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The Senses.-Notice the position of the eyes from the front and in a side view of the fish. Is the position of advantage and how? The eye is globular in shape. Such an eye has been found to be very near sighted. Thus it is unlikely that a fish is able to perceive objects at any great distance from it. The eye is unprotected by eyelids, but the position on the sides of the body affords some protection. There is much opportunity for a careful boy or girl to make simple experiments to determine how much and how far the fish can see. Feed the fish; does it appear to see the food or to go to the food by a sense of smell? 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 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. A fish rejects bits of food which it does not like.
Notice that 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 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 bgills 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 into the throat, are a series of delicate filaments of flesh, covered with a very delicate membrane or skin. Into each of these filaments pass two blood vessels, one downward and one 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. Blood passes into the gill filament, and there comes in 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. Possibly other waste products find their way into the water bathing the gill filaments, the wastes being carried off by the current of water passing over the gills.
Gills and heart of the fish, exposed by removal of gill cover on left side; a, first of the four bony arches which carry the gills; b, gills; b', lower edges of gills on the right side;
Teeth. - Notice the arrangement and number of the teeth in the stickleback. Sticklebacks are carnivorous, preying upon the eggs and young of other small fish inhabiting the brooks where they live. They also feed upon decaying and live vegetable matter, especially algæ. The teeth of
the stickleback are small and evidently useful for seizing and holding prey. This fish, like many other species, is a hunter and uses the teeth as weapons of offense as well as for defense. How are the teeth of the stickleback adapted to their functions? The tongue in most fishes is wanting or very slightly developed.
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. fishes, as the shad and menhaden, make such use of the gill rakers. 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 of considerable economic importance. Most fishes have a series of pockets, called pyloric caca, which are outgrowths from the intestine and probably serve to give more absorbing surface to that part of the digestive tract. The intestine ends at the vent, which is usually located on the ventral side of the fish, immediately in front of the anal fin.
Anatomy of the carp; br, branchiæ, or gill openings; c, heart; f, liver; vn, swimming bladder; ci, intestine.
An organ of unusual significance, called the swim bladder, is connected with the digestive tract in front of the stomach. In young fishes of many species this connection is a tube, which in some forms
persists throughout life, but which in other fish becomes closed, a thin, fibrous cord taking its place. This swim bladder aids the fish in balancing and is in some way used in aiding the fish to maintain its equilibrium in deep or shallow water. The walls of the organ are richly supplied with blood vessels, and it thus undoubtedly serves as an organ for taking in oxygen. It has been compared to a lung of the higher vertebrates.
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
Plan of circulation in fishes; a, auri
cle; b, ventricle; c, branchial artery; e, branchial veins, bringing blood from the gills, d, and uniting in the aorta, f; g, vena cava, returning blood to heart.
chambered muscular organ, a thin-walied auricle leading into a thick-walled muscular ventricle. 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 of the body, eventually breaking up into very tiny tubes called capillaries. From the capillaries the blood returns, in tubes of gradually increasing diameter, toward the heart again. During its course some of the blood passes through the kidneys and is there relieved of part of its nitrogenous waste. (See Chap. XXX.)
Circulation of blood in the body of the fish is rather slow. The temperature of the blood being nearly that of the surrounding media in which the fish lives, the animal has incorrectly been given the term cold-blooded.
NERVOUS SYSTEM. As in all vertebrate animals, the brain and spinal cord of the fish are inclosed in a series of bony structures called vertebræ. The central nervous system consists of a brain, with nerves leading to the organs of sight, taste, smell, the ear, and to such parts of the body as possess the sense of touch; a spinal cord; and spinal nerves. Nerve cells located near the outside of the body send in messages to the central system, which are there received as sensations. Cells of the central nervous system in turn send out messages which result in the movement of muscles.
We have already learned something of the senses of a fish. That of hearing is poorly developed, the ear being largely an organ of balancing. Along each side of almost every species of fish is found a line, which consists of a series of tiny pits each connected with its neighbor.1 This lateral line, as it is called, is believed to have to do with the sense of touch.
SKELETON. In the vertebrates, of which the bony fish is an example, the skeleton is under the skin and is hence called an endoskeleton. It consists of a bony framework, the vertebral column, and certain attached 1 This line is plainly visible in some fishes because of its dark color.
bones, the ribs, with other spiny bones to which the unpaired fins are attached. The paired fins are attached to the spinal column by two collections of bones, known respectively as the pectoral and pelvic girdles. The bones serve in the fish for the attachment of powerful muscles, by means of which locomotion is accomplished.
The Egg-laying Habits of the Bony Fishes. The stickleback has the rather curious habit (for a fish) of nest building. The nest is attached to waterweeds or a submerged stick. It is almost circular in outline, and in the case of the common stickleback is about two inches in diameter. A hole in the top gives access to the interior. In this nest the female deposits her eggs, which are then guarded by the male until the young hatch out. The freshwater sunfish also has the habit of nest building, its nest being scooped out in the sand of the lake or river bed.
The eggs of most bony fishes are laid in great numbers at the time of spawning. This number varies from a few thousand in the trout to many hundreds of thousands in the shad and several millions in the cod. The time of egg laying is usually spring or early summer. Usually the eggs are left to develop by themselves, sometimes attached to some submerged object, but more frequently free in the water. They are exposed to many dangers, and both eggs and developing fish are eaten, not only by birds, fish of other species, and other water inhabitants, but also by their own relatives and even parents. Consequently a very small percentage of eggs ever reach maturity.
The Relation of the Spawning Habits to Economic Importance of Fish. The spawning habits of fish are of great importance to us because of the economic value of fish to mankind, not only directly as a food, but indirectly as food for other animals in turn valuable to man. Many of our most desirable food fishes, notably the salmon, shad, sturgeon, and smelt, pass up rivers from the ocean to deposit their eggs. The salmon is said to travel thousands of miles, swimming against strong currents much of the way, leaping rapids and falls, in order to deposit her eggs in suitable localities, where the conditions of water and food are requisite, and the water shallow enough to allow the sun's rays to warm the water sufficiently to cause the eggs to develop. At the
time of the spawning migration the salmon are taken in vast numbers. The salmon fisheries net over $13,000,000 annually, the shad at least $1,500,000, the smelt fishery nearly $150,000 more. The total annual value of the fisheries of the United States is over $50,000,000. But the profits from these fisheries are steadily decreasing because of the yearly destruction of untold millions of eggs which might develop into adult fish. State and government interposition is in many cases coming too late, for at the present rate of destruction many of our most desirable food fishes will
soon be extinct. The sturgeon, the eggs of which are used in the manufacture of the delicacy known as caviare, is an example of a fish that is almost extinct in this part of the world.
Other deep-water fish do not go up rivers to spawn, but lay their eggs in the sea. In such eggs there is found a tiny oil drop, which, being lighter than water, causes the egg to come to the surface of the water, where the heat of the sun may favorably influence development. Other fish change their habitat at different times during the year, moving in vast schools northward in summer and southward in the winter. In a general way such migrations follow the coast lines. Examples of such migratory fish are the cod,