ii8 



The Organs of Sense 



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

 quently brightly colored, red, orange, Ijlack, blue, or green. 

 In fishes, Uke 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, Anahleps, 

 Dialoininus, the cornea is divided by a horizontal partition into 



Fig. 86. — Ipnops murraiji 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. 

 Aiiableps, the four-eyed fish, is a fresh-water fish of tropical 

 America, which swims at the surface like a top-minnow, feeding 

 on insects. Dialommiis is a marine blennv from the Panama 

 region, apparently of similar habit. 



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



Fig. S7. — Pond-skipper, Bokophthalmus chinensis (Osbeck). Bay of Tokj'O, 

 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 previous 

 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- 



