HO BLIND VERTEBRATES AND THEIR EYES. 



THE EYES OF THE AMBLYOPSID^E. 



The Amblyopsidae offer exceptional facilities for the study of the degeneration 

 of eyes. They furnish gradations in habits from permanent epigean species to 

 species that have for ages been established in caves. The eyes of the following are 

 considered : 



1. Chologaster cornutus Agassiz. Locally abundant in the lowland streams and swamps in 



the South Atlantic states from Virginia to Florida. Maximum length about 55 mm. 



2. Chologaster agassizii Putnam. Found in the underground streams of Kentucky and 



Tennessee. It is rare. Maximum length 62 mm. 



3. Chologaster papilliferus Forbes. Found under stones in the springs of Southwestern 



Illinois, in Union and Jackson counties. Maximum length 55 mm. 



4. Amblyopsis spelcnts De Kay. Widely distributed in the caves east of the Mississippi both 



north and south of the Ohio River. Maximum length 135 mm. 



5. Typhlichthys subterraneus Girard. Found with the latter species in the caves east of the 



Mississippi, but confined as far as known to the south side of the Ohio River. 



6. Troglichthys TOSCR Eigenmann. Found in the caves west of the Mississippi River. Maxi- 



mum length 55 mm. 



The first two species mentioned live, as far as known, altogether in terranean 

 streams; the others, altogether in subterranean streams. Chologaster has well- 

 developed eyes, the others mere vestiges. We have thus two epigean species with 

 well-developed eyes, one subterranean species with well-developed eyes, and three 

 subterranean species with greatly degenerate eyes. The three latter species are 

 descended from three distinct terranean ancestors. Amblyopsis is the only member 

 of the family possessing ventral fins, and Troglichthys has scleral cartilages which 

 are not found in the other members except Amblyopsis. 



It must be apparent that an experiment on a vast scale has been conducted by 

 nature, leaving us but to read the results. Moreover the experiment is one in 

 evolution without the assistance or intervention of natural selection. 



CHOLOGASTER PAPILLIFERUS. 



The only account of the eyes of Chologaster papilliferus Forbes, aside from the 

 measurements in the description of the species, is a note by Wright. Professor 

 Wright obtained his specimen from Prof. S. A. Forbes, and therefore had C. papil- 

 liferus. He announced that the pigment is absent in the pigmentary layer of the 

 retina of this species. But this condition was unquestionably either accidental or 

 due- to the reagents employed. Chromic acid partly or wholly removes the pig- 

 ment, leaving the cells in good condition. 



The vertical diameter of the eye in a specimen 39 mm. long is 640 [J-; in a 

 specimen 55 mm. long (the largest secured), 960 p.. The distance from the point 

 of entrance of the optic nerve to the front of the cornea is 560 fj. and 900 p., respec- 

 tively, in the two specimens. The distance from the point of entrance of the optic 

 nerve to the front of the epidermis over the eye is 600 p. in the smaller specimens, 

 i lie lens ;il ion! jbop. in diameter. For further measurements see the table, page 120. 



The eye is small when compared with that of other fishes of the same size, and 

 especially so when compared with the eyes of Zygoncctes. It is located high up 

 on the side of the head, its upper surface being nearly on a level with the top of the 

 head. It is directed outward and forward. In a specimen 35 mm. long it is 1.44 

 mm. from the tip of the snout and 0.88 mm. long. The distance between the 

 eyes is 1.60 mm. 



