484 A. M. Banta 



tions neither persisted in entering the light for any great length 

 of time. Asellus, however, after having been in a cave in darkness 

 for a time, would become positively phototactic and would then 

 have both its rheotactic and its positively phototactic impulses to 

 direct it out of the cave. 



The suggestion that cave conditions involve fewer mechanical 

 disturbances in underground waters than are found in out-door 

 streams and pools, and that the relative immunity from mechan- 

 ical disturbance within caves is a factor in determining the dis- 

 tribution of the one species within caves and the other outside, 

 seems hardly worth following up. 



The difference in the food taken by the two species seems to be 

 a factor in determining their habitats. Caecidotea eats nearly 

 exclusively dead plant tissue, even when provided with living and 

 dead tissue in equal abundance. While Asellus takes much fresh 

 plant tissue in addition to the dead plant tissue which it also feeds 

 upon, it is not shown that living plant tissue forms a neccessary 

 part of its food, so that apparently it might feed upon dead plant 

 tissue exclusively. There is an extremely small amount of organic 

 matter and a great prevalence of sand, etc., in caves as compared 

 with the amounts of organic matter and inorganic matter in the 

 situations in which Asellus h ves. In view of this fact the relatively 

 very small amount of dirt and small particles of mineral matter 

 within the digestive tract of Caecidotea as compared with that 

 of Asellus suggests that Caecidotea exercises more discrimination 

 than Asellus in taking its food. 



The importance to an animal living in a cave of a superior ability 

 in selecting its food could not be overestimated, since the organic 

 matter there is so scanty. If Caecidotea possesses such an advan- 

 tage over Asellus that alone may be a very important factor in 

 determining the suitability of Caecidotea to a cave habitat and 

 the unsuitability of Asellus for the same locality. 



To recapitulate, we find some apparent importance in the 

 factors of the animals' food and their different rheotactic responses 

 in determining the habitats of the two species, but the one factor 

 which seems of most importance, and which alone affords an 

 explanation of Caecidotea 's living in caves and Asellus not living 



