OCELLI. 



147 



the other hand, the image would be pictured in greater 

 detail by the eye A. 



Fig. 99.— Long section through the front (^) and hinder (S) dorsal eyes of Epeira 

 diadema (after Grenacher). A, Anterior eye; B, posterior eye; Hp, hypoderm ; 

 Ct, cuticle ; ct, boundary membrane ; K, nuclei of the cells of the retina ; M, mus- 

 cular fibres ; 31, M\ cross sections of ditto ; .S"^, rods ; Pg, P\ pigment cells ; 

 L, lens ; Gk\ vitreous body ; Kt, crystalline cones ; Rt, retina ; Xojj, optic nerve. 



Speaking generally, an ocellus may be regarded as 

 consisting of — 



1. A lens, forming part of the general body covering. 



2. A layer of transparent cells. 



3. A retina, or second layer of deeper lying cells, 

 each of which bears a rod in front, while their inner 

 ends pass into the filaments of the optic nerve. 



4. The pigment. 



From the convexity of the lens it would have a 

 short focus, and the comparatively small number of 

 rods would give but a very imperfect image, except 

 of very near objects. 



But though these eyes agree so far with ours, there 

 is an essential difference between them. It will be at 



