Reactions of I sopods 481 



The nicety with which even sHght sensitiveness to Hght may 

 regulate an animal's movements is well illustrated by the follow- 

 ing case. Parker ('05, p. 418) found sensitiveness to strong light 

 in the skin of the tail of Ammocoetes, and pointed out the sig- 

 nificance of this in reLation to the burrowing habit of the animal. 

 Here again, the sensitiveness was to intense light only. The eyes 

 of these animals probably subserve all light-receptive functions 

 necessary for swimming about, but in burrowing the integumen- 

 tary organs of the tail serve to distinguish intensities sufficiently 

 to direct the animal to continue burrowing until completely cov- 

 ered. Here, as m the other cases cited, the organs are not highly 

 discriminating light-receptive organs, but they suffice for the 

 regulation of the animal's movements. 



Among the higher animals those that possess degenerate, or 

 poorly developed eyes are such as in general live in dark situa- 

 tions. Their light-perceiving organs are of advantage to them 

 only in aiding them to remain within a suitable environment. 

 Highly developed light-receptive organs are of no advantage in 

 their dark habitat, and organs capable of perceiving only consider- 

 able changes in intensity of illumination are sufficient to serve 

 as a check to keep them in their proper surroundings. 



Caecidotea nearly always lives in absolute darkness and ordi- 

 narily has little occasion to discriminate between intensities of 

 illumination. If, however, in approaching the mouth of a cave 

 it becomes subjected to increased illumination, its light-receptive 

 organs ordinarily are discriminative enough to prevent its leaving 

 its subterranean abode. Whether its light-perceptive organs 

 were adapted to its cave habitat or whether its cave habitat was 

 adopted because its organs suited it for such surroundings, does 

 not here concern us. At any rate, Caecidotea's light-receptive 

 organs are sufficiently discriminating to serve their part in regu- 

 lating the movements of the organisms within caves. 



We have no reason to suppose that a species lives where there 

 is extremely little light because it is extremely responsive to light. 

 On the contrary, we have every reason to suppose the opposite — 

 that when an animal lives where there is little light and where it 

 has little opportunit\' or occasion to be influenced by light, it will 



