726 



THE EYE IN EVOLUTION 



negatively lohototactic and if exposed in a well-lit pool will immediately seek refuge 

 and hide under rocks. In Amblyopsis the eye lies deeply under the surface, the lens 

 is vestigial or absent, the iris is represented by a pigment-free membrane and the 

 retina contains only a few ill-formed cones. Similar rudimentary eyes are found in 

 the two other related genera, Typhlichthys and Troglichthys ; and in the only non- 

 cavernicolous representative of this family, Chologaster, which inhabits the swamps of 

 Kentucky and Tennessee, the eyes which lie under a patch of pigment-free epidermis 

 are reduced rather than degenerate ; the fish does not dei^end on its eyes, however, 



for detecting or securing its prey or for avoiding obstacles. 

 Although possessed of ears, experiments have shown 

 that the sense of hearing of the Amblyopsidse is limited ; 

 the tactile sense is the one on which they rely to find and 

 locate their food for which purpose they are provided 

 with numerous tactile ridges princijaally in the region of 

 the head (Eigenmann, 1909). 



Some goby fishes (Gobiidte), particularly the 

 " sleepers " living on muddy bottoms or in crevices, also 

 have degenerate eyes. Typhlogohius calif orniensis, a blind 

 fish which CO -habits rocky crevices on the Californian 

 coast with a blind species of shrimp on which it depends 

 for food, has relatively normal eyes in the larval stage 

 which become small, functionless and rudimentary in the 

 adult, lying under the thick skin (Ritter, 1893) ; they 

 lack tapetum, cones and vitreous, while, curiously, the 

 lens may be either very large indeed or absent. It is as 

 if a brave strviggle were made to collect what light there 

 is or, alternatively, the attempt has been abandoned. 

 Trypauchen and Trypmichenophrys, littoral crevice-dwellers in Japan, and other limico- 

 line gobies as Austrolethops, and the sole, Typhlachirus, have similarly minute or 

 rudimentary eyes (Fig. 868) (Franz, 1910-34). Undersized eyes are also usual in the 

 fresh-water fishes which inhabit silty rivers such as are common in the great plains of 

 America ; only occasionally, as in Lake Balaton in Western Hungary, an immense 

 shallow lake the waters of which are so turbid as to be virtually ojiaque, is an effort 

 made to increase the sensitivity of the eye by the liberal dejoosition of guanine in an 

 unusually well-developed tapetum (Wunder, 1926-30). 



AMPHIBIANS. Amongst the Urodeles, the salamanders which hve a 

 secretive existence in shallow water, in caves, in mud and under flat stones, 

 have little use for eyes. These organs are well differentiated in the larvse 

 but regress at metamorphosis (Zeller, 1888) (Fig. 869) ; as we have already 



Fig. 868. — The Eye of the 

 Goby Fish, TRi-PAUCHEy 



WAK.E. 



The eye is rudimentary 

 and functionless (after 

 Franz). 



Fig. 869. — -The Olm, Proteus a.xgujaus (Zool. Soe., London). 



