THE CRUSTACEA 



arising from the brain. In many cases there is no special refracting 



apparatus, but a refractive body, or lens (J), is sometimes formetl 



on the outer side of the retinal cells, while 



I. in the Copepoda, where the median eye may 



undergo considerable modification, cuticular 



lenses and other accessory structures may be 



developed. 



1. The compound eyes show considerable 



agreement in the details of their structure 



with those of Insects (Fig. 13). They consist 



Fig. 12. of a varying number of ommatidia or visual 



Horizontal section throuKh elements, covcred bv a transparent region 



the median eye of Ci/pris. . , . , , , . , . ° ,, 



(After ciaus.) Only t\Vo of 01 the cuticie, the covuea, Avhich IS usually 

 rr'^rt'ie^rr™ divided into lenticular facets. Typically 

 1-, pigment; r rod-uke bodies each ommatidium has the structure shown in 



contained m the retinal cells. • n t t i i 



the accompanying figure, immediately under 

 the cuticle lie a pair of corneagen cells {hy), by Avhich the cuticular 

 lens is secreted and renewed on ecdysis. Below these are a group, 

 generally two or four, of cells forming a refractive crijstalline body (cr), 



B. 



Fig. 13. 



A, horizontal section of the i^ye and ocnlar peihincle of Branchipns. B, four ommatidia of 

 same further enlarged, b, baseint-nt mrmbrane ; c, corneal cuticle, which in this case is not 

 thickened to form lenses ; cr, ciystalliiip body ; cr.e, cells of the crystalline body ; /, nerve- 

 fibrils ; g, optic ganglia in the jipduncle : Ini, hypodermis or corneagen cells : m, muscle of the 

 I)eduncle ; r, retinula cells surrounding the rhabdome, which is here concealed from view by the 

 black pigment. (After Glaus.) 



the lower end of which is embraced by the tips of the elongated 

 retmula cells (r). These surround a rod-like body, the rhahdomc, 

 of cuticular nature but penetrated by nerve -fibrils, and usually 



