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 Pecten (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 
Fic. 217. 
Sagittal section of the pallial eye of Pecten pusio. em, complementary optic nerve; co, 
cornea ; /, crystalline lens; o.n, optic nerve ; 0.p, optic peduncle ; p.e, pigmented epithelium ; 
p.l, pigmented layer ; re, retina; r.n, retinal nerve ; ro, rods ; 0, septum ; ta, tapetum. (After 
Rawitz.) 
or inferior mantle lobe, and they are of different sizes and irregularly 
arranged. ach eye is borne ona 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 (F ig. 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 
