=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. [Illustration: 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. How does the tail of a shark or a gar differ from the tail of common fishes? (Fig. 228.) Do you know of fish destitute of scales? Do you know of fish with whiplike feelers on the head? (Figs.) Why are most fishes white on the under side? =Comparative Review.=--(Copy table on one page or two facing pages of notebook.)