122 



THE GASTROPODA 



Marginellidae and Bullia; in subterranean Pulmonates, such as 

 Caecilianella and Helix hauffeni ; in abyssal Gastropods, such as 

 Lepeta, Propilidium, Bathysciadium, Puncturella, Cocculina, a species of 

 Eulima, Choristes, Oocwys, some species of Fossarus, Addisonia, a 

 species of Chrysodomus, Pleurotoma nivalis, Bathydoris, and Gonieolis ; 

 in internal parasites, such as the Entoconchidae and Ento- 

 siphon -, among pelagic Gastropoda in Janthina and the " Pteropoda." 



Fio. 101. 



Eye of PUrotrachea. 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. 11, 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 .Grenadier.) 



In addition to the cephalic eyes, certain species of Oncidiidae 

 (Peronui) 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 



