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 larvse of various insects (Fig. 191, A); 



,f 



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



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



retinula (rt). (After Lankester and Bourne.) I, lens; rZ, retinula; op, optic 



nerve. 

 D, Part of the compound eye of Pliri/r/anea ; the retinal cells are seen to bo united 



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



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



there is a single lens, the hypodermic cells that form 

 the sensitive elements, and some of which are con- 

 tinuous with filaments of the optio 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 

 D D 16 



