics are at once more specialized and more degraded. They are better eels than those of northern regions, but, as the[Pg 217] eel is a degraded type, they have gone farther in the loss of structures in which this degradation consists.

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

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

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

Degeneration of Structures.—In the lancelet, which is a primitively simple organism, the various structures of the body are formed of simple tissues and in a very simple fashion. It is probable from the structure of each of these that it has never been very much more complex. As the individual develops in the process of growth each organ goes as it were straight to its final form and structure without metamorphosis or especial alterations by the way. When this type of development occurs, the organism belongs to a type which is primitively simple. But there are other forms which in their adult state appear feeble or simple, in which are found elements of organs of high complexity. Thus in the sea-snail (Liparis), small, weak, with feeble fins and flabby skin, we find the essential anatomy of the sculpin or the rosefish. The organs of the latter are there, but each one is reduced or degenerate, the bones as soft as membranes, the spines[Pg 218] obsolete or buried in the skin. Such a type is said to be degenerate. It is very different from one primitively simple, and it is likely in its earlier stages of development to be more complex than when it is fully grown.

Fig. 158.—Liparid, Crystallias matsushimæ (Jordan and Snyder). Family Liparididæ. Matsushima Bay, Japan.

Fig. 159.—Yellow-backed Rockfish, Sebastichthys maliger Jordan and Gilbert. Sitka, Alaska.

In the evolution of groups of fishes it is a common feature that some one organ will be the center of a special stress, in view of some temporary importance of its function. By the process of natural selection it will become highly developed and highly specialized. Some later changes in conditions will ren[Pg 219]der this specialization useless or even harmful for at least a part of the species possessing it. The structure then undergoes degeneration, and in many cases it is brought to a lower estate than before the original changes. An example of this may be taken from the loricate or mailed-cheek fishes. One of the primitive members of this group is the rockfish known as priestfish (Sebastodes mystinus). In this fish the head is weakly armed, covered with ordinary scales. A slight suggestion of cranial ridges and a slight prolongation of the third suborbital constitute the chief suggestions of its close affinity with the mailed-cheek fishes. In other rockfishes the cranial ridges grow higher and sharper. The third suborbital extends itself farther and wider. It becomes itself spinous in still others. Finally it covers the whole cheek in a coat of mail. The head above becomes rough and horny and at last the whole body also is enclosed in a bony box. But while this specialization reaches an extraordinary degree in forms like Agonus and Peristedion, it begins to abate with Cottus, and thence through Cottunculus, Psychrolutes, Liparis, and the like, and the mailed cheek finds its final degradation in Parliparis. In this type no spines are present anywhere, no hard bone, no trace of scales, of first dorsal, or of ventral fins, and in the soft, limp structure covered with a fragile, scarf-like skin we find little suggestion of affinity with the strong rockfish or the rough-mailed Agonus. Yet a study of the skeleton shows that all these loricate forms constitute a continuous divergent series. The forms figured constitute only a few of the stages of specialization and degradation which the members of this group represent.

Fig. 160.—European Sculpin, Myoxocephalus scorpius (Linnæus). Cumberland Gulf, Arctic America

[Pg 220]

Fig. 161.—Sea-raven, Hemitripterus americanus (Gmelin). Halifax, Nova Scotia.

Some of the features of the habits and development of certain fresh-water fishes are mentioned in the following chapter.

Fig. 162.—Lumpfish, Cyclopterus lumpus (Linnæus). Eastport, Maine.

The degeneration of the eye of the blind fishes of the caves of the Mississippi Valley, Amblyopsis, Typhlichthys, and Troglichthys, have been very fully studied by Dr. Carl H. Eigenmann.

[Pg 221]

According to his observations

"The history of the eye of Amblyopsis spelæus may be divided into four periods:

Fig. 163.—Sleek Sculpin, Psychrolutes paradoxus (Günther). Puget Sound.

"(a) The first extends from the appearance of the eye till the embryo is 4-5 mm. long. This period is characterized by a normal palingenic development, except that the cell division is retarded and there is very little growth.

Fig. 164.—Agonoid-fish, Pallasina barbata (Steindachner). Port Mulgrave, Alaska.

"(b) The second period extends till the fish is 10 mm. long. It is characterized by the direct development of the eye from the normal embryonic stage reached in the first period to the highest stage reached by the Amblyopsis eye.

Fig. 165.—Blindfish of the Mammoth Cave, Amblyopsis spelæus (De Kay). Mammoth Cave, Kentucky.

"(c) The third, from 10 mm. to about 80 or 100 mm. It is characterized by a number of changes which are positive as con[Pg 222]trasted with degenerative. There are also distinct degenerative processes taking place during this period.

"(d) The fourth, 80-100 mm. to death. It is characterized by degenerative processes only.

"The eye of Amblyopsis appears at the same stage of growth as in normal fishes developing normal eyes. The eye grows but little after its appearance.

"All the developmental processes are retarded and some of them give out prematurely. The most important, if the last, is the cell division and the accompanying growth that provide material for the eye.

"The lens appears at the normal time and in the normal way, but its cells never divide and never lose their embryonic character.

"The lens is first to show degenerative steps and disappears entirely before the fish is 10 mm. long.

Fig. 166.—Blind Brotula, Lucifuga subterranea (Poey), showing viviparous habit. Joignan Cave, Pinar del Rio, Cuba. Photographed by Dr. Eigenmann.

"The optic nerve appears shortly before the fish reaches 5 mm. It does not increase in size with the growth of the fish and disappears in old age.

"The scleral cartilages appear when the fish is 10 mm. long; they grow very slowly, possibly till old age.

"There is no constant ratio between the extent and degree of ontogenic and phylogenic degeneration.

"The eye is approaching the vanishing point through the route indicated by the eye of Troglichthys rosæ.

"There being no causes operative or inhibitive, either within the fish or in the environment, that are not also operative or[Pg 223] inhibitive in Chologaster agassizii, which lives in caves and develops well-formed eyes, it is evident that the causes controlling the development are hereditarily established in the egg by an accumulation of such degenerative changes as are still notable in the later history of the eye of the adult.

"The foundations of the eye are normally laid, but the superstructure, instead of continuing the plan with additional material, completes it out of the material provided for the foundations. The development of the foundation of the eye is phylogenic; the stages beyond the foundations are direct."

Conditions of Evolution among Fishes.—Dr. Bashford Dean ("Fishes, Living and Fossil") has the following observations on the processes of adaptation among fishes:

"The evolution of groups of fishes must accordingly have taken place during only the longest periods of time. Their aquatic life has evidently been unfavorable to deep-seated structural changes, or at least has not permitted these to be perpetuated. Recent fishes have diverged in but minor regards from their ancestors of the Coal Measures. Within the same duration of time, on the other hand, terrestrial vertebrates have not only arisen, but have been widely differentiated. Among land-living forms the amphibians, reptiles, birds, and mammals have been evolved, and have given rise to more than sixty orders.

"The evolution of fishes has been confined to a noteworthy degree within rigid and unshifting bounds; their living medium, with its mechanical effects upon fish-like forms and structures, has for ages been almost constant in its conditions; its changes of temperature and density and currents have rarely been more than of local importance, and have influenced but little the survival of genera and species widely distributed; its changes, moreover, in the normal supply of food organisms cannot be looked upon as noteworthy. Aquatic life has built few of the direct barriers to survival, within which the terrestrial forms appear to have been evolved by the keenest competition.

"It is not, accordingly, remarkable that in their descent fishes are known to have retained their tribal features, and to have varied from each other only in details of structure. Their[Pg 224] evolution is to be traced in diverging characters that prove rarely more than of family value; one form, as an example, may have become adapted for an active and predatory life, evolving stronger organs of progression, stouter armoring, and more trenchant teeth; another, closely akin in general structures, may have acquired more sluggish habits, largely or greatly diminished size, and degenerate characters in its dermal investiture, teeth and organs of sense or progression. The flowering out of a series of fish families seems to have characterized every geological age, leaving its clearest imprint on the forms which were then most abundant. The variety that to-day maintains among the families of bony fishes is thus known to be paralleled among the carboniferous sharks, the Mesozoic Chimæroids, and the Palæozoic lung-fishes and Teleostomes. Their environment has retained their general characters, while modelling them anew into forms armored or scaleless, predatory or defenseless, great, small, heavy, stout, sluggish, light, slender, blunt, tapering, depressed.

"When members of any group of fishes became extinct, those appear to have been the first to perish which were the possessors of the greatest number of widely modified or specialized structures. Those, for example, whose teeth were adapted for a particular kind of food, or whose motions were hampered by ponderous size or weighty armoring, were the first to perish in the struggle for existence; on the other hand, the forms that most nearly retained the ancestral or tribal characters—that is, those whose structures were in every way least extreme—were naturally the best fitted to survive. Thus generalized fishes should be considered those of medium size, medium defenses, medium powers of progression, omnivorous feeding habits, and wide distribution, and these might be regarded as having provided the staples of survival in every branch of descent.

"Aquatic living has not demanded wide divergence from the ancestral stem, and the divergent forms which may culminate in a profusion of families, genera, and species do not appear to be again productive of more generalized groups. In all lines of descent specialized forms do not appear to regain by regression or degeneration the potential characters of their ancestral condition. A generalized form is like potter's clay, plastic in the[Pg 225] hands of nature, readily to be converted into a needed kind of cup or vase; but when thus specialized may never resume unaltered its ancestral condition: the clay survives; the cup perishes." (Dean.)

[Pg 226]

CHAPTER XIII
THE COLORS OF FISHES

Pigmentation.—The colors of fishes are in general produced by oil sacs or pigment cells beneath the epidermis or in some cases beneath the scales. Certain metallic shades, silvery blue or iridescent, are produced, not by actual pigment, but, as among insects, by the deflection of light from the polished skin or the striated surfaces of the scales. Certain fine striations give an iridescent appearance through the interference of light.

The pigmentary colors may be divided into two general classes, ground coloration and ornamentation or markings. Of these the ground color is most subject to individual or local variation, although usually within narrow limits, while the markings are more subject to change with age or sex. On the other hand, they are more distinctive of the species itself.

Protective Coloration.—The ground coloration most usual among fishes is protective in its nature. In a majority of fishes the back is olivaceous or gray, either plain or mottled, and the belly white. To birds looking down into the water, the back is colored like the water itself or like the bottom below it. To fishes in search of prey from below, the belly is colored like the surface of the water or the atmosphere above it. In any case the darker colored upper surface casts its shadow over the paler lower parts.

In shallow waters or in rivers the bottom is not uniformly colored. The fish, especially if it be one which swims close to the bottom, is better protected if the olivaceous surface is marked by darker cross streaks and blotches. These give the fish a color resemblance to the weeds about it or to the sand and stones on which it lies. As a rule, no fish which lies on the bottom is ever quite uniformly colored.

[Pg 227]

Fig. 167.—Garibaldi (scarlet in color), Hypsypops rubicunda (Girard). La Jolla, San Diego, California.

In the open seas, where the water seems very blue, blue colors, and especially metallic shades, take the place of olivaceous gray or green. As we descend into deep water, especially in the warm seas, red pigment takes the place of olive. At a moderate depth a large percentage of the fishes are of various shades of red. Several of the large groupers of the West Indies are represented by two color forms, a shore form in which the prevailing shade is olive-green, and a deeper-water form which is crimson. In several cases an intermediate-color form also exists which is lemon-yellow. On the coast of California is a band-shaped blenny (Apodichthys flavidus) which appears in three colors, according to its surroundings, blood-red, grass-green, and olive-yellow. The red coloration is also essentially protective, for the region inhabited by such forms is the zone of the rose-red algæ. In the arctic waters, and in lakes where rose-red algæ are not found, the red-ground coloration is almost unknown, although red may appear in markings or in nuptial colors. It is possible that the red, both of fishes and algæ, in deeper water is related to the effect of water on the waves of light, but whether this should make fishes red or violet has never been clearly under[Pg 228]stood. It is true also that where the red in fishes ceases violet-black begins.

In the greater depths, from 500 to 4000 fathoms, the ground color in most fishes becomes deep black or violet-black, sometimes with silvery luster reflected from the scales, but more usually dull and lusterless. This shade may be also protective. In these depths the sun's rays scarcely penetrate, and the fish and the water are of the same apparent shade, for black coloration is here the mere absence of light.

In general, the markings of various sorts grow less distinct with the increase of depth. Bright-red fishes of the depths are usually uniform red. The violet-black fishes of the oceanic abysses show no markings whatever (luminous glands excepted), and in deep waters there are no nuptial or sexual differences in color.

Ground colors other than olive-green, gray, brown, or silvery rarely appear among fresh-water fishes. Marine fishes in the tropics sometimes show as ground color bright blue, grass-green, crimson, orange-yellow, or black; but these showy colors are almost confined to fishes of the coral reefs, where they are often associated with elaborate systems of markings.

Protective Markings.—The markings of fishes are of almost every conceivable character. They may be roughly grouped as protective coloration, sexual coloration, nuptial coloration, recognition colors, and ornamentation, if we may use the latter term for brilliant hues which serve no obvious purpose to the fish itself.

Examples of protective markings may be seen everywhere. The flounder which lies on the sand has its upper surface covered with sand-like blotches, and these again will vary according to the kind of sand it imitates. It may be true sand or crushed coral or the detritus of lava, in any case perfectly imitated.

Equally closely will the markings on a fish correspond with rock surroundings. With granite rocks we find an elaborate series of granitic markings, with coral rocks another series of shades, and if red corals be present, red shades of like appearance are found on the fish. Still another kind of mark indicates rock pools lined with the red calcareous algæ called corallina. Black species are found in lava masses, grass-green ones[Pg 229] among the fronds of ulva, and olive-green among Sargassum or fucus, the markings and often the form corresponding to the nature of the algæ in which the species makes its home.

Fig. 168.—Gofu, or Poison Fish, Synanceia verrucosa (Linnæus). Family Scorpænidæ. Specimen from Apia, Samoa, showing resemblance to coral masses, in the clefts of which it lives.

