RUDIMENTARY EYES 



731 



with simple but massive rods, and the two nv;clear layers and the ganglion cells are 

 represented by a few rows of si:)arse cells (Kohl, 1892 ; Hanke, 1912 ; Engelhardt, 1924). 



Among Reptiles, burrowing snakes and lizards come into the same 

 category. Within the group of snakes (Ophidia) the lowest types are the 

 T\^3hlopidse. blind subterranean burrowers usually smaller than earth- 

 worms which occur in most of the w^armer parts of the earth. The eyes are 

 tiny and vestigial. It would seem that when the snakes originally went 



FiC4. 877. — The Eye and Orbit of the Blind Snake, Ttphlops. 



The globe is minute, less than 1 iTim. in diameter. The heavily pigmented uvea, a, 

 and the ill-formed retina, r, are well seen. Most of the cavity of the eye is taken 

 up by the large lens. Anterior to this is the tenuous cornea, the enclosed conjunctival 

 sac and the dermal " sijectacle." m is a tenuous extra-ocular muscle (0"Day). 



underground the eyes became vestigial, and when they again emerged from 

 the ground the eye had to be reconstructed, but those of this primitive 

 species retained their simple form (Walls, 1942)^. 



The eye of Typhlops, a blind snake widely distributed in the Southern Hemisphere 

 and South East Europe, which lives on worms and insects obtained by burrowing, has 

 a rudimentary uvea and a small embryonic cellular lens ; the retina contains few and 

 rudimentary visual cells and insignificant nuclear and ganglion-cell layers while a 

 central area is lacking (Kohl, 1892) (Fig. 877). A similarly primitive eye is seen in 

 Typhlops lumbricalis, a blind snake seen in the West Indies and Guiana (Muhse, 1903), 

 and in the uropeltid snake, Rhinophis (Baumeister, 1908). 



1 p. 383. 



