THE LAMELLIBRANCHIA 



239 



they are compound or faceted eyes, each element of which is a 

 pigmented cell or ommatidium with a cuticular cornea. In Lima 

 excavata (L. hians and L. loscombii have no eyes) there are from 

 eighteen to twenty-three eyes on the border of each mantle flap, 

 consisting of very deep pigmented fossae, at the bottom of each of 

 which there is a layer of rods and a refractive body. 



In Peden (with the exception of abyssal species) and Spondylus 

 the eyes have a more complicated structure : they are isolated and 

 always in larger number on the left or superior than on the right 



on, 



FIG. 217. 



Sagittal section of the pallial eye of Pecten puslo. c.n, complementary optic nerve ; co, 

 cornea ; I, crystalline lens ; o.n, optic nerve ; o.p, optic peduncle ; p.e, pigmented epithelium ; 

 p.l, pigmented layer ; re, retina ; r.n, retinal nerve ; ro, rods ; o, septum ; to, tapetum. (After 

 Rawitz.) 



or inferior mantle lobe, and they are of different sizes and irregularly 

 arranged. Each eye is borne on a short tentacle projecting from 

 the internal duplicature of the mantle border (Fig. 235, e) and its 

 essential structure is that of a sub-epithelial ocular globe. The more 

 superficial moiety of the ocular wall forms the retina in such fashion 

 that the transparent retinal elements have their free extremities 

 turned towards the interior of the globe (Fig. 217, re), and each is 

 capped by a cuticular rod. The deeper moiety of the ocular wall, 

 as well as the part of the tentacle surrounding it, is pigmented. In 

 the interior of the ocular cavity there is a refringent layer the 



