Reactions of Isopods 485 



in caves, is the different responses to light, — ^Caecidotea always 

 responding negatively to such intensities as it responds to at all, 

 and Asellus", after having been in darkness, being positively 

 phototactic for a time. 



Since Asellus is so closely related to Caecidotea morphologically 

 and physiologically, it would seem that if under stress of circum- 

 stances any epigeal animal could suddenly become a cave inhab- 

 itant, Asellus might be expected to be capable of undergiong such 

 change in environment. Asellus is in many ways apparently 

 nearly suited for cave habitation, and in time it may become fur- 

 ther modified so that it will be capable of living in caves, but it is 

 not fitted for cave fife. Its physiological reactions are such that 

 it is prevented from taking up cave life, and further its apparent 

 lack of discriminative ability in selecting food renders most 

 improbable the continued existence of a straggler of this species 

 within a cave. This case fails to support Lankester's ('93, p. 

 389) theory of the sudden and accidental origin of cave animals, 

 but rather lends support to the theory that cave animals must 

 become closely adapted for cave life before they are capable of 

 taking up such existence. 



V. SUMMARY 



Mechanical Stimulation 



1. Asellus and Caecidotea respond in much the same manner, 

 and corresponding parts in the two species are the regions of 

 greatest sensitiveness. 



2. Caecidotea is decidedly more responsive than Asellus, react- 

 ing more often, more vigorously, and to weaker stimuli. 



3. The threshold for Caecidotea is below that for Asellus. 



Rheotaxis 



I. Asellus and Caecidotea are rheotactic and respond to 

 currents in like manner. 



