198 



THE EYE IN EVOLUTION 



Figs. 186 to 188. — The Common Garden Snail, Helix aspebsa. 



Fig. 186. — The two eyes arc situated on the tijD of each of the long posterior 



horns. 



Fig. 



187 — The eye at the tip of the ex- 

 panded horn. 



Shell of Pterocera 



Fig. 188.— The eye (E) retracted into 

 the horn. The horn invaginates like 

 the inturned finger of a glove ; the 

 obliquity of this section gives the 

 appearance of a double cavity 

 (Norman Ashton). 



Prince (1955), the two vesicular eyes, which have an elaborate neural structure, ^ 

 are mounted on the tip of stalks (ommatophores) which also carry an olfactory 

 tentacle and a sensory node (Fig. 189). These, supplied with muscles arranged 

 round a central sinus, are retractile partly by muscular activity and partly by 

 fluid engorgement by heemolymph. Retraction can be slow and voluntary or 

 r;i!)id and reflex in response to stimuli such as touch, odour or the cutting off of 

 I':--"' i : the reaction is thus the opposite of that seen in the snail. It appears 

 als -hat a certain amount of convergence upon an object is possible. 



1 p. 142. 