[Pg 230]

Sexual Coloration.—In many groups of fishes the sexes are differently colored. In some cases bright-red, blue, or black markings characterize the male, the female having similar marks, but less distinct, and the bright colors replaced by olive, brown, or gray. In a few cases, however, the female has marks of a totally different nature, and scarcely less bright than those of the male.

Fig. 169.—Lizard-skipper, Alticus saliens (Forster). A blenny which lies out of water on lava-rocks, leaping from one to another with great agility. From nature; specimen from Point Distress, Tutuila Island, Samoa. (About one-half size.)

Nuptial Coloration.—Nuptial colors are those which appear on the male in the breeding season only, the pigment afterwards vanishing, leaving the sexes essentially alike. Such colors are found on most of the minnows and dace (Cyprinidæ) of the rivers and to a less degree in some other fresh-water fishes, as the darters (Etheostominæ) and the trout. In the[Pg 231] minnows of many species the male in spring has the skin charged with bright pigment, red, black, or bright silvery, for the most part, the black most often on the head, the red on the head and body, and the silvery on the tips of the fins. At the same time other markings are intensified, and in many species the head and sometimes the body and fins are covered with warty excrescences. These shades are most distinct on the most vigorous males, and disappear with the warty excrescences after the fertilization of the eggs.

Fig. 170.—Blue-breasted Darter, Etheostoma camurum (Cope), the most brilliantly colored of American river-fishes. Cumberland Gap, Tennessee.

Nuptial colors do not often appear among marine fishes, and in but few families are the sexes distinguishable by differences in coloration.

Recognition-marks.—Under the head of "recognition-marks" may be grouped a great variety of special markings, which may be conceived to aid the representatives of a given species to recognize each other. That they actually serve this purpose is a matter of theory, but the theory is plausible, and these markings have much in common with the white tail feathers, scarlet crests, colored wing patches, and other markings regarded as recognition-marks among birds.

Among these are ocelli, black- or blue-ringed with white or yellow, on various parts of the body; black spots on the dorsal fin; black spots below or behind the eye; black, red, blue, or yellow spots variously placed; cross-bars of red or black or green, with or without pale edges; a blood-red fin or a fin of shining blue among pale ones; a white edge to the tail; a yellow, blue, or red streamer to the dorsal fin, a black tip to the pectoral[Pg 232] or ventral; a hidden spot of emerald in the mouth or in the axil; an almost endless variety of sharply defined markings, not directly protective, which serve as recognition-marks, if not to the fish itself, certainly to the naturalist who studies it.

These marks shade off into an equally great variety for which we can devise no better name than "ornamentation." Some fishes are simply covered with brilliant spots or bars or reticulations, their nature and variety baffling description, while no useful purpose seems to be served by them, unless we stretch still more widely the convenient theory of recognition-marks.

In many cases the markings change with age, certain bands, stripes, or ocelli being characteristic of the young and gradually disappearing. In such cases the same marks will be found permanent in some related species of less differentiated coloration. In such cases it is safe to regard them as ancestral.

In case of markings on the fins and of elaborate ornamentation in general, it is best defined in the oldest and most vigorous individuals, becoming intensified by degrees. The most brilliantly colored fishes are found about the coral reefs. Here may be found species of which the ground color is the most intense blue, others are crimson, grass-green, lemon-yellow, jet-black, and each with a great variety of contrasted markings. The frontispiece of this volume shows a series of such fishes drawn from nature from specimens taken in pools of the great coral reef of Apia in Samoa. These colors are not protective. The coral masses are mostly plain gray, and the fishes which lie on the bottom are plain gray also. Nothing could be more brilliant or varied than the hues of the free-swimming fishes. What their cause or purpose may be, it is impossible to say. It is certain that their intense activity and the ease with which they can seek shelter in the coral masses enable them to defy their enemies. Nature seems to riot in bright colors where her creatures are not destroyed by their presence.

Intensity of Coloration.—In general, coloration is most intense and varied in certain families of the tropical shores, and especially about coral reefs. But in brilliancy of individual markings some fresh-water fishes are scarcely less notable, especially the darters (Etheostominæ) and sunfishes (Centrarchidæ) of the streams of eastern North America. The bright[Pg 233] hues of these fresh-water fishes are, however, more or less concealed in the water by the olivaceous markings and dark blotches of the upper parts.

Fig. 171.—Snake-eels, Liuranus semicinctus (Lay and Bennett), and Chlevastes colubrinus (Boddaert), from Riu Kiu Islands, Japan.

[Pg 234]

Fig. 172.—Coral Reef at Apia.

[Pg 235]

Coral-reef Fishes.—The brilliantly colored fishes of the tropical reefs seem, as already stated, to have no need of protective coloration. They save themselves from their enemies in most cases by excessive alertness and activity (Chætodon, Pomacentrus), or else by busying themselves in coral sand (Julis gaimard), a habit more frequent than has been suspected. Every large mass of branching coral is full of lurking fishes, some of them often most brilliantly colored.

Fading of Pigments in Spirits.—In the preservation of specimens most red and blue pigments fade to whitish, and it requires considerable care to interpret the traces which may be left of red bands or blue markings. Yet some blue pigments are absolutely permanent, and occasionally blood-red pigments persist through all conditions. Black pigment seldom changes in spirits, and olivaceous markings simply fade a little without material alteration. It is an important part of the work of the systematic ichthyologist to learn to interpret the traces of the faded pigment left on specimens he may have occasion to examine. In such cases it is more important to trace the markings than to restore the ground color, as the ground color is at once more variable with individuals and more constant in large groups.

Variation in Pattern.—Occasionally, however, a species is found in which, other characters being constant, both ground color and markings are subject to a remarkable range of variation. In such cases the actual unity of the species is open to serious question. The most remarkable case of such variation known is found in a West Indian fish, the vaca, which bears the incongruous name of Hypoplectrus unicolor. In the typical vaca the body is orange with black marks and blue lines, the fins checkered with orange and blue. In a second form the body is violet, barred with black, the head with blue spots and bands. In another form the blue on the head is wanting. In still another the body is yellow and black, with blue on the head only. In others the fins are plain orange, without checks, and the body yellow, with or without blue stripes and spots, and[Pg 236] sometimes with spots of black or violet. In still others the body may be pink or brown, or violet-black, the fins all yellow, part black or all black. Finally, there are forms deep indigo-blue in color everywhere, with cross bands of indigo-black, and these again may have bars of deeper blue on the head or may lack these altogether. I find, no difference among these fishes except in color, and no way of accounting for the differences in this regard.

Certain species of puffer (Tetraodon setosus, of Panama, and Tetraodon nigropunctatus, of Polynesia) show similar remarkable variations, being dark gray with white spots, but varying to indigo-blue, lemon-yellow, or sometimes having coarse blotches of either. Lemon-yellow varieties of several species are known, and these may be due to a failure of pigment, a sort of semi-albinism. True albinos, individuals wholly without pigment, are rare among fishes. In some cases the markings, commonly black, will be replaced by a deep crimson which does not fade in alcohol. This change happens most frequently among the Scorpænidæ. An example of this is shown in the frontispiece of Volume II of this work. The Japanese okose or poison-fish (Inimicus) is black and gray about lava-rocks. In deeper water among red algæ it is bright crimson, the color not fading in spirits, the markings remaining the same. In still deeper water it is lemon-yellow.

[Pg 237]

CHAPTER XIV
THE GEOGRAPHICAL DISTRIBUTION OF FISHES

Zoogeography.—Under the head of distribution we consider the facts of the actual location of species of organisms on the surface of the earth and the laws by which their location is governed. This constitutes the subject-matter of the science of zoogeography. In physical geography we may prepare maps of the earth or of any part of it, these bringing to prominence the physical features of its surface. Such maps show here a sea, there a plateau, here a mountain chain, there a desert, a prairie, a peninsula, or an island. In political geography the maps show their physical features of the earth as related to the people who inhabit them and the states or powers which receive or claim their allegiance. In zoogeography the realms of the earth are considered in relation to the species or tribes of animals which inhabit them. Thus series of maps could be drawn representing those parts of North America in which catfishes or trout or sunfishes are found in the streams. In like manner the distribution of any particular fish as the muskallonge or the yellow perch could be shown on the map. The details of such a map are very instructive, and their consideration at once raises a series of questions as to the cause behind each fact. In science it must be supposed that no fact is arbitrary or meaningless. In the case of fishes the details of the method of diffusion of species afford matters of deep interest. These are considered in a subsequent chapter.

The dispersion of animals may be described as a matter of space and time, the movement being continuous but modified by barriers and other conditions of environment. The tendency of recent studies in zoogeography has been to consider[Pg 238] the facts of present distribution as the result of conditions in the past, thus correlating our present knowledge with the past relations of land and water as shown through paleontology. Dr. A. E. Ortmann well observes that "Any division of the earth's surface into zoogeographical regions which starts exclusively from the present distribution of animals without considering its origin must always be unsatisfactory." We must therefore consider the coast-lines and barriers of Tertiary and earlier times as well as those of to-day to understand the present distribution of fishes.

General Laws of Distribution.—The general laws governing the distribution of all animals are reducible to three very simple propositions.

Each species of animal is found in every part of the earth having conditions suitable for its maintenance, unless

(a) Its individuals have been unable to reach this region through barriers of some sort; or,

(b) Having reached it, the species is unable to maintain itself, through lack of capacity for adaptation, through severity of competition with other forms, or through destructive conditions of environment; or else,

(c) Having entered and maintained itself, it has become so altered in the process of adaptation as to become a species distinct from the original type.

Species Absent through Barriers.—The absence from the Japanese fauna of most European or American species comes under the first head. The pike has never reached the Japanese lakes, though the shade of the-lotus leaf in the many clear ponds would suit its habits exactly. The grunt[22] and porgies[23] of our West Indian waters have failed to cross the ocean and therefore have no descendants in Europe or Asia.

Species Absent through Failure to Maintain Foothold.—Of species under (b), those who have crossed the seas and not found lodgement, we have, in the nature of things, no record. Of the existence of multitudes of estrays we have abundant evidence. In the Gulf Stream off Cape Cod are every year taken many young fishes belonging to species at home in the Bahamas and which find no permanent place in the New England fauna. In [Pg 239]like fashion, young fishes from the tropics drift northward in the Kuro Shiwo to the coasts of Japan, but never finding a permanent breeding-place and never joining the ranks of the Japanese fishes. But to this there have been, and will be, occasional exceptions. Now and then one among thousands finds permanent lodgement, and by such means a species from another region will be added to the fauna. The rest disappear and leave no trace. A knowledge of these currents and their influence is eventual to any detailed study of the dispersion of fishes.

The occurrence of the young of many shore fishes of the Hawaiian Islands as drifting plankton at a considerable distance from the shores has been lately discovered by Dr. Gilbert. Each island is, in a sense, a "sphere of influence," affecting the fauna of neighboring regions.

Species Changed through Natural Selection.—In the third class, that of species changed in the process of adaptation, most insular forms belong. As a matter of fact, at some time or another almost every species must be in this category, for isolation is a source of the most potent elements in the initiation and intensification of the minor differences which separate related species. It is not the preservation of the most useful features, but of those which actually existed in the ancestral individuals, which distinguish such species. Natural selection must include not only the process of the survival of the fittest, but also the results of the survival of the existing. This means the preservation through heredity of the traits not of the species alone, but those of the actual individuals set apart to be the first in the line of descent in a new environment. In hosts of cases the persistence of characters rests not on any special usefulness or fitness, but on the fact that individuals possessing these characters have, at one time or another, invaded a certain area and populated it. The principle of utility explains survivals among competing structures. It rarely accounts for qualities associated with geographical distribution.

Extinction of Species.—The extinction of species may be noted here in connection with their extension of range. Prof. Herbert Osborn has recognized five different types of elimination.

[Pg 240]

1. That extinction which comes from modification or progressive evolution, a relegation to the past as the result of a transmutation into more advanced forms. 2. Extinction from changes of physical environment which outrun the powers of adaptation. 3. The extinction which results from competition. 4. The extinction from extreme specialization and limitation to special conditions the loss of which means extinction. 5. Extinction as a result of exhaustion. As an illustration of No. 1, we may take almost any species which has a cognate species on the further side of some barrier or in the tertiary seas. Thus the trout of the Twin Lakes in Colorado has acquired its present characters in the place of those brought into the lake by its actual ancestors. No. 2 is illustrated by the disappearance of East Indian types (Zanclus, Platax, Toxotes, etc.) in Italy at the end of the Eocene, perhaps for climatic reasons. Extinction through competition is shown in the gradual disappearance of the Sacramento perch (Archoplitis interruptus) after the invasion of the river by catfish and carp. From extreme specialization certain forms have doubtless disappeared, but no certain case of this kind has been pointed out among fishes, unless this be the cause of the disappearance of the Devonian mailed Ostracophores and Arthrodires. It is not likely that any group of fishes has perished through exhaustion of the stock of vigor.

Barriers Checking Movement of Marine Fishes.—The limits of the distribution of individual species or genera must be found in some sort of barrier, past or present. The chief barriers which limit marine fishes are the presence of land, the presence of great oceans, the differences of temperature arising from differences in latitude, the nature of the sea bottom, and the direction of oceanic currents. That which is a barrier to one species may be an agent in distribution to another. The common shore fishes would perish in deep waters almost as surely as on land, while the open Pacific is a broad highway to the albacore or the swordfish.

Again, that which is a barrier to rapid distribution may become an agent in the slow extension of the range of a species. The great continent of Asia is undoubtedly one of the greatest of barriers to the wide movement of species of fish, yet its long shore-line enables species to creep, as it were, from bay to bay,[Pg 241] or from rock to rock, till, in many cases, the same species is found in the Red Sea and in the tide-pools or sand-reaches of Japan. In the North Pacific, the presence of a range of half-submerged volcanoes, known as the Aleutian and the Kurile Islands, has greatly aided the slow movement of the fishes of the tide-pools and the kelp. To a school of mackerel or of flying-fishes these rough islands with their narrow channels might form an insuperable barrier.

