Day. — Eye of Crayfish. 



PLATE 4. 



Figs. 7a, 8a, 9a are copies of the original diagrammatic records of eyes 140, 

 139, and 141, respectively. 



Figs. 7b, 8b, 9b are photographs of sections of the same eyes as those ex- 

 hibited in Figs. 7a, 8a, and 9a after they had been killed, to show the 

 changes induced in them by exposure to Ught. 



Fig. 7a. Observation record of eye 140, darlc 6 hr. Color, ruddy orange. 

 Shght darkening in upper half. 



Fig. 7b. Section of eye 140, X 43. On the left the pigment extends sUghtly 

 beyond tapetum, on the right there is a heavier local migration. 



Fig. 8a. Observation record of eye 139, dark 6 hr. Color, yellowish orange. 

 More darkened in upper half than in Fig. 7a. 



Fig. 8b. Section of eye 139, X 42. The pigment extends in a more definite 

 fringe beyond the tapetum than in Fig. 7b; a heavy local migra- 

 tion is observable on the left. 



