428 W. H. GASKELL. 



It shows between the layer of square-shaped, non- nucleated 

 masses {r), which I consider to be the terminations of the 

 nerve-end cells, and the layer of large nuclei [v) of the anterior 

 wall, a more or less definite layer of small cells with distinct 

 nuclei (o.w.). 



In the diagram (fig. 28, PI. XXVIII) I have depicted the 

 different elements which I see in my sections as they would 

 appear if the eye were restored ; and without making any 

 positive statement upon the meaning of the smaller nuclei in 

 the anterior wall, whether they are connective-tissue elements, 

 or form a distinct layer as figured by Patten (26), it seems to 

 me clear that the large nuclei belong to the hypodermal layer 

 of cells known by the name of the vitreous layer in the Arthro- 

 pod type of eye. 



According to the views of Patten (26, p. 165), the '' ground 

 plan of all the variations in the eyes of both Molluscs and 

 Arthropods is a three-layered eye consisting of an invaginated 

 optic vesicle, the inner wall of which becomes the retina, and 

 an overlying layer of hypodermis, the corneagen." 



According to this view the pineal eye would consist as in 

 the diagram of the following layers : 



1. (R) The inner or posterior wall of the optic vesicle or 

 retina, composed of the large nerve-end cells with their asso- 

 ciated pigment. 



2. (oc.) The cavity of the optic vesicle, containing the termi- 

 nations of the nerve-end cells with their attached rhabdites. 



3. {ow.) The outer or anterior wall of the optic cavity, com- 

 posed of a thin layer of small cells. 



4. (v) The overlying layer of hypodermis or corneagen. 



If, on the other hand, the small cell-elements be looked 

 upon as intrusive connective tissue, as described by Lankester 

 and Bourne (21) in the central eye of Limulus, then the pineal 

 eye would be described as consisting of a retina with nerve-end 

 cells associated with pigment and bearing rhabdites near their 

 terminations, and in front of the retina the vitreous layer of 

 large hypodermal cells, so that the eye would be included in 

 the group of diplostichous eyes as defined by these authors. 