Fig. 173.—Japanese filefish, Rudarius ercodes Jordan and Snyder. Wakanoura, Japan. Family Monacanthidæ.

Temperature the Central Fact in Distribution.—It has long been recognized that the matter of temperature is the central fact in all problems of geographical distribution. Few species in any group freely cross the frost-line, and except as borne by oceanic currents, not many extend their range far into waters colder than those in which the species is distinctively at home. Knowing the average temperature of the water in a given region we know in general the types of fishes which must inhabit it. It is the similarity in temperature and physical conditions which chiefly explains the resemblance of the Japanese fauna to that of the Mediterranean or the Antilles. This fact alone[Pg 242] must explain the resemblance of the Arctic and Antarctic faunæ, there being in no case a barrier in the sea that may not some time be crossed. Like forms lodge in like places.

Agency of Ocean Currents.—We may consider again for a moment the movements of the great currents in the Pacific as agencies in the distribution of species.

A great current sets to the eastward, crossing the ocean just south of the equator. It extends past Samoa and passes on nearly to the coast of Mexico, touching the Galapagos Islands, Clipperton Island, and especially the Revillagigedos. This may account for the number of Polynesian species found on these islands, about which they are freely mixed with immigrants from the mainland of Mexico.

From the Revillagigedos[24] the current moves northward and westward, passing the Hawaiian Islands and thence onward to the Ladrones. The absence in Hawaii of most of the characteristic fishes of Polynesia and Micronesia may be in part due to the long detour made by these currents, as the conditions of life in these groups of islands are not very different. Northeast of Hawaii is a great spiral current, moving with the hands of the watch, forming what is called Fleurieu's Whirlpool. This does not reach the coast of California. This fact may help to account for the almost complete distinction in the shore fishes of Hawaii and California.[25]

No other group of islands in the tropics has a fish fauna so isolated as that of Hawaii. The genera are largely the ordinary tropical types. The species are largely peculiar to these islands.

The westward current from Hawaii reaches Luzon and Formosa. It is deflected to the northward and, joining a northward current from Celebes, it forms the Kuro Shiwo or Black Stream of Japan, which strews its tropical species in the rock pools along the Japanese promontories as far as Tokio. Then, turning into the open sea, it passes northward to the Aleutian Islands, across to Sitka. Thence it moves southward as a cold [Pg 243]current, bearing Ochotsk-Alaskan types southward as far as the Santa Barbara Islands, to which region it is accompanied by species of Aleutian origin. A cold return current seems to extend southward in Japan, along the east shore perhaps as far as Matsushima. A similar current in the sea to the west of Japan extends still further to the southward, to Noto, or beyond.

It is, of course, not necessary that the movements of a species in an oceanic current should coincide with the direction of the current. Young fishes, or fresh-water fishes, would be borne along with the water. Those that dwell within floating bodies of seaweed would go whither the waters carry the drifting mass. But free-swimming fishes, as the mackerel or flying-fishes, might as readily choose the reverse direction. To a free-swimming fish the temperature of the water would be the only consideration. It is thus evident that a current which to certain forms would prove a barrier to distribution, to others would be a mere convenience in movement.

In comparing the Japanese fauna with that of Australia, we find some trace of both these conditions. Certain forms are perhaps excluded by cross-currents, while certain others seem to have been influenced only by the warmth of the water. A few Australian types on the coast of Chile seem to have been carried over by the cross-currents of the South Atlantic.

It is fair to say that the part taken by oceanic currents in the distribution of shore fishes is far from completely demonstrated. The evidence that they assist in such distribution is, in brief, as follows:

1. The young of shore fishes often swim at the surface.

2. The young of very many tropical fishes drift northward in the Gulf Stream and the Japanese Kuro Shiwo.

3. The faunal isolation of Hawaii may be correlated with the direction of the oceanic currents.

Centers of Distribution.—We may assume, in regard to any species, that it has had its origin in or near that region in which it is most abundant and characteristic. Such an assumption must involve a very large percentage of error or of doubt, but in considering the mass of species, it may represent essential truth. In the same fashion we may regard a genus as being autochthonous or first developed in the region where it shows[Pg 244] the greatest range or variety of species. Those regions where the greatest number of genera are thus autochthonous may be regarded as centers of distribution. So far as the marine fishes are concerned, the most important of these supposed centers are found in the Pacific Ocean. First of these in importance is the East-Indian Archipelago, with the neighboring shores of India. Next would come the Arctic Pacific and its bounding islands, from Japan to British Columbia. Third in importance in this regard is Australia. Important centers are found in temperate Japan, in California, the Panama region, and in New Zealand, Chili, and Patagonia. The fauna of Polynesia is almost entirely derived from the Indies; and the shore fauna of the Red Sea, the Bay of Bengal, and Madagascar, so far as genera are concerned, seems to be not really separable from the Indian fauna generally.

Fig. 174.—Globefish, Tetraodon setosus Rosa Smith. Clarion Island, Mexico.

I know of but six genera which may be regarded as autochthonous in the Red Sea, and nearly all of these are of doubtful value or of uncertain relation. The many peculiar genera described by Dr. Alcock, from the dredgings of the Investigator in the Bay of Bengal, belong to the bathybial or deep-water series, and will all, doubtless, prove to be forms of wide distribution.

In the Atlantic, the chief center of distribution is the West Indies; the second is the Mediterranean. On the shores to the northward or southward of these regions occasional genera have[Pg 245] found their origin. This is true especially of the New England region, the North Sea, the Gulf of Guinea, and the coast of Argentina. The fish fauna of the North Atlantic is derived mainly from the North Pacific, the differences lying mainly in the relative paucity of the North Atlantic. But in certain groups common to the two regions the migration must have been in the opposite direction, exceptions that prove the rule.

Distribution of Marine Fishes.—The distribution of marine fishes must be indicated in a different way from that of the fresh-water forms. The barriers which limit their range furnish also their means of dispersion. In some cases proximity overbalances the influence of temperature; with most forms questions of temperature are all-important.

Pelagic Fishes.—Before consideration of the coast-lines we may glance at the differences in vertical distribution. Many species, especially those in groups allied to the mackerel family, are pelagic—that is, inhabiting the open sea and ranging widely within limits of temperature. In this series some species are practically cosmopolitan. In other cases the genera are so. Each school or group of individuals has its breeding place, and from the isolation of breeding districts new species may be conceived to arise. The pelagic types have reached a species of equilibrium in distribution. Each type may be found where suitable conditions exist, and the distribution of species throws little light on questions of distribution of shore fishes. Yet among these species are all degrees of localization. The pelagic fishes shade into the shore fishes on the one hand and into the deep-sea fishes on the other.

Bassalian Fishes.—The vast group of bassalian or deep-sea fishes includes those forms which live below the line of adequate light. These too are localized in their distribution, and to a much greater extent than was formerly supposed. Yet as they dwell below the influence of the sun's rays, zones and surface temperatures are nearly alike to them, and the same forms may be found in the Arctic or under the equator. Their differences in distribution are largely vertical, some living at greater depths than others, and they shade off by degrees from bathybial into semi-bathybial, and finally into ordinary pelagic and ordinary shore types. Apparently all of the bassalian fishes[Pg 246] are derived from littoral types, the changes in structure being due to degeneration of the osseous and muscular systems and of structures not needed in deep-sea life.

Fig. 175.—Sting-ray, Dasyatis sabina Le Sueur. Galveston.

The fishes of the great depths are soft in substance, some of them blind, some of them with very large eyes, all black in color, and very many are provided with luminous spots or areas. A large body of species of fishes are semi-bathybial, inhabiting depths of 20 to 100 fathoms, showing many of the characters of shore fishes, but far more widely distributed. Many of the remarkable cases of wide distribution of type belong to this class. In moderate depths red colors are very common, corresponding to the zone of red algæ, and the colors in both cases are perhaps determined from the fact that the red rays of light are the least refrangible.

A certain number of species are both marine and fresh water, inhabiting estuaries and brackish waters, while some more strictly marine ascend the rivers to spawn. In none of these cases can any hard and fast line be drawn, and some groups which are shore fishes in one region will be represented by semi-bathybial or fluviatile forms in another.[26]

[Pg 247]

Littoral Fishes.—The shore fishes are in general the most highly specialized in their respective groups, because exposed to the greatest variety of selecting conditions and of competition. Their distribution in space is more definite than that of the pelagic and bassalian types, and they may be more definitely assigned to geographical areas.

Distribution of Littoral Fishes by Coast-lines.—Their distribution is best indicated, not by realms or areas, but as forming four parallel series corresponding to the four great north and south continental outlines. Each of these series may be represented as beginning at the north in the Arctic fauna, practically identical in each of the four series, actually identical in the two Pacific series. Passing southward, forms are arranged according to temperature. One by one in each series, the Arctic types disappear; subarctic, temperate, and semi-tropical types take their places, giving way in turn to south-temperate and Antarctic forms. The distribution of these is modified by barriers and by currents, yet though genera and species may be different, each isotherm is represented in each series by certain general types of fishes.

Fig. 176.—Green-sided Darter, Diplesion blennioides Rafinesque. Clinch River. Family Percidæ.

Passing southward the two American series, the East Atlantic and the East Pacific, pass on gradually through temperate to Antarctic types. These are analogous to those of the Arctic, and in a few cases they are generally identical. The West Pacific (East Asian) series is not a continuous line on account of the presence of Australia, the East Indies, and Polynesia. The irregularities of these regions make a number of subseries, which break up the simplicity expressed in the idea of four[Pg 248] parallel series. Yet the fauna of Polynesia is strictly East Indian, modified by the omission or alteration of species, and that of Australia is Indian at the north, and changes to the southward much as that of Africa does. In its marine fishes, it does not constitute a distinct "realm." The East Atlantic (Europe-African) series follows the same general lines of change as that of the West Atlantic. It extends, however, only to the South Temperate Zone, developing no Antarctic elements. The relative shortness of Africa explains in large degree, as already shown, the similarity between the tropical elements in the two Old-World series, as the similarity in tropical elements in the two American series must be due to a former depression of the connecting Isthmus. The practical unity of the Arctic marine fauna needs no explanation in view of the present shore lines of the Arctic Ocean.

Minor Faunal Areas.—The minor faunal areas of shore fishes may be grouped as follows:

East Atlantic.

• Icelandic,
• British,
• Mediterranean,
• Guinean,
• Cape.

West Atlantic.

• Greenlandic,
• New England,
• Virginian,
• Austroriparian,
• Floridian,
• Antillæan,
• Caribbean,
• Brazilian,
• Argentinan,
• Patagonian.

East Pacific.

• Arctic,
• Aleutian,
• Sitkan,
• Californian,
• San Diegan,
• Sinaloan,
• Panamanian,
• Peruvian,
• Revillagigedan,
• Galapagan,
• Chilian,
• Patagonian.

West Pacific.

• Arctic,
• Aleutian,
• Kurile,
• Hokkaido,
• Nippon,
• Chinese,
• East Indian,
• Polynesian,
• Hawaiian,
• Indian,
• Arabian,
• Madagascarian,
• Cape,
• North Australian,
• Tasmanian,
• New Zealand,
• Antarctic.

Equatorial Fishes Most Specialized.—In general, the different types are most highly specialized in equatorial waters. The processes of specific change, through natural selection or[Pg 249] other causes, if other causes exist, take place most rapidly there and produce most far-reaching modification. As elsewhere stated, the coral reefs of the tropics are the centers of fish-life, the cities in fish economy. The fresh waters, the arctic waters, the deep sea and the open sea represent forms of ichthyic backwoods, regions where change goes on more slowly, and in them we find survivals of archaic or generalized types. For this reason the study in detail of the distribution of marine fishes of equatorial regions is in the highest degree instructive.

Realms of Distribution of Fresh-water Fishes.—If we consider the fresh-water fishes alone we may divide the land areas of the earth into districts and zones not differing fundamentally with those marked out for mammals and birds. The river basin, bounded by its shores and the sea at its mouth, shows many resemblances, from the point of view of a fish, to an island considered as the home of an animal. It is evident that with fishes the differences in latitude outweigh those of continental areas, and a primary division into Old World and New World would not be tenable.

The chief areas of distribution of fresh-water fishes we may indicate as follows, following essentially the grouping proposed by Dr. Günther:[27]

Northern Zone.—With Dr. Günther we may recognize first the Northern Zone, characterized familiarly by the presence of sturgeon, salmon, trout, whitefish, pike, lamprey, stickleback, and other species of which the genera and often the species are identical in Europe, Siberia, Canada, Alaska, and most of the United States, Japan, and China. This is subject to cross-division into two great districts, the first Europe-Asiatic, the second North American. These two agree very closely to the northward, but diverge widely to the southward, developing a variety of specialized genera and species, and both of them passing finally by degrees into the Equatorial Zone.

Still another line of division is made by the Ural Mountains in the Old World and by the Rocky Mountains in the New. In both cases the Eastern region is vastly richer in genera and species, as well as in autochthonous forms, than the Western. The reason for this lies in the vastly greater extent of the river [Pg 250]basins of China and the Eastern United States, as compared with those of Europe or the Californian region.

Fig. 177.—Japanese Sea-horse, Hippocampus mohnikei Bleeker. Misaki, Japan.

[Pg 251]

Minor divisions are those which separate the Great Lake region from the streams tributary to the Gulf of Mexico; and in Asia, those which separate China from tributaries of the Caspian, the Black, and the Mediterranean.

Equatorial Zone.—The Equatorial Zone is roughly indicated by the tropics of Cancer and Capricorn. Its essential feature is that of the temperature, and the peculiarities of its divisions are caused by barriers of sea or mountains.

Dr. Günther finds the best line of separation into two divisions to lie in the presence or absence of the great group of dace or minnows,[28] to which nearly half of the species of fresh-water fishes the world over belong. The entire group, now spread everywhere except in the Arctic, South America, Australia, and the islands of the Pacific, seems to have had its origin in India, from which region its genera have radiated in every direction.

