Day.— Eye of Crayfish. 



PLATE 3. 



Fig. 5. Photograph of a median section of an eye exposed to yellow at 550 cm. 

 for 1 min., after having been 6 hr. in dark. X 40. The pigment 

 is coextensive with the tapetum (to the broken line in the left half 

 of the figure). 



Fig. 6. Photograph of a median section of an eye exposed to red at 550 cm. 

 for 1 min., after having been 6 hr. in the dark. X 45. The 

 pigment is coextensive with the tapetum only about the center; 

 on the sides (as indicated by the diminishing width of the dark 

 band sketched into the left half) it falls short of the extent of the 

 tapetum. 



