XXII. THE VERTEBRATE ANIMALS

Increasing Complexity of Structure and of Habits in Plants and Animals. In our study of biology so far we have attempted to get some notion of the various factors which act upon and interact with living things. We have learned something about the various physiological processes of plants and animals, and have found them to be in many respects identical. We have examined a number of forms of plants and have found all grades of complexity, from the one-celled plant, bacterium or pleurococcus, to the complicated flowering plants of considerable size and with many organs. So in animal life the forms we may have studied, from the Protozoa upward, there is constant change, and the change is toward greater complexity of structure and functions. A worm is simpler in structure than an insect, and in many ways, especially by its actions, shows that it is not so high in the scale of life as its more lively neighbor.

We are already awake to the fact that we, as living creatures, are better equipped in the battle for life than our more lowly neigh

In structure we also differ. Particularly is this difference seen in the skeleton. We call ourselves vertebrates, because we have a

bony vertebral column, made up of pieces of bone joined one to another, forming a flexible yet strong support for the muscles and protecting the delicate central nervous system. This kind of an endoskeleton, or inside skeleton, is possessed by fishes, frogs, turtles or snakes, and birds, and by mammals, such as the dog, cat, and man. All such animals are called vertebrates. We are now to take up the study of some types of various kinds of vertebrates, with the view to a better understanding of man.

Fishes

Problem XXXIV. A study of how a fish is fitted for the life it leads. (Laboratory Manual, Prob. XXXIV.)

The Body. One of our common fresh-water fish is the bream, or golden shiner. The body of the bream runs insensibly into the head, the neck being absent. The long, narrow body with its smooth surface fits the fish admirably for its life in the water. Certain cells in the skin secrete mucus or slime, another adaptation. The position of the scales, overlapping in a backward direction, is yet another adaptation which aids in passing through

the water. Its color, olive above and bright silver and gold below, is also protective. Can you see how?

The Appendages and their Uses. The appendages of the fish consist of paired and unpaired fins. The paired fins are four in number, and are believed to correspond in position and structure

with the paired limbs of a man. Note the Figure on page 326 and locate the paired pectoral and pelvic fins. (These are so called because they are attached to the bones forming the pectoral and pelvic girdles. See page 426.) Find, by comparison with the Figure, the dorsal, anal, and caudal fins. How many unpaired

fins are there?

The flattened, muscular body of the fish, tapering toward the caudal fin, is moved from side to side with an undulating motion which results in the movement forward of the fish. This movement is almost identical with that of an oar in sculling a boat. Turning movements are brought about by use of the lateral fins in much the same way as a boat is turned. We notice the dorsal and other single fins are evidently useful as balancing and steering organs.

The Senses. The position of the eyes at the side of the head is an evident advantage to the fish. Why? The eye is globular in shape. Such an eye has been found to be very nearsighted. Thus it is unlikely that a fish is able to perceive objects at any great distance from it. The eyes are unprotected by eyelids, but the tough outer covering and their position afford some protection.

Feeding experiments with fishes show that a fish becomes aware of the presence of food by smelling it as well as by seeing it. The nostrils of a fish can be proved to end in little pits, one under each nostril hole. Thus they differ from our own, which are connected with the mouth cavity. In the catfish, for example, the barbels, or horns, receive sensations of smell and taste. The sense of perceiving odor is not as we understand the sense of smell, for a fish perceives only substances that are dissolved in the water in which it lives. The senses of taste and touch appear to be less developed than the other senses.

Along each side of most fishes is a line of tiny pits, provided with sense organs and connected with the central nervous system of the fish. This area, called the lateral line, is believed to be sensitive to mechanical stimuli of certain sorts. The " ear of the fish is under the skin and serves partly as a balancing organ.

A fish must go after its food and seize it, but has no structures for grasping except the teeth. Consequently we find the teeth small, sharp, and numerous, well adapted for holding living prey. The tongue in most fishes is wanting or very slightly developed.

Breathing.

A fish, when swimming quietly or when at rest, seems to be biting when no food is present. A reason for this act is to be seen when we introduce a little finely powdered carmine into the water near the head of the fish. It will be found that a current of water enters the mouth at each of these biting movements and passes out through two slits found on each side of the head of the fish. Investigation shows us that under the broad, flat plate, or operculum, forming each side of the head, lie several long, feathery, red structures, the gills.

Gills.

If we examine the gills of any large fish, we find that a single gill is held in place by a bony arch, made of several pieces

of bone which are hinged in such a way as to give great flexibility to the gill arch, as the support is called. Covering the bony framework, and extending from it, are numerous delicate filaments of flesh, covered with a very delicate membrane or skin. Into each of these filaments pass two blood vessels; in one blood flows downward and in the other upward. Blood reaches the gills and is carried away from these organs by means of two large vessels which pass along the bony arch previously mentioned. In the gill filament the blood comes into contact with the free oxygen of the water bathing the gills. An exchange of gases through the walls of the gill filaments results in the loss of carbon dioxide and a gain of oxygen by the blood. Gill Rakers. If we open wide the mouth of any large fish and look inward, we find that the mouth cavity leads to a funnel-like opening, the gullet. On each side of the gullet we can see the gill arches, guarded on the inner side by a series of sharp-pointed structures, the gill rakers. In some fishes in which the teeth are not well developed, there seems to be a greater development of the gill rakers, which in this case are used to strain out small organisms from the water which passes over the gills. Many fishes make such use of the gill rakers. Such are the shad and menhaden, which feed almost entirely on plankton, a name given to the small plants and animals found by millions in the water.

The head of a fish, with the operculum cut away to show the gills.

Digestive System. - The gullet leads directly into a baglike stomach. There are no salivary glands in the fishes. There is, however, a large liver, which appears to be used as a digestive gland. This organ, because of the oil it contains, is in some fishes, as the cod, of considerable economic importance. Many fishes have outgrowths like a series of pockets from the intestine. These structures, called the pyloric caca, are believed to secrete a digestive fluid. The intestine ends at the vent, which is usually located on the ventral side of the fish, immediately in front of the anal fin.

Swim Bladder. An organ of unusual significance, called the swim bladder, occupies the region just dorsal to the food tube. In young fishes of many species this is connected by a tube with the anterior end of the digestive tract. In some forms this tube persists throughout life, but in other fish it becomes closed, a thin, fibrous cord taking its place. The swim bladder aids in giving the fish nearly the same weight as the water it displaces, thus buoying it up. The walls of the organ are richly supplied with blood vessels, and it thus undoubtedly serves as an organ for supplying oxygen to the blood when all other sources fail. In some fish (the dipnoi, p. 284) it has come to be used as a lung.

Circulation of the Blood. In the vertebrate animals the blood is said to circulate in the body, because it passes through a more or less closed system of tubes in its course around the body. In the fishes the heart is a two-chambered muscular organ, a thin-walled auricle, the receiving chamber, leading into a thick-walled muscular ventricle from which the blood is forced out. The blood is pumped from the heart to the gills; there it loses some of its carbon dioxide; it then passes on to other parts