There is little doubt that the sense of smell in fishes is relatively acute, and that the odor of their prey attracts them to it. It is known that flesh, blood, or a decaying carcass will attract sharks, and other predatory fish are drawn in a similar manner. At the same time the strength of this function is yet to be tested by experiments.
Fig. 83.—Dismal Swamp Fish, Chologaster cornutus Agassiz. Supposed ancestor of Typhlichthys. Virginia.
Fig. 84.—Blind Cavefish, Typhlichthys subterraneus Girard. Mammoth Cave, Kentucky.
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.
Fig. 85.—Four-eyed Fish, Anableps dovii Gill. Tehuantepec, Mexico.
In the eyes of the fishes there is no lachrymal gland. True eyelids no fishes possess; the integuments of the head pass over the eye, becoming transparent as they cross the orbit. In some fishes part of this integument is thickened, covering the eye fully although still transparent. This forms the adipose eyelid characteristic of the mullet, mackerel, and ladyfish. Many of the sharks possess a distinct nictitating membrane or special eyelid, moved by a set of muscles. The iris in most fishes surrounds a round pupil without much power of contraction. It is frequently brightly colored, red, orange, black, blue, or green. In fishes, like rays or flounders, which lie on the bottom, a dark lobe covers the upper part of the pupil—a curtain to shut out light from above. The cornea is little convex, leaving small space for aqueous humor. In two genera of fishes, Anableps, Dialommus, the cornea is divided by a horizontal partition into two parts. This arrangement permits these fishes, which swim at the surface of the water, to see both in and out of the medium. Anableps, the four-eyed fish, is a fresh-water fish of tropical America, which swims at the surface like a top-minnow, feeding on insects. Dialommus is a marine blenny from the Panama region, apparently of similar habit.