The Cyprinoid division of the Equatorial Zone forms two districts, the Indian and the African. The Acyprinoid division includes South America, south of Mexico, and all the islands of the tropical Pacific lying to the east of Wallace's line. This line, separating Borneo from Celebes and Bali from Lompoe, marks in the Pacific the western limit of Cyprinoid fishes, as well as that of monkeys and other important groups of land animals. This line, recognized as very important in the distribution of land animals, coincides in general with the ocean current between Celebes and Papua, which is one of the sources of the Kuro Shiwo.

In Australia, Hawaii, and Polynesia generally, the fresh-water fishes are derived from marine types by modification of one sort or another. In no case, so far as I know, in any island to the eastward of Borneo, is found any species derived from fresh-water families of either the Eastern or the Western Continent. Of course, minor subdivisions in these districts are formed by the contour lines of river basins. The fishes of the Nile differ from those of the Niger or the Congo, or of the streams of Madagascar
[Pg 252]or Cape Colony, but in all these regions the essential
character of the fish fauna remains the same.

Southern Zone.—The third great region, the Southern Zone, is scantily supplied with fresh-water fishes, and the few it possesses are chiefly derived from modifications of the marine fauna or from the Equatorial Zone to the north. Three districts are recognized—Tasmania, New Zealand, and Patagonia.

Origin of the New Zealand Fauna.—The fact that certain peculiar groups are common to these three regions has attracted the notice of naturalists. In a critical study of the fish fauna of New Zealand,[29] Dr. Gill discusses the origin of the four genera and seven species of fresh-water fishes found in these islands, the principal of these genera (Galaxias) being represented by nearly related species in South Australia, in Patagonia,[30] the Falkland Islands, and in South Africa.

According to Dr. Gill, we can account for this anomaly of distribution only by supposing, on the one hand, that their ancestors were carried for long distances in some unnatural manner, as (a) having been carried across entombed in ice, or (b) being swept by ocean currents, surviving their long stay in salt water, or else that they were derived (c) from some widely distributed marine type now extinct, its descendants restricted to fresh water.

On the other hand, Dr. Gill suggests that as "community of type must be the expression of community of origin," the presence of fishes of long-established fresh-water types must imply continuity or at least contiguity of land. The objections raised by geologists to the supposed land connection of New Zealand and Tasmania do not appear to Dr. Gill insuperable. It is well known, he says, "that the highest mountain chains are of comparatively recent geological age. It remains, then, to consider which is the more probable, (1) that the types now common in distant regions were distributed in some unnatural manner by the means referred to, or (2) that they are descendants of forms once wide-ranging over lands now submerged." After considering questions as to change of type in other groups, Dr. Gill is inclined to postulate, from the occurrence of species of the [Pg 253]trout-like genus Galaxias, in New Zealand, South Australia, and South America, that "there existed some terrestrial passage-way between the several regions at a time as late as the close of the Mesozoic period. The evidence of such a connection afforded by congeneric fishes is fortified by analogous representatives among insects, mollusca, and even amphibians. The separation of the several areas must have occurred little later than the late Tertiary, inasmuch as the salt-water fishes of corresponding isotherms found along the coast of the now widely separated lands are to such a large extent specifically different. In general, change seems to have taken place more rapidly among marine animals than fresh-water representatives of the same class."

In this case, when one guess is set against another, it seems to me that the hypothesis first suggested, rather than the other, lies in the line of least logical resistance. I think it better to adopt provisionally some theory not involving the existence of a South Pacific Antarctic Continent, to account for the distribution of Galaxias. For this view I may give five reasons:

1. There are many other cases of the sort equally remarkable and equally hard to explain. Among these is the presence of species of paddle-fish and shovel-nosed sturgeon,[31] types characteristic of the Mississippi Valley, in Central Asia. The presence of one and only one of the five or six American species of pike[32] in Europe; of one of the three species of mud-minnow in Austria,[33] the others being American. Still another curious case of distribution is that of the large pike-like trout of the genus Hucho, one species (Hucho hucho) inhabiting the Danube, the other (Hucho blackistoni) the rivers of northern Japan. Many such cases occur in different parts of the globe and at present admit of no plausible explanation.

2. The supposed continental extension should show permanent traces in greater similarity in the present fauna, both of rivers and of sea. The other fresh-water genera of the regions in question are different, and the marine fishes are more [Pg 254]different than they could be if we imagine an ancient shore connection. If New Zealand and Patagonia were once united other genera than Galaxias would be left to show it.

3. We know nothing of the power of Galaxias to survive submergence in salt water, if carried in a marine current. As already noticed, I found young and old in abundance of the commonest of Japanese fresh-water fishes in the open sea, at a distance from any river. Thus far, this species, the hakone[34] dace, has not been recorded outside of Japan, but it might well be swept to Korea or China. Two fresh-water fishes of Japanese origin now inhabit the island of Tsushima in the Straits of Korea.

4. The fresh-water fishes of Polynesia show a remarkably wide distribution and are doubtless carried alive in currents. One river-goby[35] ranges from Tahiti to the Riu Kiu Islands. Another species,[36] originally perhaps from Brazil through Mexico, shows an equally broad distribution.

5. We know that Galaxias with its relatives must have been derived from a marine type. It has no affinity with any of the fresh-water families of either continent, unless it be with the Salmonidæ. The original type of this group was marine, and most of the larger species still live in the sea, ascending streams only to spawn.

When the investigations of geologists show reason for believing in radical changes in the forms of continents, we may accept their conclusions. That geological evidence exists which seems to favor the existence of a former continent, Antarctica, is claimed on high authority. If this becomes well established we may well explain the distribution of Galaxias with reference to it. But we cannot, on the other hand, regard the anomalous distribution of Galaxias alone constituting proof of shore connection. There can be no doubt that almost every case of anomalies in the distribution of fishes admits of a possible explanation through "the slow action of existing causes."

Real causes are always simple when they are once known. All anomalies in distribution cease to be such when the facts necessary to understand them are at our disposal.

[Pg 255]

CHAPTER XV.
ISTHMUS BARRIERS SEPARATING FISH FAUNAS

The Isthmus of Suez.—In the study of the effect of the Isthmus of Suez on the distribution of fishes we may first consider the alleged resemblance between the fauna of the Mediterranean and that of Japan. Dr. Günther claims that the actual identity of genera and species in these two regions is such as to necessitate the hypothesis that they have been in recent times joined by a continuous shore-line. This shore-line, according to Prof. A. Ortmann and others, was not across the Isthmus of Suez, but farther to the northward, probably across Siberia.

The Fish Fauna of Japan.—For a better understanding of the problem we may give a brief analysis of the fish fauna of Japan.

The group of islands which constitute the empire of Japan is remarkable for the richness of its animal life. Its variety in climatic and other conditions, its nearness to the great continent of Asia and to the chief center of marine life, the East Indian Islands, its relation to the warm Black Current or Kuro Shiwo from the south and to the cold currents from the north, all tend to give variety and richness to the fauna of its seas. Especially is this true in the group of fishes. In spite of the political isolation of the Japanese Empire, this fact has been long recognized and the characteristic types of Japanese fishes have been well known to naturalists.

At present about 900 species of fishes are known from the four great islands which constitute Japan proper—Hondo, Hokkaido, Kiusiu, and Shikoku. About 200 others are known from the volcanic islands to the north and south. Of these 1100 species, about fifty belong to the fresh waters. These are all closely allied to forms found on the mainland of Asia, from which re[Pg 256]gion all of them were probably derived. In general the same genera appear in China and with a larger range of species.

Fresh-water Faunas of Japan.—Two faunal areas of fresh waters may be fairly distinguished, although broadly overlapping. The northern region includes the island of Hokkaido and the middle and northern part of the great island of Hondo. In a rough way, its southern boundary may be defined by Fuji Yama, and the Bay of Matsushima. It is characterized by the presence of salmon, trout, and sculpins, and northward by sturgeon and brook lampreys. The southern area loses by degrees the trout and other northern fishes, while in its clear waters abound various minnows, gobies, and the famous ayu, or Japanese dwarf salmon, one of the most delicate of food fishes. Sculpins and lampreys give place to minnows, loaches, and chubs. Two genera, a sculpin[37] and a perch,[38] besides certain minnows and catfishes, are confined to this region and seem to have originated in it, but, like the other species, from Chinese stock.

Origin of Japanese Fresh-water Fishes.—The question of the origin of the Japanese river fauna seems very simple. All the types are Asiatic. While most of the Japanese species are distinct, their ancestors must have been estrays from the mainland. To what extent river fishes may be carried from place to place by currents of salt water has never been ascertained. One of the most widely distributed of Japanese river fishes is the large hakone dace or chub.[39] This has been repeatedly taken by us in the sea at a distance from any stream. It would evidently survive a long journey in salt water. An allied species[40] is found in the midway island of Tsushima, between Korea and Japan.

Faunal Areas of Marine Fishes in Japan.—The distribution of the marine fishes of Japan is mainly controlled by the temperature of the waters and the motion of the ocean currents. Five faunal areas may be more or less clearly recognized, and these may receive names indicating their scope—Kurile, Hokkaido, Nippon, Kiusiu, Kuro Shiwo, and Riu Kiu. The first or Kurile district is frankly subarctic, containing species characteristic of the Ochotsk Sea on the one hand, and of Alaska on [Pg 257]the other. The second or Hokkaido[41] district includes this northern island and that part of the shore of the main island of Hondo[42] which lies to the north of Matsushima and Noto. Here the cold northern currents favor the development of a northern fauna. The herring and the salmon occupy here the same economic relation as in Norway, Scotland, Newfoundland, and British Columbia. Sculpins, blennies, rockfish, and flounders abound of the rocky shores and are seen in all the markets.

South of Matsushima Bay and through the Island Sea as far as Kobe, the Nippon fauna is distinctly one of the temperate zone. Most of the types characteristically Japanese belong here, abounding in the sandy bays and about the rocky islands.

About the islands of Kiusiu and Shikoku, the semi-tropical elements increase in number and the Kiusiu fauna is less characteristically Japanese, having much in common with the neighboring shores of China, while some of the species range northward from India and Java. But these faunal districts have no sharp barriers. Northern fishes[43] unquestionably of Alaskan origin range as far south as Nagasaki, while certain semi-tropical[44] types extend their range northward to Hakodate and Volcano Bay. The Inland Sea, which in a sense bounds the southern fauna, serves at the same time as a means of its extension. While each species has a fairly definite northern or southern limit, the boundaries of a faunal district as a whole must be stated in the most general terms.

The well-known boundary called Blackiston's Line, which passes through the Straits of Tsugaru, between the two great islands of Hondo and Hokkaido, marks the northern boundary of monkeys, pheasants, and most tropical and semi-tropical birds and mammals of Japan. But as to the fishes, either marine or fresh water, this line has no significance. The northern fresh-water species probably readily cross it; the southern rarely reach it.

We may define as a fourth faunal area that of the Kuro

[Pg 258]

Shiwo district itself, which is distinctly tropical and contrasts strongly with that of the inshore bays behind it. This warm "Black Current," analogous to our Gulf Stream, has its origin in part from a return current from the east which passes westward through Hawaii, in part from a current which passes between Celebes and New Guinea. It moves northward by way of Luzon and Formosa, touching the east shores of the Japanese islands Kiusiu and Shikoku, to the main island of Hondo, flooding the bays of Kagoshima and Kochi, of Waka, Suruga, and Sagami. The projecting headlands reach out into it and the fauna of their rock-pools is distinctly tropical as far to the northward as Tokio.

Fig. 178.—Sacramento Perch, Archoplites interruptus Girard. Family Centrarchidæ. Sacramento River.

These promontories of Hondo, Waka, Ise, Izu, Misaki, and Awa have essentially the same types of fishes as are found on the reefs of tropical Polynesia. The warmth of the off-shore currents gives the fauna of Misaki its astonishing richness, and the wealth of life is by no means confined to the fishes. Corals, crustaceans, worms, and mollusks show the same generous profusion of species.

A fifth faunal area, closely related to that of the Black Current, is formed by the volcanic and coral reefs of the Riu Kiu Archipelago. This fauna, so far as known, is essentially East Indian, the genera and most of the species being entirely identical with those of the islands about Java and Celebes.

[Pg 259]

Resemblance of the Japanese and Mediterranean Fish Faunas.—It has been noted by Dr. Günther that the fish fauna of Japan bears a marked resemblance to that of the Mediterranean. This likeness is shown in the actual identity of genera and species, and in their relation to each other. This resemblance he proposes to explain by the hypothesis that at some recent period the two regions, Japan and the Mediterranean, have been united by a continuous shore-line. The far-reaching character of this hypothesis demands a careful examination of the data on which it rests.

The resemblance of the two faunal areas, so far as fishes are concerned, may be stated as follows: There are certain genera[45] of shore fishes, tropical or semi-tropical, common to the Mediterranean and Japan, and wanting to California, Panama, and the West Indies, and in most cases to Polynesia also. Besides these, certain others found in deeper water (100 to 200 fathoms) are common to the two areas,[46] and have been rarely taken elsewhere.

Significance of Resemblance.—The significance of these facts can be shown only by a fuller analysis of the fauna in question, and those of other tropical and semi-tropical waters. If the resemblances are merely casual, or if the resemblances are shown by other regions, the hypothesis of shore continuity would be unnecessary or untenable. It is tenable if the resemblances are so great as to be accounted for in no other way.

Of the genera regarded as common, only two[47] or three are represented in the two regions by identical species, and these have a very wide distribution in the warm seas. Of the others, nearly all range to India, to the Cape of Good Hope, to Australia, or to Brazil. They may have ranged farther in the past; they may even range farther at present. Not one is confined to the two districts in question. As equally great resemblances exist between Japan and Australia or Japan and the West Indies, the case is not self-evident without fuller comparison. I shall [Pg 260]therefore undertake a somewhat fuller analysis of the evidence bearing on this and similar problems with a view to the conclusions which may be legitimately drawn from the facts of fish distribution.

