. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 322 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, pg,ri 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
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 322 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, pg,ri 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 cf 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 conditio
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