=The Organs of Sight.=--The eyes of fishes differ from those of the higher vertebrates mainly in the spherical form of the crystalline lens. This extreme convexity is necessary because the lens itself is not very much denser than the fluid in which the fishes live. The eyes vary very much in size and somewhat in form and position. They are larger in fishes living at a moderate depth than in shore fishes or river fishes. At great depths, as a mile or more, where all light is lost, they may become aborted or rudimentary, and may be covered by the skin. Often species with very large eyes, making the most of a little light or of light from their own luminous spots, will inhabit the same depths with fishes having very small eyes or eyes apparently useless for seeing, retained as vestigial structures through heredity. Fishes which live in caves become also blind, the structures showing every possible phase of degradation. The details of this gradual loss of eyes, whether through reversed selection or hypothetically through inheritance of atrophy produced by disuse, have been given in a number of memoirs on the blind fishes of the Mississippi Valley by Dr. Carl H. Eigenmann. In some fishes the eye is raised on a short, fleshy stalk and can be moved about at the will of the fish. It is said that the vision of the pond-skipper, _Periophthalmus_, when hunting insects on the mud flats of Japan or India is "quite equal to that of a frog." It is known also that trout possess keen eyesight, and that they show a marked preference for one sort or another of real or artificial fly. Nevertheless the vision of fishes in general is probably not very precise. They apparently notice motion rather than outline, changes rather than objects, while the extreme curvature of the crystalline lens would seem to render them all near-sighted. [Illustration: FIG. 85.--Four-eyed Fish, _Anableps dovii_ Gill. Tehuantepec, Mexico.]