Differences between Japanese and Mediterranean Fish Faunas.—We may first, after admitting the alleged resemblances and others, note that differences are equally marked. In each region are a certain number of genera which we may consider as autochthonous. These genera are represented by many species or by many individuals in the region of their supposed origin, but are more scantily developed elsewhere. Such genera in Mediterranean waters are Crenilabrus, Labrus, Spicara, Pagellus, Mullus, Boops, Spondyliosoma, Oblata. None of these occurs in Japan, nor have they any near relatives there. Japanese autochthonous types, as Pseudoblennius, Vellitor, Duymæria, Anoplus, Histiopterus, Monocentrus, Oplegnathus, Plecoglossus, range southward to the Indies or to Australia, but all of them are totally unknown to the Mediterranean. The multifarious genera of Gobies of Japan show very little resemblance to the Mediterranean fishes of this family, while blennies, labroids, scaroids, and scorpænoids are equally diverse in their forms and alliances. To the same extent that likeness in faunas is produced by continuity of means of dispersion is it true that unlikeness is due to breaks in continuity. Such a break in continuity of coast-line, in the present case, is the Isthmus of Suez, and the unlikeness in the faunas is about what we might conceive that such a barrier should produce.

Sources of Faunal Resemblances.—There are two main sources of faunal resemblances: first, the absence of any barriers permitting the actual mingling of the species; second, the likeness of temperature and shore configuration on either side of an imperfect barrier. Absolute barriers do not exist and apparently never have existed in the sea. If the fish faunas of different regions have mingled in recent times, the fact would be shown by the presence of the same species in each region. If the union were of a remote date, the species would be changed, but the genera might remain identical.

In case of close physical resemblances in different regions, as in the East Indies and West Indies, like conditions would favor[Pg 261] the final lodgement of like types, but the resemblance would be general, the genera and species being unlike. Without doubt part of the resemblance between Japan and the Mediterranean is due to similarity of temperature and shores. Is that which remains sufficient to demand the hypothesis of a former shore-line connection?

Effects of Direction of Shore-line.—We may first note that a continuous shore-line produces a mingling of fish faunas only when not interrupted by barriers due to climate. A north and south coast-line, like that of the East Pacific, however unbroken, permits great faunal differences. It is crossed by the different zones of temperature. An east and west shore-line lies in the same temperature. In all cases of the kind which now exist on the earth (the Mediterranean, the Gulf of Mexico, the Caribbean Sea, the shores of India), even species will extend their range as far as the shore-line goes. The obvious reason is because such a shore-line rarely offers any important barrier to distribution, checking dispersion of species. We may, therefore, consider the age and nature of the Isthmus of Suez and the character of the faunas it separates.

Numbers of Genera in Different Faunas.—For our purposes the genera must be rigidly defined, a separate name being used in case of each definable difference in structure. The wide-ranging genera of the earlier systematists were practically cosmopolitan, and their geographical distribution teaches us little. On the other hand, when we come to the study of geological distribution, the broad definition of the genus is the only one usually available. The fossil specimens are always defective. Minor characters may be lost past even the possibility of a guess, and only along broad lines can we achieve the classification of the individual fossil.

Using the modern definition of genus, we find in Japan 483 genera of marine fishes; in the Red Sea, 225; in the Mediterranean, 231. In New Zealand 150 are recorded; in Hawaii, 171; 357 from the West Indies, 187 from the Pacific coast of tropical America, 300 from India, 450 from the East-Indian islands, and 227 from Australia.

Of the 483 genera ascribed to Japan, 156 are common to the Mediterranean also, 188 to the West Indies and Japan, 169 to[Pg 262] the Pacific coast of the United States and Mexico. With Hawaii Japan shares 90 genera, with New Zealand 62; 204 are common to Japan and India, 148 to Japan and the Red Sea, most of these being found in India also. Two hundred genera are common to Japan and Australia.

From this it is evident that Japan and the Mediterranean have much in common, but apparently not more than Japan shares with other tropical regions. Japan naturally shows most likeness to India, and next to this to the Red Sea. Proportionately less is the resemblance to Australia, and the likeness to the Mediterranean seems much the same as that to the West Indies or to the Pacific coast of America.

But, to make these comparisons just and effective, we should consider not the fish fauna as a whole; we should limit our discussion solely to the forms of equatorial origin. From the fauna of Japan we may eliminate all the genera of Alaskan-Aleutian origin, as these could not be found in the other regions under comparison. We should eliminate all pelagic and all deep-sea forms, for the laws which govern the distribution of these are very different from those controlling the shore fishes, and most of the genera have reached a kind of equilibrium over the world.

Significance of Rare Forms.—We may note also, as a source of confusion in our investigation, that numerous forms found in Japan and elsewhere are very rarely taken, and their real distribution is unknown. Some of these will be found to have, in some unexpected quarter, their real center of dispersion. In fact, since these pages were written, I have taken in Hawaii representatives of three[48] genera which I had enumerated as belonging chiefly to Japan and the West Indies. Numerous other genera common to the two regions have since been obtained by Dr. Gilbert. Such species may inhabit oceanic plateaus, and find many halting places in their circuit of the tropical oceans. We have already discovered that Madeira, St. Helena, Ascension, and other volcanic islands constitute such halting places. We shall find many more such, when the deeper shore regions are explored, the region between market-fishing and the deep-sea dredgings of the Challenger and [Pg 263]the Albatross. In some cases, no doubt, these forms are verging on extinction and a former wide distribution has given place to isolated colonies.

The following table shows the contents, so far as genera are concerned, of those equatorial areas in which trustworthy catalogues of species are accessible. It includes only those fishes of stationary habit living in less than 200 fathoms. It goes without saying that considerable latitude must be given to these figures, to allow for errors, omissions, uncertainties, and differences of opinion.

Distribution of Shore Fishes.—

Extension of Indian Fauna.—From the above tables it is evident that the warm-water fauna of Japan, as well as that of Hawaii, is derived from the great body of the fauna of the East Indies and Hindostan; that the fauna of the Red Sea is derived in the same way; that the fauna of the Mediterranean bears no especial resemblance to that of Japan, rather than to other[Pg 266] elements of the East Asiatic fauna in similar conditions of temperature, and no greater than is borne by either to the West Indies; that the faunas of the sides of the Isthmus of Suez have relatively little in common, while those of the two sides of the Isthmus of Panama show large identity of genera, although few species are common to the two sides. Of the 255 genera recorded from the Panama region, 179, or over 70 per cent., are also in the West Indies, while 68, or more than 30 per cent. of the number, are limited to the two regions in question.

The Isthmus of Suez as a Barrier to Distribution.—With the aid of the above table we may examine further the relation of the fauna of Japan to that of the Mediterranean. If a continuity of shore-line once existed, it would involve the obliteration of the Isthmus. With free connection across this isthmus the fauna of the Red Sea must have been once practically the same as that of the Mediterranean. The present differences must be due to later immigrations to one or the other region, or to the extinction of species in one locality or the other, through some kind of unfitness. In neither region is there evidence of extensive immigration from the outside. The present conditions of water and temperature differ a little, but not enough to explain the difference in faunæ. The Red Sea is frankly tropical and its fauna is essentially Indian, much the same, so far as genera are concerned, as that of southern Japan. The Mediterranean is at most not more than semi-tropical and its fishes are characteristically European. Its tropical forms belong rather to Guinea than to the East Indies. With the Red Sea the Mediterranean has very little in common, not so much, for example, as has Hawaii. Forty genera of shore fishes (and only fifty of all fishes) are identical in the two regions, the Mediterranean and the Red Sea. Of those, every one is a genus of wide distribution, found in nearly all warm seas. Of shore fishes, only one genus in seven is common to the two regions. Apparently, therefore, we cannot assume a passage across the Isthmus of Suez within the lifetime of the present genera. Not one of the types alleged to be peculiar to Japan and the Mediterranean is thus far known in the Red Sea. Not one of the characteristically abundant[Pg 267] Mediterranean types[52] crosses the Isthmus of Suez, and the distinctive Red Sea and Indian types[53] are equally wanting in the Mediterranean. The only genera which could have crossed the Isthmus are certain shallow-water or brackish-water forms, sting-rays, torpedoes, sardines, eels, and mullets, widely diffused through the East Indies and found also in the Mediterranean. The former channel, if one ever existed, had, therefore, much the same value in distribution of species as the present Suez Canal.

Geological Evidence of Submergence of the Isthmus of Suez.—Yet, from geological data, there is strong evidence that the Isthmus of Suez was submerged in relatively recent times. The recognized geological maps of the Isthmus show that a broad area of post-Pliocene or Pliocene deposits constitutes the Isthmus and separates the nummulitic hills of Suez from their fellows about thirty miles to the eastward. The northern part of the Isthmus is alluvium from the Nile, and its western part is covered with drifting sands. The Red Sea once extended farther north than now and the Mediterranean farther to the southeast. Assuming the maps to be correct, the Isthmus must have been open water in the late Pliocene or post-Pliocene times.

Admitting this as a fact, the difference in the fish fauna would seem to show that the waters over the submerged area were so shallow that the rock-loving forms did not and could not cross it. Moreover, the region was very likely overspread with silt-bearing fresh waters from the Nile. To such fishes as Chætodon, Holocentrus, Thalassoma of the Red Sea, or to Crenilabrus, Boops, and Zeus of the Mediterranean, such waters would form a barrier as effective as the sand-dunes of to-day.

Conclusions as to the Isthmus of Suez.—We are led, therefore, to these conclusions:

1. There is no evidence derivable from the fishes of the recent submergence of the Isthmus of Suez.

2. If the Isthmus was submerged in Pliocene or post-Pliocene times, the resultant channel was shallow and muddy, so [Pg 268]that ordinary marine fishes or fishes of rock bottoms or of deep waters did not cross it.

3. It formed an open water to brackish-water fishes only.

4. The types common to Japan and the Mediterranean did not enter either region from the other by way of the Red Sea.

5. As most of these are found also in India or Australia or both, their dispersion was probably around the south coast of Africa or by the Cape of Good Hope.

6. In view of the fact that numerous East Indian genera, as Zanclus, Enoplosus, Toxotes, Ephippus, Platax, Teuthis, Acanthurus (Monoceros), Myripristis occur in the Eocene rocks of Tuscany, Syria, and Switzerland, we may well suppose that an open waterway across Africa then existed. Perhaps these forms were destroyed in European waters by a wave of glacial cold, perhaps after the Miocene. As our knowledge of the Miocene fish faunæ of Europe is still imperfect, we cannot locate accurately the period of their disappearance. About half the species found in the Eocene of Italy belong to existing genera, and these genera are almost all now represented in the Indian fauna, and those named above with others are confined to it.

The study of fishes alone furnishes no adequate basis for mapping the continental masses of Tertiary times. The known facts in regard to their distribution agree fairly with the provisional maps lately published by Dr. Ortmann (Bull. Philos. Soc., XLI). In the Eocene map (Fig. 179) the Mediterranean extends to the northward of Arabia, across to the mouth of the Ganges. This extension would account for the tropical, Eocene, and Miocene fish fauna of Southern Europe.

The Cape of Good Hope as a Barrier to Fishes.—The fishes of the Cape of Good Hope are not well enough known for close comparison with those of other regions. Enough is known of the Cape fauna to show its general relation to those of India and Australia. The Cape of Good Hope lies in the South Temperate Zone. It offers no absolutely impassable barrier to the tropical fishes from either side. It bears a closer relation to either the Red Sea or the Mediterranean than they bear to each other. It is, therefore, reasonable to conclude that the transfer of tropical shore fishes of the Old World between the Atlantic and Pacific, in recent times, has taken place mainly[Pg 269] around the southern point of Africa. To pelagic and deep-sea fishes the Cape of Good Hope has offered no barrier whatever. To ordinary fishes it is an obstacle, but not an impassable one. This the fauna itself shows. It has, however, not been passed by many tropical species, and by these only as the result of thousands of years of struggle and point-to-point migration.

Relations of Japan to Mediterranean Explainable by Present Conditions.—We may conclude that the resemblance of the Mediterranean fish fauna to that of Japan or India is no more than might be expected, even had the present contour of the continents been permanent for the period of duration of the present genera and species. An open channel in recent times would have produced much greater resemblances than actually exist.

The Isthmus of Panama as a Barrier to Distribution.—Conditions in some regards parallel with those of the Isthmus of Suez exist in but one other region—the Isthmus of Panama. Here the first observers were very strongly impressed by the resemblance of forms. Nearly half the genera found on the two sides of this isthmus are common to both sides. Taking those of the Pacific shore for first consideration, we find that three-fourths of the genera of the Panama fauna occur in the West Indies as well.

This identity is many times greater than that existing at the Isthmus of Suez. Moreover, while the Cape of Good Hope offers no impassable barrier to distribution, the same is not true of the southern part of South America. The subarctic climate of Cape Horn has doubtless formed a complete check to the movements of tropical fishes for a vast period of geologic time.

Unlikeness of Species on the Shores of the Isthmus of Panama.—But, curiously enough, this marked resemblance is confined chiefly to the genera and does not extend to the species on the two shores.

Of 1400 species of fishes recorded from tropical America north of the Equator, only about 70 are common to the two coasts. The number of shore fishes common is still less. In this 70 are included a certain number of cosmopolitan types which might have reached either shore from the Old World.

[Pg 270]

Fig. 179.—Map of the Continents, Eocene time. (After Ortmann.)

[Pg 271]

A few others invade brackish or fresh waters and may possibly have found their way, in one way or another, across the Isthmus of Nicaragua. Of fishes strictly marine, strictly littoral, and not known from Asia or Polynesia, scarcely any species are left as common to the two sides. This seems to show that no waterway has existed across the Isthmus within the lifetime, whatever that may be, of the existing species. The close resemblance of genera shows apparently with almost equal certainty that such a waterway has existed, and within the period of existence of the groups called genera. How long a species of fish may endure unchanged no one knows, but we know that in this regard great differences must exist in different groups. Assuming that different species crossed the Isthmus of Panama in Miocene times, we should not be surprised to find that a few remain to all appearances unchanged; that a much larger number have become "representative" species, closely related forms retaining relations to the environment to those of the parent form, and, finally, that a few species have been radically altered.

