. A guide to the study of fishes. Fishes; Zoology; Fishes. 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 A)iablcps 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 haA'e any commercial value although the flesh is Fig. 152a.—Four-eT. ed Fi^h, inahhpb doi i Gill Tehuautcpec, Mex
. A guide to the study of fishes. Fishes; Zoology; Fishes. 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 A)iablcps 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 haA'e any commercial value although the flesh is Fig. 152a.—Four-eT. ed Fi^h, inahhpb doi i Gill Tehuautcpec, Mexico. The unique structure of the eye of this curious fish has been carefully studied by Mr. j\I. 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
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