276 THE INVERTEBRATA 



each probably representing a narrowed and deepened cup. Compound 

 eyes of this type are found in crustaceans and insects . They vary much 

 in detail, but essentially the structure of an ommatidium is as follows 

 (Fig. 200 D). At its outer end is a transparent portion of the general 



Fig. 200. 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.nie. basement membrane of retinular layer; ex. central cell; 

 cgn, corneagen 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. 



cuticle of the body, usually thickened to form for the ommatidium a 

 biconvex lens. Under this lie the epidermal cells which secrete it 

 {corneagen cells) : the lens is one of the facets of the eye. Under the 



