u8 



The Organs of Sense 



round pupil without much power of contraction. It is fre- 

 quently 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 



FIG. 86. Ipnops murrayi Giinther. 



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. 



In one genus of deep-sea fishes, Ipnops, the eyes are spread 



FIG. 87. Pond-skipper, Boleophthalmus chinensis (Osbeck). Bay of Tokyo, 

 Japan; from nature. K. Morita. (Eye-stalks shrunken in preservation.) 



out to cover the whole upper surface of the head, being modi- 

 fied as luminous areas. Whether these fishes can see at all is 

 not known. 



The position of the optic nerves is described in a previoiis 

 chapter. 



In ordinary fishes there is one eye on each side of the head, 

 but in the flounders, by a distortion of the cranium, both ap- 



