The Organs of Sense 



117 



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 Missis- 

 sippi 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 



FIG. 85. Four-eyed Fish, Anableps dovii Gill. Tehuantepec, Mexico. 



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. 



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 char- 

 acteristic of the mullet, mackerel, and lady-fish. 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 



