SUMMARIAL ACCOUNT OF THE EYE OF THE AMBLYOPSID^E. 175 



GENERAL SUMMARIAL ACCOUNT OF THE EYES OF THE AMBLYOPSIDjE. 



As in all organs no longer of use or hindrance, and therefore no longer under 

 the control of selection, the individual variations in the structure of the eye of 

 Amblyopsis, Troglichthys, and Typhllchthys are very great. 1 There is also 

 a marked change in the eye with age. It is therefore necessary to distinguish be- 

 tween individual variations and stages in ontogenetic and phylogenetic degeneration. 

 The eye of each species has a general structure which is typical for the species. 

 The individual variations have been sufficiently described under the respective 

 species. 



PHYLETIC DEGENERATION OF THE EYE OF THE AM BLYOPSID/E. 



The steps in degeneration in the Amblyopsidae are indicated in figure 66. The 

 most highly developed eye is that of Chologastcr papilliferus. The parts of this 

 eye are well proportioned, but the eye as a whole is small, measuring less than 

 i mm. in a specimen 55 mm. long. The proportions of this eye are symmetrically 

 reduced if it has been derived from a fish eye of the average size. The retina is 

 much simpler than in Zygonectes. The simplifications in the retina have taken 

 place between the outer nuclear and the ganglionic layers. The pigment layer 

 has not been materially affected. These facts are exactly opposed to the 

 supposition of Kohl that the retina and the optic nerve are the last to be affected, 

 and that the vitreous body and the lens cease to develop early. In Chologaster 

 papilliferus (b) the latter parts are normal, while the retina is simplified. That 

 the retina is affected first is proved beyond cavil by cornutus (a). The vitreous 

 body and the lens are here larger than in papilliferus, but the retina is very greatly 

 simplified. Cornutus, it must be borne in mind, lives in the open. The eye of 

 Chologaster agassizii (c) differs from that of papilliferus largely in size. There 

 is little difference in the retinas except the pigmented layer, which is about 26 

 per cent thinner in agassizii than in papilliferus. 



If we bear in mind that no two of the eyes represented here are members of 

 a phyletic series, we may be permitted to state that from an eye like that of cornutus, 

 but possessing scleral cartilages, both the eyes of Amblyopsis and Troglichthys 

 have been derived, and that the eye of Amblyopsis represents one of the stages 

 through which the eye of Troglichthys passed. The eye of Amblyopsis (h) is the 

 eye of Chologaster cornutus minus a vitreous body with the pupil closed and with 

 a minute lens or more probably none at all. The nuclear layers have gone a step 

 farther in their degeneration than in cornutus, but the greatest modification has taken 

 place in the dioptric apparatus. 



In Troglichthys (z) even the mass of ganglionic cells present in the center of 

 the eye as the result of the collapsing after the removal of the vitreous body has 

 vanished. The pigmented epithelium, and in fact all the other layers, are repre- 

 sented by mere fragments. 



The eye of Typhllchthys (g) has degenerated along a different line. There is 

 an almost total loss of the lens and vitreous body in an eye like that of papilliferus 



1 This is also true of the eye of Luci/uga and Stygicola. 



