Day. — Eye Of Crayfish. 



PLATE 1. 



Fig. la. Standard light-eye: diagrammatic representation of external ap- 

 pearance in daytime showing pseudopupilla. 



Fig. lb. Standard light-eye: exposed 6 hr. to light of north window; photo- 

 graph of median section, X 40. A single ommatidium sketched 

 in left half: 



hm, basement membrane dp, distal retinular cells 



c, cone pp, proximal " " 

 cr, corneal hypodermis rh, rhabdome 



d, cuticula t, tapetum 



Pigment of the proximal cells has migrated from the region below the base- 

 ment membrane and has become concentrated around the nuclei in the tips 

 of the cells. 



Fig. 2a. Standard dark-eye; diagrammatic representation of eye after 6 hr. 



in the dark when examined with a flash-light. 

 Fig. 2b. Standard dark-eye : adapted to dark for 6 hr. and killed in the dark; 



photograph of median section. 



npp, nuclei of proximal retinular cells (drawn in ink). 

 rp, pigment " " " " 



Retinal pigment hes mostly behind the basement membrane. The left 

 half of the photograph is retouched to show the blunt processes of pigment of 

 the proximal retinular cells distal to the basement membrane, which were not 

 differentiated from the tapetal pigment by the camera. 



