RHEOTAXIS IN ISOPODA 311 



bon dioxide than has been found in any isopod habitat so far 

 studied. In this case the action of the carbon dioxide in twelve 

 hours cut the positive reaction to 28 per cent, while the control 

 in low carbon dioxide stayed at 78 per cent. The control and 

 experimental animals were then changed. In twenty-one hours 

 the isopods in the experiment gave an 18 per cent positive response 

 while those that had been changed from the experiment to the 

 control had increased from 28 to 57 per cent positive. The 

 lowering of the oxygen in this case is due to bubbling the carbon 

 dioxide through the water in order to increase the amount of 

 carbon dioxide present. 



Eight days later the response in the two conditions was about 

 the same but the breeding season had set in and one pair of the 

 control animals was copulating. This complication makes the 

 cause of the increased response in the experiment problematical. 

 It may be due to acclimatization, or to an interaction of the car- 

 bon dioxide and the breeding season, similar to that given with 

 decreased oxygen. 



Part 1 of table 16 .shows two experiments run during the breed- 

 ing season of 1911. The first was started April 6, and the second 

 twenty-four days later. The presence of the carbon dioxide 

 caused an increase in the positive rheotactic response from 38 

 to 58 per cent in that time. In the second experiment the con- 

 trol animals increased their positiveness due to the close of the 

 breeding activities, and the animals in the increased amount of 

 carbon dioxide did the same, although there was twice the amount 

 of carbon dioxide present that was ever found in the ponds studied. 



The second part of table 16 gives the results of experiments 

 run at the same time as those given for low oxygen* in table 12. 

 In this case there was first a slight increase followed by a decrease 

 in the positive rheotactic response, and as the amount of carbon 

 dioxide was increased the positive reaction • decreased. The in- 

 crease in positiveness however was not so great as that given with 

 low oxygen under similar conditions. 



The results obtained correspond exactly with the known effects 

 of carbon dioxide as a drug. Cushny ('10, p. 588) says in part, 

 that in mammals, carbon dioxide unmixed with oxygen produces 



