oue BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
they struck the reflecting layer, they were returned as filtered light. 
After exposure to light for several minutes the original metallic- 
orange faded to a dull yellow hue and, pgri pussu 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 
Fig. F. Diagram of apparatus for exposing crayfish to colored light. At extreme 
right is the end of the colored-light generator; at the extreme left a dish containing 
the crayfish; between these positions are three diaphragms. 
ee eee a EO ti NO eC GONE OES 
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