SUMMARY OF AMBLYOPSID^. 



145 



SUMMARY OF THE EYES OF THE AMBLYOPSID£. 



i. There are at least 8 species of "blind fishes," Amblyopsicke, inhabiting 

 North America; 3 with well-developed eyes and 5 with mere vestiges. 



2. The 5 species with vestigial eyes are descended from 3 generically distinct 

 ancestors with well-developed eyes. 



3. The genera can be more readily distinguished by the structure of their eyes 

 than by any other characteristic. 



4. The most highly developed eye is much smaller and simpler than the eye of 

 normal-eyed fishes. 



5. The structure of their eyes may be represented by the following key to the 

 genera and species of Chologaster: 



a. Vitreous body and lens normal, the eye functional. No scleral cartilages. 

 Eye permanently connected with the brain by the optic nerve. Eye mus- 

 cles normal. No optic-fiber layer. Minimum diameter of the eye 700 fi. Chologaster 

 b. Eye in adult more than 1 mm. in longitudinal diameter. Lens over 0.5 

 mm. in diameter. Retina very simple, its maximum thickness 83.5 

 p in the old; the outer and inner nuclear layers consisting of a 

 single series of cells each; the ganglionic layer of isolated cells. 

 Maximum thickness of the outer nuclear layer 5 /*; of the inner 

 layer 8 /j.. . . . . . . . . . comutus 



bb. Eye in adult less than 1 mm. in longitudinal diameter. Lens less than 



0.4 mm. Outer nuclear layer composed of at least 3 layers of 



cells; the inner nuclear layer of at least 3 layers of cells, the former 



at least 10 /j. thick, the latter at least 18 (J.. 



c. Pigment epithelium 65 fi thick in the middle-aged, 102 in the old . papilliferus 



cc. Pigment 40 (*■ thick in the middle-aged, 74 in the old ; 24-30 per 



cent thinner than in papilliferus. Eye smaller . . . agassizii 



aa. The eye a vestige, not functional ; vitreous body and lens mere vestiges ; the 

 eye collapsed, the inner faces of the retina in contact; maximum diame- 

 ter of eye about 200 fx.. 



d. No scleral cartilages; no pigment in the pigment epithelium; 

 a minute vitreal cavity ; hyaloid membrane with blood- 

 vessels. Pupil not closed. Outer nuclear, outer reticu- 

 lar, inner nuclear, inner reticular, ganglionic, and pig- 

 ment epithelial layers differentiated. Cones probably none. 

 No eye muscles. Maximum diameter of eye 180 p. Eye 

 probably connected with brain throughout life . . Typhlichtkys 



dd. Scleral cartilages; pigment in the pigment epithelium ; vitreal 

 cavity obliterated ; no hyaloid membrane. Pupil closed. 

 Some of the eye muscles developed. No outer reticular 

 layer. Outer and inner nuclear layers merged into one. 

 Eye in adult not connected with the brain. 

 e. Pigment epithelium well developed; cones well developed; 

 ganglionic cells forming a funnel-shaped mass through 

 the center of the eye. Pigment epithelium over the 

 front of the eye without pigment. Maximum diameter 

 of eye about 200 /j- ..... Amblyopsis 



ee. Pigment epithelium developed on distal face of the eye, 

 rarely over the sides and back. No cones. Nuclear 

 layers mere vestiges ; the ganglionic layer restricted to 

 the anterior face of the eye just within the pigmented 

 epithelium. Maximum diameter of eye about 85 p . Troglichlhys 



