OCELLI. 



147 



the other hand, the Image wouhl be pictured in greater 

 detail by the eye A. 



Ct. l[p 



^^'j?'T " '"^'='"'" throngh the front C^) and hinder (fl) dorsal eyes of Epeira 

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

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

 cular fibres ; M, M', cross sections of ditto ; St, rods ; Pg, P', pigment cells ; 

 i. lens j ak\ vitreous body ; Kt, crystalline cones ; Et, retina ; Nop, optic nerve. 



Speaking generally, an ocellu.s 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 cell-s 

 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 tltem. It will be at 



