i8 



THE CRUSTACEA 



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

 apparatus, but a refractive body, or lens (I), is sometimes formed 

 on the outer side of the retinal cells, while 

 in the Copepoda, where the median eye may 

 undergo considerable modification, cuticular 

 lenses and other accessory structures may be 

 developed. 



The compound eyes show considerable 

 agreement in the details of their structure 

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

 of a varying number of orrvmatidia or visual 

 elements, covered by a transparent region 



, . . , , , . , . a 



of the cuticle, the cwnea, which is usually 

 divided into lenticular facets. Typically 

 eacn ommatidiura has the structure shown in 



. 



the accompanying figure. Immediately under 

 the cuticle lie a pair of wrneagen cells (by), 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 crystalline body (cr), 



FIG. 12. 

 Horizontal section through 



the median eye of Cypris. 



(After ciaus.) Only two of 



P, pigment; r, rod-like bodies 



contained in the retinal cells. 



B. 



m. 



FIG. 13. 



A, horizontal section of the eye and ocular peduncle of Bmnfhiptt*. B, four onimatidia of 

 same further enlarged, b, basement membrane ; c, corneal cuticle, which in this case is not 

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

 fibrils ; g, optic ganglia in the peduncle ; hy, hypodermis or corneagen cells ; m, muscle of tin; 

 peduncle ; r, retinula cells surrounding the rhabdome, which is here concealed from view by thu 

 black pigment. (After Glaus.) 



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

 retinula cells (r). These surround a rod-like body, the rhabdome, 

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



