278 COMPARATIVE ANATOMY 



Cyclostomes. The eye of Cyclostomes is at a very low stage 

 of development, and has evidently undergone partial degeneration : 

 this is indicated, not only by the structure of the retina, but also, 

 in Myxinoids, in which the choroid fissure persists, by the absence 

 of the lens, iris, and of a differentiated sclerotic and cornea as well 

 as of eye-muscles. Moreover, the eye in Myxinoids and in the 



" Eyes " Dorsal zone oj " optic cells " Anterior pigment-spot 



A 



Pig 



FIG. 204. A. THE ANTERIOR PORTION OF Amphioxm lanceolatu*. 

 (Modified from H. Joseph. ) 



B. ONE OF THE EYE-LIKE ORGANS IN A. (After H. Joseph.) 



GK, capsule, with neuroglia elements ; K, nucleus of optic cell ; A r , its nerve- 

 process, and f, its granular border ; Pig, pigment-cell, showing three layers ; 

 St, striated border of optic cell. 



larval Petromyzon lies beneath the skin and subdermal connec- 

 tive tissue, but in the latter the skin covering the eyes becomes 

 thinned out, and thus the animal, which was blind, or nearly blind, 

 in the larval state, can see after undergoing metamorphosis : at the 

 same time the eye becomes more highly organised, though the 

 primary lumen in the lens (cf. p. 274) does not entirely disappear. 



