MOLLUSCA 



199 



In Onchidium, a naked littoral Pulmonate which creeps on rocks near the 

 high-water mark, a unique type of vesicular eye with an inverted retina is found 

 arranged on papillae scattered over the skin of the back in groups of six or up 

 to a total which inay reach a hundred (Fig. 122).^ 



An interesting elaboration is seen in some marine heteropods {Carinaria, 



tCMBORY NOOC 



NAOIAL MU«CL£ 



Carinaria 



RITIOULO-CHOOTHELIAL SYCTIM. 



Fig. 189. — The Stalked Eye of the Spider-shell, Pterocera lambis. 



Showing the sensory tentacle, sensory node, sinus, and muscular systems 

 (after J. H. Prince). 



Pterotrachea) which ha\e tubular eyes containing a large spherical lens; the 

 available visual field is increased by the provision of lateral " windows " wherein 

 pigment is lacking, opposite which the posterior retina is prolonged up the side 

 of the eye. Pterotracliea coronata which swims with its belly in the air has an 

 eye at the extremity of each of its two tentacles ; images in front are focused 

 on the posterior retina by the enormovis lens, while movements and changes in 



1 p. 148. 