This is exactly what has taken place at the Isthmus of Panama with the marine shore fishes. Curiously enough, the movement of genera seems to have been chiefly from the Atlantic to the Pacific. Certain characteristic genera[54] of the Panama region have not passed over to the Pacific. On the other hand, most of the common genera[55] show a much larger number of species on the Atlantic side. This may be held to show their Atlantic origin.

Of the relatively small number of genera which Panama has received from Polynesia[56] few have crossed the Isthmus to appear in the West Indian fauna.

Views of Earlier Writers on the Fishes of the Isthmus of Panama.—The elements of the problem at Panama may be better understood by a glance at the results of previous investigations.

[Pg 272]

In 1869 Dr. Günther, after enumerating the species examined by him from Panama, reaches the conclusion that nearly one-third of the marine fishes on the two shores of tropical America will be found to be identical. He enumerates 193 such species as found on the two coasts; 59 of these, or 31 per cent. of the total, being actually identical. From this he infers that there must have been, at a comparatively recent date, a depression of the Isthmus and intermingling of the two faunas.[57]

Catalogue of Fishes of Panama.—In an enumeration of the fishes of the Pacific coast in 1885,[58] the present writer showed that Dr. Günther's conclusions were based on inadequate data.

In my list 407 species were recorded from the Pacific coast of tropical America—twice the number enumerated by Dr. Günther. Of these 71 species, or 17½ per cent., were found also in the Atlantic. About 800 species are known from the Caribbean and adjacent shores, so that out of the total number of 1,136 species but 71, or 6 per cent. of the whole, are common to the two coasts. This number does not greatly exceed that of the species common to the West Indies and the Mediterranean, or even the West Indies and Japan. It is to be noted also that the number 71 is not very definitely ascertained, as there must be considerable difference of opinion as to the boundaries of species, and the actual identity in several cases is open to doubt.

This discrepancy arises from the comparatively limited representation of the two faunas at the disposal of Dr. Günther. He enumerates 193 marine or brackish-water species as found on the two coasts, 59 of which are regarded by him as specifically identical, this being 31 per cent. of the whole. But in 30 of these 59 cases I regard the assumption of complete identity as erroneous, so that taking the number 193 as given I would reduce the percentage to 15. But these 193 species form but a fragment of the total fauna, and any conclusion based on such narrow data is certain to be misleading.

Of the 71 identical species admitted in our list, several (e.g., Mola, Thunnus) are pelagic fishes common to most warm seas.

[Pg 273]

Still others (e.g., Trachurus, Carangus, Diodon sp.) are cosmopolitan in the tropical waters. Most of the others (e.g., Gobius, Gerres, Centropomus, Galeichthys sp., etc.) often ascend the rivers of the tropics, and we may account for their diffusion, perhaps, as we account for the dispersion of fresh-water fishes on the Isthmus, on the supposition that they may have crossed from marsh to marsh at some time in the rainy season.

In very few cases are representatives of any species from opposite sides of the Isthmus exactly alike in all respects. These differences in some cases seem worthy of specific value, giving us "representative species" on the two sides. In other cases the distinctions are very trivial, but in most cases they are appreciable, especially in fresh specimens.

Further, I expressed the belief that "fuller investigation will not increase the proportion of common species. If it does not, the two faunas show no greater resemblance than the similarity of physical conditions on the two sides would lead us to expect." This similarity causes the same types of fishes to persist on either side of the Isthmus while through isolation or otherwise these have become different as species.

This conclusion must hold so far as species are concerned, but the resemblance of the genera on the sides has a significance of its own.

In 1880[59] Dr Günther expressed his views in still stronger language, claiming a still larger proportion of the fishes of tropical America to be identical on the two sides of the continent. He concluded that "with scarcely any exceptions the genera are identical, and of the species found on the Pacific side, nearly one-half have proved to be the same as those of the Atlantic. The explanation of this fact has been found in the existence of communications between the two oceans by channels and straits which must have been open till within a recent period. The isthmus of Central America was then partially submerged, and appeared as a chain of islands similar to that of the Antilles; but as the reef-building corals flourished chiefly north and east of these islands and were absent south and west of them, reef fishes were excluded from the Pacific shores when the communications were destroyed by the upheaval of land."

[Pg 274]

Conclusions of Evermann and Jenkins.—This remark led to a further discussion of the subject on the part of Dr. B. W. Evermann and Dr. O. P. Jenkins. From their paper on the fishes of Guaymas[60] I make the following quotations:

"The explorations since 1885 have resulted (1) in an addition of about 100 species to one or other of the two faunas; (2) in showing that at least two species that were regarded as identical on the two shores[61] are probably distinct; and (3) in the addition of but two species to those common to both coasts.[62]

"All this reduces still further the percentage of common species.

"Of the 110 species obtained by us, 24, or less than 21 per cent., appear to be common to both coasts. Of these 24 species, at least 16, from their wide distribution, would need no hypothesis of a former waterway through the Isthmus to account for their presence on both sides. They are species fully able to arrive at the Pacific shores of the Americas from the warm seas west. It thus appears that not more than eight species, less than 8 per cent. of our collection, all of which are marine species, require any such hypothesis to account for their occurrence on both coasts of America. This gives us, then, 1,307 species that should properly be taken into account when considering this question, not more than 72 of which, or 5.5 per cent., seem to be identical on the two coasts. This is very different from the figures given by Dr. Günther in his 'Study of Fishes.'

"Now, if from these 72 species, admitted to be common to both coasts, we subtract the 16 species of wide distribution—so wide as to keep them from being a factor in this problem—we have left but 56 species common to the two coasts that bear very closely upon the waterway hypothesis. This is less than 4.3 per cent. of the whole number.

"But the evidence obtained from a study of other marine life of that region points to the same conclusion.

[Pg 275]

"In 1881, Dr. Paul Fischer discussed the same question in his 'Manual de Conchyliologie,' pp. 168, 169, in a section on the Molluscan Fauna of the Panamic Province, and reached the same general conclusions. He says: 'Les naturalistes Américians se sont beaucoup preéoccupés des espèces de Panama qui paraissent identiques avec celles des Antilles, ou qui sont représentatives. P. Carpenter estime qu'il en existe 35. Dans la plupart des cas, l'identite absolue n'a pu être constantée et on a trouvé quelques caractères distinctifs, ce qui n'a rien d'ètonnant, puisque dans l'hypothèse d'une origine commune, les deux races pacifique et atlantique sont séparée depuis la periode Miocène. Voici un liste de ces espèces représentatives ou identiques.' Here follows a list of 20 species. 'Mais ces formes semblables,' he says, 'constituent un infime minorité (3 per cent.).'

"These facts have a very important bearing upon certain geological questions, particularly upon the one concerning the cold of the Glacial period.

"In Dr. G. Frederick Wright's recent book, 'The Ice Age in North America,' eight different theories as to the cause of the cold are discussed. The particular theory which seems to him quite reasonable is that one which attributes the cold as due to a change of different parts of the country, and a depression of the Isthmus of Panama is one of the important changes he considers. He says: 'Should a portion of the Gulf Stream be driven through a depression across the Isthmus of Panama into the Pacific, and an equal portion be diverted from the Atlantic coast of the United States by an elevation of the sea-bottom between Florida and Cuba, the consequences would necessarily be incalculably great, so that the mere existence of such a possible cause for great changes in the distribution of moisture over the northern hemisphere is sufficient to make one hesitate before committing himself unreservedly to any other theory; at any rate, to one which has not for itself independent and adequate proof.'

"In the appendix to the same volume Mr. Warren Upham, in discussing the probable causes of glaciation, says: 'The quaternary uplifts of the Andes and Rocky Mountains and of the West Indies make it nearly certain that the Isthmus of Panama[Pg 276] has been similarly elevated during the recent epoch.... It may be true, therefore, that the submergence of this isthmus was one of the causes of the Glacial period, the continuation of the equatorial oceanic currents westward into the Pacific having greatly diminished or wholly diverted the Gulf Stream, which carries warmth from the tropics to the northern Atlantic and northwestern Europe.'

Fig. 180.—Caulophryne jordani Goode and Bean, a deep-sea fish of the Gulf Stream. Family Ceratiidæ.

Fig. 181.—Exerpes asper Jenkins and Evermann, a fish of the rock-pools, Guaymas, Mexico. Family Blenniidæ.

"Any very recent means by which the fishes could have passed readily from one side to the other would have resulted in making the fish faunas of the two shores practically identical; but the time that has elapsed since such a waterway could have[Pg 277] existed has been long enough to allow the fishes of the two sides to become practically distinct. That the mollusks of the two shores are almost wholly distinct, as shown by Dr. Fischer, is even stronger evidence of the remoteness of the time when the means of communication between the two oceans could have existed, for 'species' among the mollusks are probably more persistent than among fishes.

"Our present knowledge, therefore, of the fishes of tropical America justifies us in regarding the fish faunas of the two coasts as being essentially distinct, and believing that there has not been, at any comparatively recent time, any waterway through the Isthmus of Panama."

It is thus shown, I think, conclusively, that the Isthmus of Panama could not have been depressed for any great length of time in a recent geological period.

Conclusions of Dr. Hill.—These writers have not, however, considered the question of generic identity. To this we may find a clue in the geological investigations of Dr. Robert T. Hill.

In a study of "The Geological History of the Isthmus of Panama and Portions of Costa Rica," Dr. Hill uses the following language:

"By elimination we have concluded that the only period of time since the Mesozoic within which communication between the seas could have taken place is the Tertiary period, and this must be restricted to the Eocene and Oligocene epochs of that period. The paleontologic evidence upon which such an opening can be surmised at this period is the occurrence of a few California Eocene types in the Atlantic sides of the tropical American barrier, within the ranges of latitude between Galveston (Texas) and Colon, which are similar to others found in California. There are no known structural data upon which to locate the site of this passage, but we must bear in mind, however, that this structure has not been completely explored.

"Even though it was granted that the coincidence of the occurrence of a few identical forms on both sides of the tropical American region, out of the thousands which are not common, indicates a connection between the two seas, there is still an absence of any reason for placing this connection at the Isthmus of Panama, and we could just as well maintain that the[Pg 278] locus thereof might have been at some other point in the Central American region.

"The reported fossil and living species common to both oceans are littoral forms, which indicate that if a passage existed it must have been of a shallow and ephemeral character.

"There is no evidence from either a geologic or a biologic standpoint for believing that the oceans have ever communicated across the Isthmian regions since Tertiary time. In other words, there is no evidence for these later passages which have been established upon hypothetical data, especially those of Pleistocene time.

"The numerous assertions, so frequently found in literature, that the two oceans have been frequently and recently connected across the Isthmus, and that the low passes indicative of this connection still exist, may be dismissed at once and forever and relegated to the domain of the apocryphal. A few species common to the waters of both oceans in a predominantly Caribbean fauna of the age of the Claiborne epoch of the Eocene Tertiary is the only paleontologic evidence in any time upon which such a connection may be hypothesized.

"There has been a tendency in literature to underestimate the true altitude of the isthmian passes, which, while probably not intentional, has given encouragement to those who think that this Pleistocene passage may have existed. Maack has erroneously given the pass at 186 feet. Dr. J. W. Gregory states 'that the summit of the Isthmus at one locality is 154 feet and in another 287 feet in height.' The lowest isthmian pass, which is not a summit, but a drainage col, is 287-295 feet above the ocean.

"If we could lower the isthmian region 300 feet at present, the waters of the two oceans would certainly commingle through the narrow Culebra Pass. But the Culebra Pass is clearly the headwater col of two streams, the Obispo flowing into the Chagres, and the Rio Grande flowing into the Pacific, and has been cut by fluviatile action, and not by marine erosion, out of a land mass which has existed since Miocene time. Those who attempt to establish Pleistocene interoceanic channels through this pass on account of its present low altitude must not omit from their calculations the restoration of former rock[Pg 279] masses which have been removed by the general levelling of the surface by erosion."

Fig. 182.—Xenocys jessiæ Jordan and Bollman. Galapagos Islands. Family Lutianidæ.

In conclusion, Dr. Hill asserts that "there is considerable evidence that a land barrier in the tropical region separated the two oceans as far back in geologic history as Jurassic time, and that that barrier continued throughout the Cretaceous period. The geological structure of the Isthmus and Central American regions, so far as investigated, when considered aside from the paleontology, presents no evidence by which the former existence of a free communication of oceanic waters across the present tropical land barriers can be established. The paleontologic evidence indicates the ephemeral existence of a passage at the close of the Eocene period. All lines of inquiry—geologic, paleontologic, and biologic—give evidence that no connection has existed between the two oceans since the close of the Oligocene. This structural geology is decidedly opposed to any hypothesis by which the waters of the two oceans could have been connected across the regions in Miocene, Pliocene, Pleistocene, or recent times."

Final Hypothesis as to Panama.—If we assume the correctness of Dr. Hill's conclusions, they may accord in a remarkable degree with the actual facts of the distribution of the fishes about the Isthmus. To account for the remarkable identity of genera and divergence of species I may suggest the following hypothesis:

[Pg 280]

During the lifetime of most of the present species, the Isthmus has not been depressed. It was depressed in or before Miocene time, during the lifetime of most of the present genera. We learn from other sources that few of the extant species of fishes are older than the Pliocene. Relatively few genera go back to the Eocene, and most of the modern families appear to begin in the Eocene or later Cretaceous. In general the Miocene may be taken as the date of the origin of modern genera. The channel formed across the Isthmus was relatively shallow, excluding forms inhabiting rocky bottoms at considerable depths. It was wide enough to permit the infiltration from the Caribbean Sea of numerous species, especially of shore fishes of sandy bays, tide pools, and brackish estuaries. The currents set chiefly to the westward, favoring the transfer of Atlantic rather than Pacific types.

