SARGENT : THE OPTIC KEFLEX APPARATUS OF VERTEBRATES. 225 



is sedentary in its habits, and more dependent upon tactile than visual 

 sensations for obtaining its food, the apparatus is degenerate and lieiss- 

 ner's fibre inconspicuous. 



3. Effect of Degeneration of the Eye on. the Apparatus. 



The eifect of disuse of the eye, and the consequent degeneration of 

 the optic reflex apparatus, is still more evident in those vertebrates 

 which are blind or nearly so. For use in this study Prof. C. H. Eigen- 

 mann has kindly given me access to his extensive collection of prepara- 

 tions of cave vertebrates having degenerate eyes. Eigenmann ('99) has 

 described the degenerate eyes of four species of Amblyopsidae, a group 

 of small cave-inhabiting teleosts. In Chologaster papilliferus, in which 

 the eyes, though functional, are much reduced, having a maximum 

 diameter of 800 micra, I find a corresponding reduction in the optic 

 reflex apparatus. Reissner's fibre is with difliculty distinguishable, hav- 

 ing an approximate diameter of 0.5 micron. The torus is small, and in 

 a state of arrested development. In the nearly related Fundulus, which 

 is of the same size, and has normal eyes, the fibre has a diameter of one 

 micron. 



The three other species described by Eigenmann are totally blind and 

 the whole optic apparatus, peripheral and central, degenerate. In 

 Amblyopsis spelaeus, the vestigial eye has, according to Eigenmann 

 ('99), a maximum diameter of 200 micra; in Typhlichthys, 180 micra; 

 in Troglichthys, 85 micra. In the two latter species I have been unable 

 to distinguish the slightest trace of Reissner's fibre. Ramsey (:0l) has 

 investigated the optic lobes and optic tracts of Amblyopsis, and finds 

 them distinctly degenerate. The optic lobes are reduced greatly in size 

 and the tectum is very thin (Fig. G). In Amblyopsis I find Reissner's 

 fibre to be an exceedingly delicate filament which, in young specimens 

 6 to 10 mm. long, has a diameter of 0.1 micra. In larger specimens, 

 30 mm. long, the diameter was 0.8 mm. 



In Amblyopsis the eye reaches a certain stage of development and then 

 " with advancing age undergoes a distinct ontogenetic degeneration from 

 the mature structure" (Eigenmann, '99). The presence of the rudiment 

 of Reissner's fibre may be merely the persistence of a well-established 

 structure, which in older individuals disappears ; or it may be made up 

 wholly of the axons which have their origin in the olfactory centres of 

 the ganglia habenulae, and have been described by me in other species. 



The Mauthner's fibres in Amblyopsis are of relatively large size, having 

 a diameter aVjout twice as great as in Fundulus. These fibres run from 



