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The tail disappears, and with it the notochord and the greater part of the nervous system. The sense-organs vanish, the pharynx becomes remodeled, and numerous other changes occur, leaving the animal in its adult condition, with little in its motionless, sac-like body to remind one of a vertebrate.
151. The vertebrates.—Since the remainder of this volume is concerned with the vertebrates it will be well at the outset to gain some knowledge of their more important characteristics. One of the most apparent is the presence of a jointed vertebral column, composed of cartilage or bone, which supports the nervous system. To it are also usually attached several pairs of ribs, two pairs of limbs, either fins, legs, or wings, and in front it terminates in a more or less highly developed skull. In the space partially enclosed by the ribs, the body-cavity, a digestive system is located, which consists of the stomach and intestine, together with the attached liver and pancreas. The circulatory system is also highly organized, and consists of a muscular heart, arteries, and veins which ramify throughout the body. Breathing, in the aquatic animals, is carried on by means of gills, and in the air-breathing forms by means of lungs, which, like the gills, effect the removal of carbonic-acid gas and the absorption of oxygen. The nervous system, consisting of the brain situated in the head and the spinal cord extending through the body above the back-bone, even in the lower vertebrates, is far more complex than in the invertebrates. The sense-organs also attain to a high degree of acuteness, and in connection with the highly organized nervous system enable these forms to lead far more varied and complex lives than in any of the animals heretofore considered.
152. General characters.-In a general way the name fish is applied to all vertebrates which spend the whole of their life in the water, which undergo no retrograde metamorphosis, and which do not develop fingers or toes. Of other aquatic chordates or vertebrates the ascidians undergo a retrograde metamorphosis, losing the notochord, and with it all semblance of fish-like form. The amphibians, on the other hand, develop jointed limbs with fingers and toes, instead of paired fins with fin rays. A further comparison of the animals called fishes reveals very great differences among them-differences of such extent that they cannot be placed in a single class. At least three great groups or classes must be recognized : the Lancelets, the Lampreys, and the True Fishes. The general characters of all these groups will be better understood after the study of some typical fish, that is one possessing as many fish-like features as possible, unmodified by peculiar habits. Such an example is found in the bass, trout, or perch. In either fish the pointed head is united, without any external sign of a neck, to the smooth, spindle-shaped body, which is thus fitted for easy and rapid cleaving of the water when propelled by the waving of the powerful tail (Fig. 100). A keel also has been provided, enabling the fish to steer true to its course. This consists of folds of skin arising along the middle line of the body, supported by numerous bony spines or cartilaginous rays. These are the unpaired fins, as distinguished from the paired ones, which correspond to the limbs of the higher vertebrates. In the bass or perch the latter are of much service in swimming, and are also most important organs in directing the course of the fish upward or downward, or for
Fig. 100.—Yellow perch (Perca flavescens). df, dorsal fins ; pc, pectoral fin ; v, ven
tral fin; a, anal fin; c, caudal fin. aiding the tail in changing the course from side to side; or they may be used to support the animal as it rests upon the bottom in wait for food; and, finally, they may serve to keep the body suspended at a definite point.
In addition to an internal skeleton the bass or perch, like the greater number of fishes, is more or less enclosed and protected by an external one, consisting of a beautifully arranged series of overlapping scales, which afford protection to the underlying organs, and at the same time admit of great freedom of movement. These usually consist of a horny substance, to which lime is sometimes added, and are peculiar modifications of the skin, something like the feathers, nails, and hoofs of higher forms.
153. The air-bladder.—Naturally a fish's body is heavier than the water in which it lives, and there are reasons for thinking that the air-bladder (Fig. 106, a.bl.) acts in the
bass and perch and many other fishes as a float to enable them, without much effort, to remain suspended at a definite level. By compressing this sac, partly by its own muscles and partly by those of the body-wall, the bulk of the fish is made less, and it sinks; upon the relaxation of these same muscles the body expands and rises again. Deep-sea fishes, when brought to the surface, where the pressure is relatively slight, are found with their air-bladders so distended that they can not sink again, and the float of surface fishes would be as useless if they were to be carried into the depths below, so that such fishes are compelled to keep within tolerably definite limits of depth. Morphologically considered, the air-bladder is a modified or degenerate lung, and in many fishes it is lost altogether.
154. Respiration. Looking down the throat of the perch, or any other fish, a series of slits (the gill-openings), usually four or five in number, may be seen on each side communicating with the exterior. In the sharks these outer openings are readily seen, but in the bony fishes they open into a chamber on each side of the head, covered by a bony plate or gill-cover that is open behind. On raising these flaps the gills may be seen composed of great numbers of brightred filaments attached to the bars between each slit. During life the fish may be seen to open its mouth at regular intervals, and, after gulping in a quantity of water, to close it again, contracting the sides of the throat to force it out of the gill-openings and over the gill-filaments to the exterior. During this process the blood traversing the excessively thin filaments extracts the oxygen from the water and carries it to other parts of the body.
With this information, let us return to the study of the three classes of fishes.
155. The lancelet (Branchiostoma).—The lancelet, sometimes called amphioxus (Fig. 101), the type of the class Leptocardii, is a little creature, half an inch to four inches long, in the different species, transparent and colorless, living chiefly
in sand in warm seas, the ten species being found in as many different regions. A lancelet may be regarded as a vertebrate reduced to its lowest terms. Instead of a jointed back-bone, it has a cartilaginous notochord, running from the head to the tail. A nervous cord lies above it, enclosed in a membranous sheath. No skull is present, and the nerve-cord does not swell into a brain. There are no eyes and no scales. The mouth is a vertical slit, without jaws. There is no trace of the shoulder-girdle (shoulderblade and collar-bone) or pelvis (hip-bone) from which
Fig. 101.-The California lancelet (Branchiostoma californiense). Twice the natural
size. g, gills; l, liver ; m, mouth ; n, nerve-cord ; nc, notochord.
spring the paired fins, which, in true fishes, correspond to arms and legs. The circulatory system is fish-like, but there is no heart, the blood being driven about by the contraction of the walls of the vessels. /Along the edge of the back and tail is a rudimentary fin, made of fin-rays connected by membrane. In the character and arrangement of its organs the lancelet is certainly like a fish, but in degree of development it differs more from the lowest fish than the fish does from a mammal.
156. Lampreys (or Cyclostomes).—The class of lampreys stands next in development (Fig. 102). The notochord gives way anteriorly to a cartilaginous skull, in which is contained the brain, of the ordinary fish type. There are eyes, and the heart is developed, and consists of an auricle and a ventricle. As distinguished from the true fish, the lampreys show no trace whatever of limbs or of the bones which would support them. The lower jaw is wholly wanting, the mouth being a roundish sucking disk. The fins