122 THE GASTROPODA 
Marginellidae and Sullia; in subterranean Pulmonates, such as 
Caecilianella and Helix hauffeni; im abyssal Gastropods, such as 
Lepeta, Propilidium, Bathysciadium, Puncturella, Cocculina, a species of 
Eulima, Choristes, Oocorys, some species of Fossarus, Addisonia, a 
species of Chrysodomus, Plewrotoma nivalis, Bathydoris, and Gonieolis ; 
in internal parasites, such as the Entoconchidae and Ento- 
siphon ; among pelagic Gastropoda in Janthina and the “ Pteropoda.” 
Fic. 101. 
Eye of Pterotrachea. A, the whole left eye. I, retractor muscle; II, optic nerve; III, 
carina ; IV, the pellucida, or inner cornea, lacerated in order to show the lens; V, lens; VI, 
outline of the pellucida. B, sagittal section of the deeper part of the eye. I, lens; II, 
vitreous body ; III, limiting membrane; IV, retina; V, carina; VI, optic nerve; VII, retinal 
rods on their stands; VIII, retractor muscle ; IX, pigmented epithelium. (After Grenacher.) 
in addition to the cephalic eyes, certain species of Oncidiidae 
(Peronia) possess a large number of pallial eyes, situated on tubercles 
on the dorsal surface. Structurally these organs are character- 
ised by the fact that the retinal cells are reversed and their free 
ends are directed towards the interior of the body, the optic nerve 
traversing the retina, just as is the case in the vertebrate eye. The 
optic cavity is filled by a crystalline lens formed of a few large 
transparent cells. Another example of a pallial eye is found in 
Cerithidea obtusa ; in this case it is single, situated in the interior of 