Fig. 183.—Channel Catfish, Ictalurus punctatus (Rafinesque). Illinois River. Family Siluridæ.

Since the date of the closing of this channel the species left on the two sides have been altered in varying degrees by the processes of natural selection and isolation. The cases of actual specific identity are few, and the date of the establishment as species, of the existing forms, is subsequent to the date of the last depression of the Isthmus.

We may be certain that none of the common genera ever found their way around Cape Horn. Most of them disappear to the southward, along the coasts of Brazil and Peru.

While local oscillations, involving changes in coast-lines, have doubtless frequently taken place and are still going on, the past and present distribution of fishes does not alone give adequate data for their investigation.

[Pg 281]

Further, it goes without saying that we have no knowledge of the period of time necessary to work specific changes in a body of species isolated in an alien sea. Nor have we any data as to the effect on a given fish fauna of the infiltration of many species and genera belonging to another. All such forces and results must be matters of inference.

The present writer does not wish to deny that great changes have taken place in the outlines of continents in relatively recent times. He would, however, insist that the theory of such changes must be confirmed by geological evidence, and evidence from groups other than fishes, and that likeness in separated fish faunas may not be conclusive.

Fig. 184.—Drawing the net on the beach of Hilo, Hawaii. Photograph by Henry W. Henshaw.

[Pg 282]

CHAPTER XVI
DISPERSION OF FRESH-WATER FISHES[63]

Dispersion of Fishes.—The methods of dispersion of fishes may be considered apart from the broader topic of distribution or the final results of such dispersion. In this discussion we are mainly concerned with the fresh-water fishes, as the methods of distribution of marine fishes through marine currents and by continuity of shore and water ways are all relatively simple.

The Problem of Oatka Creek.—When I was a boy and went fishing in the brooks of western New York, I noticed that the different streams did not always have the same kinds of fishes in them. Two streams in particular in Wyoming County, not far from my father's farm, engaged in this respect my special attention. Their sources are not far apart, and they flow in opposite directions, on opposite sides of a low ridge—an old glacial moraine, something more than a mile across. The Oatka Creek flows northward from this ridge, while the East Coy runs toward the southeast on the other side of it, both flowing ultimately into the same river, the Genesee.

It does not require a very careful observer to see that in these two streams the fishes are not quite the same. The streams themselves are similar enough. In each the waters are clear and fed by springs. Each flows over gravel and clay, through alluvial meadows, in many windings, and with elms and alders "in all its elbows." In both streams we were sure of finding trout,[64] and in one of them the trout are still abundant. In both we used to catch the brook chub,[65] or, as we [Pg 283]called it, the "horned dace"; and in both were large schools of shiners[66] and of suckers.[67] But in every deep hole, and especially in the millponds along the East Coy Creek, the horned pout[68] swarmed on the mucky bottoms. In every eddy, or in the deep hole worn out at the root of the elm-trees, could be seen the sunfish,[69] strutting in green and scarlet, with spread fins keeping intruders away from its nest. But in the Oatka Creek were found neither horned pout nor sunfish, nor have I ever heard that either has been taken there. Then besides these nobler fishes, worthy of a place on every schoolboy's string, we knew by sight, if not by name, numerous smaller fishes, darters[70] and minnows,[71] which crept about in the gravel on the bottom of the East Coy, but which we never recognized in the Oatka.

There must be a reason for differences like these, in the streams themselves or in the nature of the fishes. The sunfish and the horned pout are home-loving fishes to a greater extent than the others which I have mentioned; still, where no obstacles prevent, they are sure to move about. There must be, then, in the Oatka some sort of barrier, or strainer, which keeping these species back permits others more adventurous to pass; and a wider knowledge of the geography of the region showed that such is the case. Farther down in its course, the Oatka falls over a ledge of rock, forming a considerable waterfall at Rock Glen. Still lower down its waters disappear in the ground, sinking into some limestone cavern or gravel-bed, from which they reappear, after some six miles, in the large springs at Caledonia. Either of these barriers might well discourage a quiet-loving fish; while the trout and its active associates have some time passed them, else we should not find them in the upper waters in which they alone form the fish fauna. This problem is a simple one; a boy could work it out, and the obvious solution seems to be satisfactory.

[Pg 284]

Generalizations as to Dispersion.—Since those days I have been a fisherman in many waters,—not an angler exactly, but one who fishes for fish, and to whose net nothing large or small ever comes amiss; and wherever I go I find cases like this.

We do not know all the fishes of America yet, nor all those well that we know by sight; still this knowledge will come with time and patience, and to procure it is a comparatively easy task. It is also easy to ascertain the more common inhabitants of any given stream. It is difficult, however, to obtain negative results which are really results. You cannot often say that a species does not live in a certain stream. You can only affirm that you have not yet found it there, and you can rarely fish in any stream so long that you can find nothing that you have not taken before. Still more difficult is it to gather the results of scattered observations into general statements regarding the distribution of fishes. The facts may be so few as to be misleading, or so numerous as to be confusing, and the few writers who have taken up this subject in detail have found both these difficulties to be serious. Whatever general propositions we may maintain must be stated with the modifying clause of "other things being equal"; and other things are never quite equal. The saying that "Nature abhors a generalization" is especially applicable to all discussions of the relations of species to environment.

Still less satisfactory is our attempt to investigate the causes on which our partial generalizations depend,—to attempt to break to pieces the "other things being equal" which baffle us in our search for general laws. The same problems, of course, come up on each of the other continents and in all groups of animals or plants; but most that I shall say will be confined to the question of the dispersion of fishes in the fresh waters of North America. The broader questions of the boundaries of faunæ and of faunal areas I shall bring up only incidentally.

Questions Raised by Agassiz.—Some of the problems to be solved were first noticed by Prof. Agassiz in 1850, in his work on Lake Superior. Later (1854), in a paper on the fishes of the Tennessee River,[72] he makes the following statement:

[Pg 285]

"The study of these features [of distribution] is of the greatest importance, inasmuch as it may eventually lead to a better understanding of the intentions implied in this seemingly arbitrary disposition of animal life....

"There is still another very interesting problem respecting the geographical distribution of our fresh-water animals which may be solved by the further investigation of the fishes of the Tennessee River. The water-course, taking the Powell, Clinch, and Holston Rivers as its head waters, arises from the mountains of Virginia in latitude 37°; it then flows S.W. to latitude 34° 25', when it turns W. and N.W., and finally empties into the Ohio, under the same latitude as its source in 37°.

Fig. 185.—Horned Dace, Semotilus atromaculatus (Mitchill). Aux Plaines River, Ills. Family Cyprinidæ.

"The question now is this: Are the fishes of this water system the same throughout its extent? In which case we should infer that water communication is the chief condition of geographical distribution of our fresh-water fishes. Or do they differ in different stations along its course? And if so, are the differences mainly controlled by the elevation of the river above the level of the sea, or determined by climatic differences corresponding to differences of latitude? We should assume that the first alternative was true if the fishes of the upper course of the river differed from those of the middle and lower courses in the same manner as in the Danube, from its source to Pesth, where this stream flows nearly for its whole length under the same parallel. We would, on the contrary, suppose the second alternative to be well founded if marked differences were observed between the fish of such tracts of the river as do not[Pg 286] materially differ in their evolution above the sea, but flow under different latitudes. Now, a few collections from different stations along this river, like that sent me by Dr. Newman from the vicinity of Huntsville, would settle at once this question, not for the Tennessee River alone, but for most rivers flowing under similar circumstances upon the surface of the globe. Nothing, however, short of such collections, compared closely with one another, will furnish a reliable answer.... Whoever will accomplish this survey will have made a highly valuable contribution to our knowledge."

Conclusions of Cope.—Certain conclusions were also suggested by Prof. Cope in his excellent memoir on the fishes of the Alleghany region[73] in 1868. From this paper I make the following quotations:

"The distribution of fresh-water fishes is of special importance to the questions of the origin and existence of species in connection with the physical conditions of the waters and of the land. This is, of course, owing to the restricted nature of their habitat and the impossibility of their making extended migrations. With the submergence of land beneath the sea, fresh-water fish are destroyed in proportion to the extent of the invasion of salt water, while terrestrial vertebrates can retreat before it. Hence every inland fish fauna dates from the last total submergence of the country.

"Prior to the elevation of a given mountain chain, the courses of the rivers may generally have been entirely different from their later ones. Subsequent to this period, they can only have undergone partial modifications. As subsequent submergences can rarely have extended to the highlands where such streams originate, the fishes of such rivers can only have been destroyed so far as they were unable to reach those elevated regions, and preserve themselves from destruction from salt water by sheltering themselves in mountain streams. On the other hand, a period of greater elevation of the land, and of consequent greater cold, would congeal the waters and cover their courses with glaciers. The fishes would be driven to the neighborhood of the coast, though no doubt in more [Pg 287]southern latitudes a sufficient extent of uncongealed fresh waters would flow by a short course into the ocean, to preserve from destruction many forms of fresh-water fishes. Thus, through many vicissitudes, the fauna of a given system of rivers has had opportunity of uninterrupted descent, from the time of the elevation of the mountain range, in which it has its sources....

"As regards the distinction of species in the disconnected basins of different rivers, which have been separated from an early geologic period, if species occur which are common to any two or more of them, the supporter of the theory of distinct creations must suppose that such species have been twice created, once for each hydrographic basin, or that waters flowing into the one basin have been transferred to another. The developmentalist, on the other hand, will accept the last proposition, or else suppose that time has seen an identical process and similar result of modification in these distinct regions.

Fig. 186.—Chub of the Great Basin, Leuciscus lineatus (Girard). Heart Lake, Yellowstone Park. Family Cyprinidæ.

"Facts of distribution in the eastern district of North America are these. Several species of fresh-water fishes occur at the same time in many Atlantic basins from the Merrimac or from the Hudson to the James, and throughout the Mississippi Valley, and in the tributaries of the Great Lakes. On the other hand, the species of each river may be regarded as pertaining to four classes, whose distribution has direct reference to the character of the water and the food it offers: first, those of the tide-waters, of the river channels, bayous, and sluggish waters near them, or in the flat lands near the coast; second, those of the river channels of its upper course, where the currents are[Pg 288] more distinct; third, those of the creeks of the hill country; fourth, those of the elevated mountain streams which are subject to falls and rapids."

In the same paper Prof. Cope reaches two important general conclusions, thus stated by him:

"I. That species not generally distributed exist in waters on different sides of the great watershed.

"II. That the distribution of the species is not governed by the outlet of the rivers, streams having similar discharges (Holston and Kanawha, Roanoke and Susquehanna) having less in common than others having different outlets (Kanawha, or Susquehanna and James).

Fig. 187.—Butterfly-sculpin, Melletes papilio Bean, a fish of the rock-pools. St. Paul, Pribilof Islands.

"In view of the first proposition, and the question of the origin of species, the possibility of an original or subsequent mingling of the fresh waters suggests itself as more probable than that of distinct origin in the different basins."

Questions Raised by Cope.—Two questions in this connection are raised by Prof. Cope. The first question is this: "Has any destruction of the river faunæ taken place since the first elevation of the Alleghanies, when the same species were thrown into waters flowing in opposite directions?" Of such destruction by submergence or otherwise, Prof. Cope finds no evidence. The second question is, "Has any means of communication[Pg 289] existed, at any time, but especially since the last submergence, by which the transfer of species might occur?" Some evidence of such transfer exists in the wide distribution of certain species, especially those which seek the highest streamlets in the mountains; but except to call attention to the cavernous character of the Subcarboniferous and Devonian limestones, Prof. Cope has made little attempt to account for it.

Prof. Cope finally concludes with this important generalization:

"It would appear, from the previous considerations, that the distribution of fresh-water fishes is governed by laws similar to those controlling terrestrial vertebrates and other animals, in spite of the seemingly confined nature of their habitat."

Views of Günther.—Dr. Günther[74] has well summarized some of the known facts in regard to the manner of dispersion of fishes:

"The ways in which the dispersal of fresh-water fishes has been affected were various. They are probably all still in operation, but most work so slowly and imperceptibly as to escape direct observation; perhaps they will be more conspicuous after science and scientific inquiry shall have reached a somewhat greater age. From the great number of fresh-water forms which we see at this present day acclimatized in, gradually acclimatizing themselves in, or periodically or sporadically migrating into, the sea, we must conclude that under certain circumstances salt water may cease to be a barrier at some period of the existence of fresh-water species, and that many of them have passed from one river through salt water into another. Secondly, the headwaters of some of the grandest rivers, the mouths of which are at opposite ends of the continents which they drain, are sometimes distant from each other a few miles only. The intervening space may have been easily bridged over for the passage of fishes by a slight geological change affecting the level of the watershed or even by temporary floods; and a communication of this kind, if existing for a limited period only, would afford the ready means of an exchange of a number of species previously peculiar to one or the other of these river or lake systems. Some fishes provided with gill-openings [Pg 290]so narrow that the water moistening the gills cannot readily evaporate, and endowed, besides, with an extraordinary degree of vitality, like many Siluroids (Chlarias, Callichthys), eels, etc., are enabled to wander for some distance over land, and may thus reach a water-course leading them thousands of miles from their original home. Finally, fishes or their ova may be accidentally carried by water-spouts, by aquatic birds or insects, to considerable distances."

Fresh-water Fishes of North America.—We now recognize about six hundred species[75] of fishes as found in the fresh waters [Pg 291]of North America, north of the Tropic of Cancer, these representing thirty-four of the natural families. As to their habits, we can divide these species rather roughly into the four categories proposed by Prof. Cope, or, as we may call them,

(1) Lowland fishes; as the bowfin,[76] pirate-perch,[77] large-mouthed black bass,[78] sunfishes, and some catfishes.

