The Scyphophori, Haplomi, and Xenomi 195 



extraordinary structure of the eye. This organ is prominent 

 and is divided by a horizontal partition into two parts, the 

 upper, less convex, adopted for sight in the air, the lower in the 

 water. The few species of Anableps are found in tropical 

 America. The species of some genera swim near the bottom, 

 but always in very shallow waters. All are very tenacious of life, 

 and none have any commercial value although the flesh is good. 



FIG. 152a. Four-eyed Fish, Anableps dovii Gill. Tehuantepec, Mexico. 



The unique structure of the eye of this curious fish has been 

 carefully studied by Mr. M. C. Marsh, pathologist of the U. S. 

 Fish Commission, who furnishes the following notes published 

 by Evermann & Goldsborough : 



"The eye is crossed by a bar, like the diameter of a circle, 

 and parallel with the length of the body. This bar is darker 

 than the other external portions of the eyeball and has its edges 

 darker still. Dividing the external aspect of the eye equally, 

 it has its lower edge on the same level as the back of the fish, 

 which is flat and straight from snout to dorsal, or nearly the 

 whole length of the fish ; so that when the body of the fish is just 

 submerged the level of the water reaches to this bar, and the 

 lower half of the eye is in water, the upper half in the air. Upon 

 dissecting the eyeball from the orbit, it appears nearly round. 

 A membranous sheath covers the external part and invests most 

 of the ball. It may be peeled off, when the dark bar on the 

 external portion of the eye is seen to be upon this, membrane, 

 which may correspond to the conjunctiva. The back portion 

 of the eyeball being cut off, one lens is found. The lining of the 

 ball consists, in front, of one black layer, evidently choroid. 

 Behind there is a retinal layer. The choroid layer turns up 

 anteriorly, making a free edge comparable to an iris. The 

 free edge is chiefly evident in the lower part of the eye. A large 

 pupil is left, but is divided by two flaps, continuations of the 

 choroid coat, projecting from either side and overlapping. 



