206 



THE EYE IN EVOLUTION 



the simple type, cupulate in form with a corneal lens formed by the 

 cuticle and hypodermal cells (Fig. 103). Eyes so simple as this serve 

 merely as a means of orientation away from light, and two cave- 

 dwelling species are blind ^ (Dakin, 1921). 



CRUSTACEA 



The CRUSTACEANS (lobsters, crabs, shrimps, water-fleas, barnacles, 

 etc.) with few exceptions (land-crabs, wood-lice, sand-hoppers) are 

 aquatic in habit and in most the eyes are prominent ; some pelagic 

 forms are transparent except for the eyes which are highly coloured or 

 phosphorescent. Compound eyes are usually present, occasionally 

 supplemented by eyes of the simple type, but in sessile or parasitic 

 forms the visual organs may be vestigial or lacking. Most forms 



Q 



Fig. 201. — The Woodlouse, Sph.sroma lanceolata. 



The compound eyes, C, are sessile, lying on the extreme lateral aspects 

 of the head segment (specimen from Natural History Museum, London). 



Homams 



Phronima 



commence life as a nauplius larva with an oval body, three pairs of 

 limbs and a single eye in the middle of the head. 



Of the larger forms (the sub-class malacostraca) the Decapods 

 (lobsters, shrimps, prawns, crabs) have the most elaborate eyes ; of 

 these the common lobster, Homarus vulgaris, may be taken as repre- 

 sentative. It possesses two typical compound eyes, each with a multi- 

 tude of ommatidia, associated with the procephalic lobes of the cerebral 

 ganglion. They stand out prominently on muscular eye-stalks to 

 protrude on either side of the median rostrum and are capable of some 

 degree of movement (Fig. 198). In crabs a similar pair of compound 

 eyes with relatively few but large ommatidia are set on eye-stalks in 

 sockets in the carapace (Figs. 199-200). The fact that the eye-stalks 

 L. ^Hi in the crab and in the crayfish exhibit optomotor reactions as when 

 tJir- animal turns or is confronted by a black and white striped rotating 

 dr^ ', indicates that their movements are optically determined 



1 p. 724. 



