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bulletin: museum of comparative zoology. 



they struck the reflecting layer, they were returned as filtered hght. 

 After exposure to Hght for several minutes the original metallic- 

 orange faded to a dull yellow hue and, pari passu with this change, 

 there appeared a dark area in the center (cf. Plates 4, 5, Figs. 7a, 8a, 9a). 

 The alteration of color is believed to be due to a partial bleaching of 

 the visual-red substance in the rhabdomes. The darkening of the 

 center of the eye is explained as the result of the migration of the 

 pigment to cover the retinal tapetum (Plate 5, Fig. 9b), whereby re- 

 flection is prevented. Through acquaintance with this fact, it was 

 conceived that a judgment might be made as to the amount of darken- 

 ing corresponding to the difference in influence of the colored lights. 



(b). Procedure. Individual records for each crayfish were kept 

 and contained the following: — the sex; exposure data including 

 color of the light, intensity in terms of centimeters distant; the 

 appearance of the glow at the beginning and at the end of exposure; 

 period of exposure; and temperature of the water. Since the initial 

 glow could be seen in the colored light, every animal was rejected 

 from a given experiment if the eye was not in full glow. The animals 

 were put in the dark at noon and left until evening, when the experi- 

 ments were conducted. The reason for this procedure was to obtain 

 the glowing condition as nearly as possible in accordance with the 

 normal change induced in the eye by the transition from day to night; 

 in other words, if there were any physiological rhythm in the pigment- 

 migration, such as Kiesel ('94) observed in Plusia gamma, my object 

 was to work with, and not counter to it. Whether the migration is 

 periodic in the crayfish I can not say. So far as the evening hours 

 were concerned, the eyes of the animals were then nearly always in 

 good glowing condition. 



The arrangement for making the exposure is shown in Fig. F. The 

 crayfish was held by hand partly submerged in a pan of water in such 



A 



A 



K 



V 



Fig. F. Diagram of apparatus for exposing crayfish to colored light. At extreme 

 right is the end of tlie colored-Ught generator; at the extreme left a dish containing 

 the crayfish; between these positions are tliree diaphragms. 



