CHAPTER XIII

THE CHORDATES

143. General characters.-Up to the present time we have been studying the representatives of a vast assemblage of animals whose skeletons, if they have any at all, are located on the outside of the body. In the corals, the mighty company of arthropods, and the echinoderms, it is external. On the other hand, we shall find that the animals we are now about to consider, the fishes, frogs, lizards, birds, and mammals, are in possession of an internal skeleton. In some of the simpler fishes and in a number of more lowly forms (Fig. 96) it is exceedingly simple, and consists merely of a gristlelike rod, the notochord (Fig. 98, nc), extending the length of the body and serving to support the nervous system, which is always dorsal. This is also the type of skeleton found in the young of the remaining higher animals, but as they grow older the notochord gives way to a more highly developed cartilaginous or bony, jointed skeleton, the vertebral column.

In the young of all these back-boned or chordate animals, the sides of the throat are invariably perforated to form a number of gill-slits. In the lower forms these persist and serve as respiratory organs, but in the higher animals they disappear in the adult. The chordates are thus seen to be distinguished by the possession of a dorsal nervous cord supported by an internal skeleton and by the presence of gill-slits, characters which separate them widely from all invertebrates.

The chordates may be divided into ten classes, seven of

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which (the lancelets, lampreys, fishes, amphibians, reptiles, birds, and mammals) are true vertebrates, while the others embrace several peculiar animals of much simpler organization.

144. The ascidians.-Among the latter are a number of remarkable species belonging to the class of ascidians or

sea-squirts (Fig. 96). These are abundantly represented along our coasts, and are readily distinguished by their sac-like bodies, which are often attached at one end to shells or rocks. On the opposite extremity two openings exist, through which a constant stream of water passes, bearing minute organisms serving as food. When disturbed. they frequently expel the water from these pores with considerable force, whence the name "sea-squirt."

While

many lead solitary lives,

numerous individuals of other species are often closely packed together in a jelly-like pad attached to the rocks, and others not distantly related are fitted to float on the surface of the sea.

The young when hatched resemble small tadpoles both in their shape and in the arrangement of some of the more important systems of organs. For a few hours each swims about, then selecting a suitable spot settles down and adheres for life. From this point on degeneration ensues.

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.

145. 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.

CHAPTER XIV

THE FISHES

146. 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. 97). 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. 97. Yellow perch (Perca flavescens). df, dorsal fins; pc, pectoral fin; pf, pelvic fin; v, ventral 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.

147. 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