74 



and forms tlie retina. The cavity remains open to the 

 exterior. 



In the higher gastropods the open pit is succeeded 

 by a closed vesicle, and this ontogenetic sequence is 

 probably an indication of the phylogeny. As we should 

 expect therefore, the Buccinum eye is a closed vesicle cut 

 off from the surface. The general epithelium extends 

 over it, forming an outer cornea (fig. 47, Cor.). The wall 

 of the vesicle is formed of a single layer of cells, once 

 part of the bounding epithelium of the head. These 

 cells are modified in various regions. The area immed- 

 iately below the outer cornea, through which the light 

 has to pass, is formed of cells free from pigment — the 

 area itself is the inner cornea (fig. 47, Ps. cor.). The rest 

 of the vesicle wall forms the retina and is made up of 

 pigment-containing cells. 



The cells of the outer cornea differ only from 

 those of the epidermis in shape. As the optic 

 vesicle is approached, the deep and narrow epidermal 

 cells with slender nuclei become less deep and of 

 greater thickness. There is a well-developed cuticular 

 margin present. With this change there is an alteration 

 in the form of the nucleus, a spherical shape taking the 

 place of the slender compressed form. The optic vesicle 

 is imbedded in a mass of connective tissue (fig. 47, 

 Con.) and muscle fibres, in which numerous blood 

 spaces occur irregularly. This tissue, which is usually 

 more compact near the surface, is continued in so as to 

 form a layer between the vesicle and the outer cornea. 



The cells of the optic vesicle appear to be of 

 two kinds, and only two kinds of cells make up the 

 entire vesicle — retinal and corneal areas inclusive. In 

 sections, the retina, or area directly opposite the cornea, 

 is built up of very large cells, which can be distinctly 



