Chap. XII.] 



OPTIC ORGANS. 



449 



lie in different planes it follows that light passes to 

 the rods from very various points. 



Among the Arthropoda the simplest cases are 

 seen in the larvae of various insects (Fig. 191, A) ; 



,f 



rl 



^%*t 



B 5pf Vac 



Figs. 191 B, 191 D. -Figures of Eyes of Arthropoda. 



B, A single cuticular lens of Limulus, to show the aggregation of cells to form a 

 retinula (rl). (After Laukester and Bourne.) I, lens ; rl, retinula ; op, optic 

 nerve. 



D, Part of the compound eye of Phryganen ; the retinal cells are seen to be united 

 into a retinula (r), which is differentiated into a rhabdom (z) posteriorly; 

 cc, crystalline cone; /.facet of compound eye; pg, pigment. (.After Grenadier.) 



there is a single lens, the hypodermic cells that form 

 the sensitive elements, and some of which are con- 

 tinuous with filaments of the optic nerve, are simple 

 and separate ; these cells may be called the retinal 

 cells. This condition may, as in Limulus (Fig. 191 B), 

 be complicated by the cells, instead of remaining 

 separate from one another, becoming aggregated into 

 DD 16 



