164 



THE EYE IN EVOLUTION 



Astacus 



retinule, the whole being more or less encased by pigmentary cells. 

 As a general rule the ommatidia are fewer than in the eyes of Insects, 

 but many variations in detail exist ^ ; a typical example is seen in 

 Fig. 146 which illustrates the eye of the crayfish, Astacus. The 

 cuticular cornea is not invariably faceted as is usually the case in 

 Insects, but, for example, in Amphipods appears as a flat extension 

 of the cuticle of the integument. Underneath the cuticle is invaginated 

 a layer of hypodermal cells (Fig. 148). The crystalline cone, in 

 contradistinction to its variability in the eyes of Insects, is never 

 lacking and is often composite and divided into three segments, a 



Fig. 146. — The Eye of a Crayfish. 

 Showing the faceted appearance of the compound eye (Norman Ashton). 



Fig. 147. — Hemisection of the Eye of the Lobster (see Fig. 69.3) 



(Norman Ashton). 



- (1916), 

 Heber'loy and Kupka (1942) 



