138 COMPARATIVE ANATOMY 



nerve. The external cuticle may either be flat and continuous, 

 as in Apus, or may be divided into as many facets as there are 

 ommatidea, as is the case in Astacus. In either case the 

 cuticle is transparent, and beneath it is a layer of hypodermis 

 cells, which are grouped together in fours above each omma- 

 tideum in those eyes in which corneal facets are developed, 



Fig. 32 



A. Two ommatidea from the paired eyes of Apus cancriformis. 



B. Two ommatidea from the eye of Palaunonetes varians. 



C, Transverse section through the retinula of Palezruon 

 squilla, showing seven retinular cells surrounding four 

 rhabdoms. co, cornea; Ic, lentigen cells; c.c, crystalline 

 cones ; V-V, vitrellae ; o.r.b, outer refractive body ; i.r.b, inner 

 refractive body^ ; Rt, retinulae ; Rh, rhabdoms ; nr, retinular 

 nuclei ; /!, / 2 , / 3 , pigment cells. (B original, A and C after 

 Grenacher.) 



and are known as Semper's cells, or better as lentigen cells. 

 Turning now to the actual eye of Apus. The transparent 

 external cuticle or cornea is not facetted, and the lemigen cells 

 beneath are attached to it by their broader ends, their narrower 



