Habits, Reactions, and Associations in Ocypoda arcnaria. 21 



In conclusion it may be said that the eyes of Ocypoda are the most 

 highly developed of crustacean eyes ; that they are stimulated by differences 

 in the intensity of light when these are large enough ; that they are quite 

 sensitive in this respect; that they do not react to different colors; that they 

 aid much in the search for food, in the detection of enemies, and in the 

 accuracy of locomotion. My observations and experiments afford no proof 

 that Ocypoda has vision, such as exists in the human eye, but its behavior 

 leads me to believe the eyes are so well developed that it almost amounts to 

 the same thing. While they probably do not see the color of an object or 

 the finer characters of its surface, they undoubtedly see its outlines and 

 possibly some of the more evident irregularities of the surface, made 

 evident by the differences in lighting. 



COLOR-CHANGES. 



It has long been well-known that some species of Crustacea change in 

 color when placed under different conditions, but these observations seem 

 to be almost entirely confined to those forms that live in the water. Cer- 

 tain species of Hippolyte, PalcFmon, Crangon, Idothea, Nika, Gammarus, 

 and others have been studied, but as far as I know observations on the color 

 changes of only one of the Brachyura have been made. 



Color changes or pigment migrations have been investigated in the verte- 

 brate skin, especially that of lizards and frogs, in the hypodermis of crus- 

 taceans, in the retina of vertebrates, crustaceans, insects, and cephalopods. 

 Various causes have been suggested which might produce these changes, 

 such as light, heat, color environment, emotional states, and other nervous 

 conditions. 



The carapace of Ocypoda is very lightly colored and shows practically no 

 color-pattern. Any pigment which it does contain apparently undergoes no 

 change when exposed to different intensities of light and heat, but the hypo- 

 dermis underneath is rich in dark pigment-cells. 7\s the carapace is almost 

 colorless, and is translucent, the pigment-cells of the hypodermis, arranged in 

 the form of a pattern, show through very plainly under certain conditions. 



The writer had been observing ocypodas and experimenting with them 

 for a considerable period before it was seen that the color-pattern under 

 the carapace changed from time to time, although it was often noticed that 

 some individuals had a definite pattern, while others were almost free from 

 it. This lack of color in some specimens was supposed to be due to the fact 

 that they had recently molted, until finally, while testing the effect of differ- 

 ent color-screens on the behavior of Ocypoda, new light was thrown on the 

 subject. In this experiment 4 ocypodas were placed in a box which had 

 colored windows made of gelatin. After being confined for about an hour 

 they were taken out and by chance placed in a dish of cool sea-water where, 

 much to my surprise, they began to turn dark, showing a very distinct color- 



