312 



THE INVERTEBRATA 



thought, from details of their structure, to have been formed by the 

 degeneration of compound eyes resembling the lateral eyes of 

 Limulus. The median eye of the Crustacea (Fig. 226) is composed of 



Fig. 211. Diagrams of a series of eyes of arthropoda. A, Hypothetical start- 

 ing point of the series. B, Cells have sunk in to form a retinula. The units of 

 the lateral eyes of Limulus are substantially in this condition. C, C", Cells 

 from the sides have closed in over the retinula. C, Hypothetical stage in the 

 evolution of an ommatidium from a cup with a single retinula. C", Actual 

 condition of many ocelli of insects, etc. : the cup has several retinulae. D, An 

 ommatidium. b.me. basement membrane of retinular layer; c.c. central cell; 

 cgn. comeagen cells; en. crystalline cone; cu. cuticle; Is. lens; n. nerve fibre; 

 pig. pigmented cells which form a ring in the outer part of the ocellus; pig.' 

 outer iris cells; pig." inner iris cells; rd. "visual rods"; ret. retinular cells; 

 rh. rhabdona; vit. vitrellae; vit.hu. vitreous humour. 



three cups, which may (some copepods) separate widely. The paired 

 eyes probably do not, as has been suggested, represent a pair of ap- 

 pendages. The foremost, or preantennal, somite, to which they would 

 in that case belong, possesses, in Peripatus and as a rudiment in 



