RHEOTAXIS IN ISOPODA 309 



In the third, fourth, seventh and eighth trials hsted the response 

 was probably shortened by the change in temperature. In all 

 but the second and fifth trials it may have been lengthened by the 

 presence of a large amount of oxygen in the water. In spite of 

 these difficulties the reactions are fairly consistent with the other 

 straight current tests, that is there is an apparent correlation 

 between the length of the positive response and the per cent of 

 positive reactions in the pan tests. This means that the animals 

 in the stream if caught in a sudden current, after a period of low 

 oxygen in summer pools, would maintain themselves against the 

 current for only a short time and must be washed down stream 

 much sooner than isopods from the usual stream conditions. 



If the low oxygen should be long continued, however, the 

 animals would tend to become acclimated. This has already been 

 shown to be true for the circular current and is illustrated here 

 by the respo-nse given in the tenth trial shown in table 14. This 

 response was given by the same isopods that showed acclimatiza- 

 tion to the circular current in the first part of tabl,e 11. 



b. Carbon dioxide 



Not enough experimental work has been done on the effect of 

 increasing the amount of carbon dioxide present in the water to 

 enable this discussion to be critical. However, there are some 

 lines of evidence that appear significant. In the streams the 

 amount of carbon dioxide present in the free state is low, that is it 

 averages from 2 to 3 cc. per liter. In the ponds it has been taken 

 as high as 40 cc. per liter but the average is about one-fourth this 

 amount. Since water will absorb over 900 cc. of carbon dioxide 

 per liter, it will be seen that only a relatively small amount is 

 present even in the ponds. 



Part 1 in table 15 gives the effect of low oxygen and increased 

 carbon dioxide, the latter being present in the amount that was 

 found in nature in the most extreme cases. The positive rheo- 

 tactic response gradually fell under these conditions, but by 

 referring to table 11, part 4, it will be seen that the decrease is 

 not so rapid as when only low oxygen was acting. The second 

 part of the table gives the effect of a much larger amount of car- 



