234 
PALLIAL AND DORSAL EYES — STRUCTURE AND ORIGIN. 
enclosed by six cylindrical pigment cells and a cuticnlar cornea, with 
bliform interstitial cells interposed between the oniinatidia; or, as 
in Pecten, the pallial eyes by specialization may become of very com- 
plicated and elaborate strnctnre, a subepithelial cellular lens which 
originates fi-om the emlnyonic ectoderm covering the eye and the optic 
nerve spreading out around and above the retina, or as in the dorsal 
eyes of Onclihlium, piercing the retina posteriorly, the iioint of entry 
of the retinal rods forming the “ blind spot,” as in the Vertebrate 
Fig. 401. Fig. 4G2. Fig, 403. 
Sections through the Pallial and Dor.sal eyes of Area, Pveten and Onchidium, to illustrate 
their differences in organization from that of the cephalic eyes. 
Fig. 401. — The composite eye of Area harbata, higlily magnified (adapted from Rawilz), 
showing its complex structure. i,c. interstitial cells ; p.c. pigmented cells ; r.c. retinal cells, with 
rod-like bodies ; r.r, retinal rods. 
Fig. 462. — The pallial eye of Peeten, highly magnified (after Patten), showing the diffusion of the 
nerve fibres above the retina, e. cornea ; ep. pigmented epithelium ; g.e, ganglion cells ; 1. lens ; 
m.t. muscular and connective tissue with blood spaces; op.n. optic nerve ; r. retina ; r.r. retinal 
rods; se. tapetum, pigmented epithelium and sclerotica. 
Fig. 463. — The dorsal Onchidium vemiculatmn, highly magnified (after Semper), showing 
the piercing of the retina by the optic nerve and the consequent formation of the “blind spot” 
and the distribution of the nerve fibres above the retina, c. cornea ; 1. lens ; op.n. optic nerve ; 
*>. pigment layer ; r. retina. 
eye, the resemblance tn which ami the divergence from the molluscan 
cephalic eye being, in both forms, further emidiasized by the visual 
rods being turned towards the body away from the light, and the 
retina generally being reversed in its arrangement to that obtaining 
in the cephalic eye. 
Pallial eyes, with few exceptions, a2)pear to be especially develo})ed 
amongst littoral species, and the more recent relimpiishment by 
I ) reh^nnsla , in comparison with the Naiads and other more ancient 
tluviatile forms, of a littoral for a Huviatile life has been held to 
account for the more perceptible presence of the pigmented visual 
cells with highly refractive cuticle, which are concentrated on the 
siphonal margin of that species. 
In addition to and independent of the more or less definite iconoptic 
vision by the more specialized eyes of the mollusk, the integument 
of many species also possesses a certain degree of Permatoptic 
