Reactions of I so pods 479 



IV. GENERAL DISCUSSION 



The significance of the various separate results obtained in the 

 different series of experiments has in several cases been pointed 

 out in summarizing these results. 



Cascidotea stygia and Asellus communis have quite similar 

 habits, except for the fact that the former lives almost exclusiveK' 

 in subterranean waters, whereas the latter, though occasionally 

 found in cave water, is extremely rare there. Its occurrence in 

 caves has been mentioned only a few times. When collecting 

 in caves I have never seen either Asellus communis or Mancasellus 

 tenax dilata, although both species occurred near caves in which I 

 did much collecting, and the latter species was present in immense 

 numbers immediately outside the caves. Caecidotea, when found 

 above ground, has occurred only in water near and immediately 

 connected with underground waters. 



These two animals, though differing in habitat, are much alike 

 in many of their reactions to stimuli. Both are negatively photo- 

 tactic to such intensities of light as they respond to at all; both 

 respond in like manner to various tactile and mechanical stimuli, 

 and the regions of the body most highly sensitive to these stimuli, 

 are nearly the same in the two species; both are rheo tactic, re- 

 sponding to currents of water in hke manner. When subjected to 

 the influence of a current of water, in a trough one end of which 

 was strongly illuminated, both species, though for a time re- 

 sponding to the rheotactic stimulus alone, soon, in most cases, 

 reacted to the light alone. Their food is nearly the same, both 

 species feeding largely upon decaying plant tissue. 



The following differences in the reactions of the two species 

 were noted : 



1. Asellus is decidedly more responsive to light stimuli and 

 responds to much lower light intensities (2.5 C. M.) than Caecido- 

 tea (80 C. M.); after retention in darkness Asellus is for a time 

 positive to a considerable range of intensities (2.5 C. M. to 80 or 

 more C. M.). Caecidotea is never positive. 



2. Caecidotea, on the other hand, is decidedly the more respon- 



