RHEOTAXIS IN ISOPODA 301 



From these results it will be seen that a decrease in the oxygen 

 supply decreases the positive response of the isopods to current 

 and that this decrease may be only temporary, or in other words 

 that the Aselli may become acclimated to the new conditions and 

 give a high positive response in an oxygen content that at first 

 caused a marked decrease in their normal reactions. However, 

 at no time in the progress of these experiments, have the isopods 

 showed any sign of an acclimatization when kept in an oxygen 

 content of about 1 cc. per liter. Thus the stream isopod if 

 kept continuously in the average amount of oxygen to be found 

 in the ponds would in time come to respond positively to currents 

 under the new conditions, yet, if they were subjected to the ex- 

 tremes of low oxygen that are sometimes maintained for fairly 

 long periods of time, there is no evidence that adults would ever 

 become acclimated. 



These results are of importance when considered in the light 

 of the' conditions which the stream isopods may have to meet in 

 nature. As has already been said, they live in pools of small 

 streams. In summer these streams sometimes cease to run and 

 in this condition, the pools often take on the characteristics of a 

 pond for the time that the condition lasts. The cycle of reactions 

 under these conditions is easily seen. The isopods used to a super- 

 saturated oxygen supply would become less positive to a chance 

 current as the pool water became low in oxygen. Then unless the 

 deoxygenation went too far, there would be an acclimatization 

 so that on the average when the stream began flowing again the 

 isopods would be positive to the current and would thus be better 

 equipped to escape being carried out of the stream. Of course 

 even when they are negative to the current, many would be kepit 

 in the stream by being stranded against piles of drift. 



(2) Breeding season. But one controlled experiment was run 

 on the effect of lowered oxygen supply on the breeding season 

 response, but this appears significant in that the experimental 

 results confirmed the prediction that was made concerning them. 

 They also ran parallel with the effect of an increase in the amount 

 of carbon dioxide as will be shown in a later section. The ani- 

 mals used in this experiment were large males about 15 mm. in 



