Make a list of the different species of fish found in the waters of your neighbourhood; in the markets of your town.
Fig. 227.—Newly Hatched Trout, with yolk-sac adhering, eyes large, and fins mere folds of the skin. (Enlarged.)
Reproduction.—The female fish deposits the unfertilized eggs, or ova, in a secluded spot on the bottom. Afterward the male fish deposits the sperms in the same place (see Fig. [225]). The eggs, thus unprotected, and newly hatched fish as well, are used for food by fish of the same and other species. To compensate for this great destruction, most fish lay (spawn) many thousands of eggs, very few of which reach maturity. Higher vertebrates (e.g. birds) have, by their superior intelligence, risen above this wasteful method of reproduction. Some kinds of marine fish, notably cod, herring, and salmon, go many miles up fresh rivers to spawn. It is possible that this is because they were originally fresh-water species; yet they die if placed in fresh water except during the spawning season. They go because of instinct, which is simply an inherited habit. Rivers may be safer than the ocean for their young. They are worn and exhausted by the journey, and never survive to lay eggs the second time.
Fig. 228.—A Shark (Acanthias vulgaris).
The air bladder is developed from the food tube in the embryo fish, and is homologous with lungs in the higher vertebrates. Are their functions the same?
Fish that feed on flesh have a short intestine. Those that eat plants have a long intestine. Which kind of food is more quickly digested?
There are mucous glands in the skin of a fish which supply a secretion to facilitate movement through the water; hence a freshly caught fish, before the secretion has dried, feels very slippery.
The air bladder, although homologous to lungs, is not a breathing organ in common fishes. It is filled by the formation of gases from the blood, and can be made smaller by the contraction of muscles along the sides of the body; this causes the fish to sink. In the gar and other ganoids, the air bladder contains blood vessels, is connected with the gullet, and is used in breathing. Organs serving the same purpose in different animals are said to be analogous. To what in man are the gills of the fish analogous? Organs having a like position and origin are said to be homologous. The air bladders of a fish are homologous with the lungs of man; but since they have not the same use they are not analogous.