Reactions of I so pods 487 



5. Caecidotea, being less sensitive to light stimulation than 

 Asellus and more sensitive to mechanical stimulation, affords an 

 illustration of compensative sensitiveness to one influence for a 

 partial loss in sensitiveness to another influence. 



Habitat 



There are several possible factors which determine the habitat 

 of one of these species to be within and the other outside of caves. 



1. There is a remote possibility that the relative freedom from 

 mechanical disturbance within cave waters is a factor in deter- 

 mining the existence of Caecidotea in caves. 



2. The difference in the reactions of Asellus and Caecidotea to 

 light aflbrds an explanation of the occurrence of Caecidotea in 

 caves, and subterranean waters in general, and the virtual non- 

 occurrence of Asellus in such situations; for the negative response 

 of Caecidotea to light would aid in directing it into caves and 

 keeping it there; but Asellus after being in darkness becomes 

 positive, and therefore would move toward the light, /.^., out of 

 a cave in case it had bv chance made its wav into one. 



3. Caecidotea, being more persistently rheotactic than Asellus, 

 and being able in creeping to hold its own against a current better 

 than Asellus, would not readily be swept away by a stream flowing 

 out of a cave. Asellus, on entering a cave under such conditions, 

 would soon lose its rheotactic response and further would become 

 positively photo tactic in a few hours. Hence, if it by chance came 

 near the mouth, it would react to the light and escape. 



4. The reactions to the influence of a current of water com- 

 bined with the influence of light afford an additional factor in 

 determining the distribution of the two animals. 



5. An apparently superior discrimination on the part of 

 Caecidotea in selecting its food may make it possible for Caecido- 

 tea, rather than Asellus to live in caves where organic matter is 

 extremely scanty. 