(2) Channel-fishes; as the channel catfish,[79] the mooneye,[80] garpike,[81] buffalo-fishes,[82] and drum.[83]

(3) Upland fishes; as many of the darters, shiners, and suckers, and the small-mouthed black bass.[84]

(4) Mountain-fishes; as the brook trout and many of the darters and minnows.

To these we may add the more or less distinct classes of (5) lake fishes, inhabiting only waters which are deep, clear, and cold, as the various species of whitefish[85] and the Great Lake trout;[86] (6) anadromous fishes, or those which run up from the sea to spawn in fresh waters, as the salmon,[87] sturgeon,[88] shad,[89] and striped bass;[90] (7) catadromous fishes, like the eel,[91] which pass down to spawn in the sea; and (8) brackish-water fishes, which thrive best in the debatable waters of the river-mouths, as most of the sticklebacks and the killifishes.

As regards the range of species, we have every possible gradation from those which seem to be confined to a single river, and are rare even in their restricted habitat, to those which are [Pg 292]in a measure cosmopolitan,[92] ranging everywhere in suitable waters.

Characters of Species.—Still, again, we have all degrees of constancy and inconstancy in what we regard as the characters of a species. Those found only in a single river-basin are usually uniform enough; but the species having a wide range usually vary much in different localities. Such variations have at different times been taken to be the indications of as many different species. Continued explorations bring to light, from year to year, new species; but the number of new forms now discovered each year is usually less than the number of recognized species which are yearly proved to be untenable. Four complete lists of the fresh-water fishes of the United States (north of the Mexican boundary) have been published by the present writer. That of Jordan and Copeland,[93] published in 1876, enumerates 670 species. That of Jordan[94] in 1878 contains 665 species, and that of Jordan and Gilbert[95] in 1883, 587 species. That of Jordan and Evermann[96] in 1898 contains 585 species, although upwards of 130 new species were detected in the twenty-two years which elapsed between the first and the last list. Additional specimens from intervening localities are often found to form connecting links among the nominal species, and thus several supposed species become in time merged in one. Thus the common channel catfish[97] of our rivers has been described as a new species not less than twenty-five times, on account of differences real or imaginary, but comparatively trifling in value.

[Pg 293]

Where species can readily migrate, their uniformity is preserved; but whenever a form becomes localized its representatives assume some characters not shared by the species as a whole. When we can trace, as we often can, the disappearance by degrees of these characters, such forms no longer represent to us distinct species. In cases where the connecting forms are extinct, or at least not represented in collections, each form which is apparently different must be regarded as a distinct species.

The variations in any type become, in general, more marked as we approach the tropics. The genera are represented, on the whole, by more species there, and it would appear that the processes of specific change go on more rapidly under the easier conditions of life in the Torrid Zone.

We recognize now in North America twenty-five distinct species of fresh-water catfishes,[98] although nearly a hundred (93) nominal species of these fishes have been from time to time described. But these twenty-five species are among themselves very closely related, and all of them are subject to a variety of minor changes. It requires no strong effort of the imagination to see in them all the modified descendants of some one species of catfish, not unlike our common "bullhead,"[99] an immigrant probably from Asia, and which has now adjusted itself to its surroundings in each of our myriad of catfish-breeding streams.

Meaning of Species.—The word "species," then, is simply a term of convenience, including such members of a group similar to each other as are tangibly different from others, and are not known to be connected with these by intermediate forms. Such connecting links we may suppose to have existed in all cases. We are only sure that they do not now exist in our collections, so far as these have been carefully studied.

When two or more species of any genus now inhabit the same waters, they are usually species whose differentiation is of long standing,—species, therefore, which can be readily distinguished from one another. When, on the other hand, we have "representative species,"—closely related forms, neither of which is found within the geographical range of the other,—we [Pg 294]can with some confidence look for intermediate forms where the territory occupied by the one bounds that inhabited by the other. In very many such cases the intermediate forms have been found; and such forms are considered as subspecies of one species, the one being regarded as the parent stock, the other as an offshoot due to the influences of different environment. Then, besides these "species" and "subspecies," groups more or less readily recognizable, there are varieties and variations of every grade, often too ill-defined to receive any sort of name, but still not without significance to the student of the origin of species. Comparing a dozen fresh specimens of almost any kind of fish from any body of water with an equal number from somewhere else, one will rarely fail to find some sort of differences,—in size, in form, in color. These differences are obviously the reflex of differences in the environment, and the collector of fishes seldom fails to recognize them as such; often it is not difficult to refer the effect to the conditions. Thus fishes from grassy bottoms are darker than those taken from over sand, and those from a bottom of muck are darker still, the shade of color being, in some way not well understood, dependent on the color of the surroundings. Fishes in large bodies of water reach a larger size than the same species in smaller streams or ponds. Fishes from foul or sediment-laden waters are paler in color and slenderer in form than those from waters which are clear and pure. Again, it is often true that specimens from northern waters are less slender in body than those from farther south; and so on. Other things being equal, the more remote the localities from each other, the greater are these differences.

Fig. 188.—Scartichthys enosimæ Jordan and Snyder, a fish of the rock-pools of the sacred island of Enoshima, Japan. Family Blenniidæ.

[Pg 295]

In our fresh-water fishes each species on an average has been described as new from three to four times, on account of minor variations, real or supposed. In Europe, where the fishes have been studied longer and by more different men, upwards of six or eight nominal species have been described for each one that is now considered distinct.

Special Creation Impossible.—It is evident, from these and other facts, that the idea of a separate creation for each species of fishes in each river-basin, as entertained by Agassiz, is wholly incompatible with our present knowledge of the specific distinctions or of the geographical distribution of fishes. This is an unbroken gradation in the variations from the least to the greatest,—from the peculiarities of the individual, through local varieties, geographical subspecies, species, sub-genera, genera, families, super-families, and so on, until all fish-like vertebrates are included in a single bond of union.

Origin of American Species of Fishes.—It is, however, evident that not all American types of fishes had their origin in America, or even first assumed in America their present forms. Some of these are perhaps immigrants from northern Asia, where they still have their nearest relatives. Still others are evidently modified importations from the sea; and of these some are very recent immigrants, land-locked species which have changed very little from the parent stock.

The problems of analogous variation or parallelism without homology are very often met with among fishes. In shallow, swift brooks in all lands there are found small fishes which hug the bottom—large-finned, swift of movement, with speckled coloration, and with the air-bladder reduced in size. In the eastern United States these fishes are darters, dwarf perches; in northern India they are catfishes; in Japan, gobies or loaches; in Canada, sculpins; in South America, characins. Members of various groups may be modified to meet the same conditions of life. Being modified to look alike, the thought of mutual affinity is naturally suggested, but in such cases the likeness is chiefly external. The internal organs show little trace of such modifications. The inside of an animal tells what it really is, the outside where it has been. In other words, it is the external characters which are most readily affected by the environ[Pg 296]ment. Throughout all groups of animals and plants, there are large branches similarly affected by peculiarities of conditions.

This is the basis of the law of "Adaptive Radiation." Prof. H. F. Osborn thus states this law:

"It is a well-known principle of zoological evolution that an isolated region, if large and sufficiently varied in its topography, soil, climate, and vegetation, will give rise to a diversified fauna according to the law of adaptive radiation from primitive and central types. Branches will spring off in all directions to take advantage of every possible opportunity of securing food. The modifications which animals undergo in this adaptive radiation are largely of mechanical nature; they are limited in number and kind by hereditary stirp or germinal influences, and thus result in the independent evolution of similar types in widely separated regions under the law of parallelism or homoplasy."

[Pg 297]

CHAPTER XVII
BARRIERS TO DISPERSION OF RIVER FISHES

The Process of Natural Selection.—We can say, in general, that in all waters not absolutely uninhabitable there are fishes. The processes of natural selection have given to each kind of river or lake species of fishes adapted to the conditions of life which obtain there. There is no condition of water, of bottom, of depth, of speed of current, but finds some species with characters adjusted to it. These adjustments are, for the most part, of long standing; and the fauna of any single stream has as a rule been produced by immigration from other regions or from other streams. Each species has an ascertainable range of distribution, and within this range we may be reasonably certain to find it in any suitable waters.

Fig. 189.—Slippery-dick or Doncella, Halichœres bivittatus Bloch, a fish of the coral reefs, Key West. Family Labridæ.

But every species has beyond question some sort of limit to its distribution, some sort of barrier which it has never passed in all the years of its existence. That this is true becomes evident when we compare the fish fauna of widely separated rivers. Thus the Sacramento, Connecticut, Rio Grande, and[Pg 298] St. John's Rivers have not a single species in common; and with one or two exceptions, not a species is common to any two of them. None of these[100] has any species peculiar to itself, and each shares a large part of its fish fauna with the water-basin next to it. It is probably true that the faunas of no two distinct hydrographic basins are wholly identical, while on the other hand there are very few species confined to a single one. The supposed cases of this character, some twenty in number, occur chiefly in the streams of the South Atlantic States and of Arizona. All of these need, however, the confirmation of further exploration. It is certain that in no case has an entire river fauna[101] originated independently from the divergence into separate species of the descendants of a single type.

The existence of boundaries to the range of species implies, therefore, the existence of barriers to their diffusion. We may now consider these barriers and in the same connection the degree to which they may be overcome.

Local Barriers.—Least important to these are the barriers which may exist within the limits of any single basin, and which tend to prevent a free diffusion through its waters of species inhabiting any portion of it. In streams flowing southward, or across different parallels of latitude, the difference in climate becomes a matter of importance. The distribution of species is governed very largely by the temperature of the water. Each species has its range in this respect,—the free-swimming fishes, notably the trout, being most affected by it; the mud-loving or bottom fishes, like the catfishes, least. The latter can reach the cool bottoms in hot weather, or the warm bottoms in cold weather, thus keeping their own temperature more even than that of the surface of the water. Although water communication is perfectly free for most of the length of the Mississippi, there is a material difference between the faunæ of the stream in Minnesota and in Louisiana. This difference is caused chiefly by the difference in temperature occupying the difference in latitude. That a similar difference in longitude, with free [Pg 299]water communication, has no appreciable importance, is shown by the almost absolute identity of the fish faunæ of Lake Winnebago and Lake Champlain. While many large fishes range freely up and down the Mississippi, a majority of the species do not do so, and the fauna of the upper Mississippi has more in common with that of the tributaries of Lake Michigan than it has with that of the Red River or the Arkansas. The influence of climate is again shown in the paucity of the fauna of the cold waters of Lake Superior, as compared with that of Lake Michigan. The majority of our species cannot endure the cold. In general, therefore, cold or Northern waters contain fewer species than Southern waters do, though the number of individuals of any one kind may be greater. This is shown in all waters, fresh or salt. The fisheries of the Northern seas are more extensive than those of the tropics. There are more fishes there, but are far less varied in kind. The writer once caught seventy-five species of fishes in a single haul of the seine at Key West, while on Cape Cod he obtained with the same net but forty-five species in the course of a week's work. Thus it comes that the angler, contented with many fishes of few kinds, goes to Northern streams to fish, while the naturalist goes to the South.

Fig. 190.—Peristedion miniatum Goode and Bean, a deep-red colored fish of the depths of the Gulf Stream.

But in most streams the difference in latitude is insignificant, and the chief differences in temperature come from differences in elevation, or from the distance of the waters from the colder source. Often the lowland waters are so different in character as to produce a marked change in the quality of their fauna. These lowland waters may form a barrier to the free movements[Pg 300] of upland fishes; but that this barrier is not impassable is shown by the identity of the fishes in the streams[102] of the uplands of middle Tennessee with those of the Holston and French Broad. Again, streams of the Ozark Mountains, similar in character to the rivers of East Tennessee, have an essentially similar fish fauna, although between the Ozarks and the Cumberland range lies an area of lowland bayous, into which such fishes are never known to penetrate. We can, however, imagine that these upland fishes may be sometimes swept down from one side or the other into the Mississippi, from which they might ascend on the other side. But such transfers certainly do not often happen. This is apparent from the fact that the two faunas[103] are not quite identical, and in some cases the same species are represented by perceptibly different varieties on one side and the other. The time of the commingling of these faunæ is perhaps now past, and it may have occurred only when the climate of the intervening regions was colder than at present.

The effect of waterfalls and cascades as a barrier to the diffusion of most species is self-evident; but the importance of such obstacles is less, in the course of time, than might be expected. In one way or another very many species have passed these barriers. The falls of the Cumberland limit the range of most of the larger fishes of the river, but the streams above it have their quota of darters and minnows. It is evident that the past history of the stream must enter as a factor into this discussion, but this past history it is not always possible to trace. Dams or artificial waterfalls now check the free movement of many species, especially those of migratory habits; while conversely, numerous other species have extended their range through the agency of canals.[104]

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Every year fishes are swept down the rivers by the winter's floods; and in the spring, as the spawning season approaches, almost every species is found working its way up the stream. In some cases, notably the Quinnat salmon[105] and the blue-back salmon,[106] the length of these migrations is surprisingly great. To some species rapids and shallows have proved a sufficient barrier, and other kinds have been kept back by unfavorable conditions of various sorts. Streams whose waters are always charged with silt or sediment, as the Missouri, Arkansas, or Brazos, do not invite fishes; and even the occasional floods of red mud such as disfigure otherwise clear streams, like the Red River or the Colorado (of Texas), are unfavorable. Extremely unfavorable also is the condition which obtains in many rivers of the Southwest, as, for example, the Red River, the Sabine, and the Trinity, which are full from bank to bank in winter and spring, and which dwindle to mere rivulets in the autumn droughts.

Favorable Waters have Most Species.—In general, those streams which have conditions most favorable to fish life will be found to contain the greatest number of species. Such streams invite immigration; and in them the struggle for existence is individual against individual, species against species, and not a mere struggle with hard conditions of life. Some of the conditions most favorable to the existence in any stream of a large number of species of fishes are the following, the most important of which is the one mentioned first: Connection with a large hydrographic basin; a warm climate; clear water; a moderate current; a bottom of gravel (preferably covered by a growth of weeds); little fluctuation during the year in the volume of the stream or in the character of the water.