﻿SUMMAEIAL ACCOUNT OF THE EYE OF THE AMBLYOPSID^. 177 



dioptric apparatus disappeared rather suddenly, and the eye, as a consequence, 

 collapsed with equal suddenness in those members which, long ago, took up their 

 abode in total darkness. The eye not only collapsed, but the number of elements 

 decreased very much. The reduction was in the horizontally repeated elements. 

 The vertical complexity, on which the function of the retina really depends, was not 

 greatly modified at first. 



In those species which took up their abode in total darkness the degeneration 

 in the dioptric apparatus was out of proportion to the degeneration of the retina, 

 while in those remaining above ground the retinal structures degenerated out of 

 proportion to the changes in the dioptric apparatus, which, according to this view, 

 degenerates only under conditions of total disuse or total darkness which would 

 necessitate total disuse. This view is upheld by the conditions found in Typhlo- 

 gobius, as Ritter's drawings and my own preparations show. In Typhlogobius 

 the eye is functional in the young and remains a light-perceiving organ throughout 

 life. The fish live under rocks between high and low tide. We have here an 

 eye in a condition of partial use and the lens is not affected. The retina has, 

 on the other hand, been horizontally reduced much more than in the Amblyopsidae, 

 so that, should the lens disappear, and Ritter found one specimen in which it was 

 gone, the type of eye found in Troglichthys would be reached without passing 

 through a stage found in Amblyopsis ; it would be simply a horizontal contracting 

 of the retina, not a collapsing of the entire eye. 



The question may with propriety be asked here : Do the most degenerate eyes 

 approach the conditions of the pineal eye ? It must be answered negatively. 



RESULTS OF THE PHYLETIC DEGENERATION ON THE DIFFERENT PARTS OF THE 



EYES OF THE AMBLYOPSID.E. 



The different structures of the eye may now be taken up in detail. 



(a) The eye muscles are normally developed in Chologaskr. They are present 

 to a greater or less extent in Amblyopsis. They have been reduced in number in 

 Troglichthys, where the half nearest the eye has been replaced by bundles of fibrous 

 tissue. In Typhlichthys they have vanished. 



(6) The scleras of the different members are not comparable on account of the 

 presence of cartilage in some species and not in others. Both this layer and the 

 choroid are insignificant in Chologastcr and Typhlichthys. In Amblyopsis cartilages 

 different in size and number are found anywhere about the eye, being frequently 

 present in shape and position to suggest a displaced lens. In thickness the cartilages 

 are disproportionate to the size of the eye. In Troglichthys we have a still more 

 evident misfit, for the scleral cartilages are both too long and too thick. Evidently 

 the scleral cartilages have not decreased in size in the same ratio as the eye, or, 

 what amounts to the same thing, they develop beyond the present needs of the 

 eye. (See also Lucifuga.) 



(c) The choroid is thin in all cases except where pigment cells are situated. 

 These are frequently several times as thick as the rest of the choroid. In Ambly- 

 opsis the pigmentation of the choroid is inversely proportional to the pigmentation 

 of the retina. 



(d) The lens has already received sufficient attention. It is merely necessary to 

 insist again that, as long as an eye is functional to any extent, the lens — in fact the 

 dioptric apparatus in general — does not degenerate and that when absolute disuse 



