ORGANS OF VISWiV. 



389 



with the exterior. The cells lining the posterior face of the vesicle 

 form a retina (R); and are continuous with the fibres of the optic 

 nerve {N.op). We have no knowledge of the development of this 

 eye. 



In the Gasteropods the eye (fig 277 B) has the form of a closed 

 vesicle: the cells lining the inner side form the retina, while the outer 

 wall of the vesicle constitutes the cornea. A cuticular lens is placed 

 in the cavity, on the side adjoining the cornea. This eye originates 

 from the ectoderm, within the velar area, and close to the supra-oeso - 

 phageal ganglia, usually at the base of the tentacles. According to 

 Rabl (Vol. I. No. 268) it is formed as an invagination, the openir.g 

 of which soon closes; while according to Bobretsky (Vol. I. No. 242; 

 and Fol it arises as a thickening of the epiblast, which becoming 

 detached takes the form of a vesicle. It is quite possible that both 

 types of development may occur, the second being no doubt abbre- 

 viated. The vesicle, however formed, soon acquires a covering of 

 pigment, except for a small area of its outer wall, where the lens 

 becomes formed as a small body projecting into the lumen of the 

 vesicle. The lens seems to commence as a cuticular deposit, and 

 to grow by the addition of concentric layers. The inner wall of the 

 vesicle gives rise to the retina. 



The most highly differentiated mollnscan eye is that of the 

 Dibranchiate Cephalopoda, which is hi fact more highly organized 

 than any other invertebrate eye. 



A brief description of its adult structure' will perhaps render more clear 

 uiy account of the development. The most important features of the eye 

 are shewn in fig. 277 C. The outermost laypv of the optic bulb forms a kind 



I' 



Nop 



J^.OJJ Cop 



Fig. 277. Thukk inAGn.\MMATic sections of the eyes of Mollusca. 



(After Grenacher.) 



A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate Cephalopod. 



Pal. eyelid; Co. cornea; Co.ep. epithelium of ciliary body; Tr. iris; Int, IiitK.. 



Int*. different parts of the integument ; /. lens; l^. outer segment of lens; R. retma; 



iV.op. optic nerve ; G. op. optic ganglion ; x. inner layer of retina; xV.S. nervous stratum 



of retina. 



1 Vide Hensen, Zeit. f. mjj.s.?. ZooJ. Bd. xv. 



