RHEOTAXIS IN ASELLUS. 95 



retains its depressing power, which laboratory tests similar to 

 those just cited renders improbable, its action would be offset 

 by the sodium which has an antagonistic effect and which is 

 present in sufficient quantity to neutralize any possible depressing 

 action of the nagnesium and to antagonize partially the action 

 of the calcium, if that were needed. 



Theoretically then, from the analyses of salt content, one would 

 not expect the pond water to be less favorable for positive rheo- 

 tactic reaction than the water from the stream ; and this expecta- 

 tion was verified by laboratory tests. Stream isopods were kept 

 in the laboratory for three days in water brought from their 

 stream habitat. Then they were divided and part were put into 

 pond water from the Osborn, Indiana, pond. Both lots of- iso- 

 pods were kept under identical conditions of temperature, aera- 

 tion, and food supply for twenty days, during which time the 

 eighteen tests shown in Fig. i were made. The graphs there 

 show the percentage of positive reactions given daily by groups 

 of five isopods chosen at random from the stocks under observa- 

 tion. The variation in positiveness is due in part to this chance 

 selection of individuals for the daily tests ('13^) and in part to the 

 setting in of an abnormal breeding season, brought on by the 

 transfer from freezing stream water to a laboratory of about 15 

 degrees centigrade. The decline of positiveness is the usual 

 effect of the breeding season. 



For the eighteen clays when comparative tests were made the 

 stream isopods in pond water averaged 73 per cent, positive while 

 their mates in stream water were 70 per cent, positive. This 

 means that the difference in salt content of the two waters did 

 not affect rheotaxis. 



The most obvious difference in the two environments is the 

 difference in the oxygen and carbon dioxide tension in the water. 

 In the more extended work of five years ago, the oxygen tension of 

 County Line Creek was found to lie normally between 5 and 10 

 c.c. per liter and the carbon dioxide tension was about 2 c.c. per 

 liter. In the ponds the oxygen was usually under 3 c.c. per liter 

 and the free carbon dioxide usually over 10 c.c. per liter. The 

 amount of half bound carbon dioxide is also consistently greater 

 in the pond than in the stream. 



