CONTROL OF RHEOTAXIS IN ASELLUS 179 



whose initial response was under 71 per cent positive with one 

 exception where the reaction was 90 per cent positive at the 

 start. This individual was made more positive by the action 

 of the acid and so may fairly be included (p. 178). Each space 

 in the abscissae represents one hour and in the ordinates 5 per 

 cent change in the positive rheotaxis. The location of the cross 

 gives then the increase or decrease in the positive rheotaxis and 

 the time that the isopod had been subjected to the treatment. 

 The crosses on the base line indicate in this experiment that 

 neither before nor after the treatment did the isopods give a 

 positive response. 



In brief the results show that 14 tests of 5 isopods showed an 

 increased positiveness averaging 15 per cent, 6 tests showed 

 no change in the positive reaction though considering the change 

 in negative, indefinite and zero responses each indicated a de- 

 pression; and that in 16 tests 9 isopods showed a decrease aver- 

 aging 21 per cent. As a final summary 39 per cent of the trials 

 showed some increase but the average amount of change in 

 the positive reaction was a decrease of 13 per cent. This is 

 similar to the results obtained with magnesium chloride and 

 indicates a depression. 



In the more concentrated solutions acids are decidedly toxic 

 as is shown by the following records for hydrochloric acid: 



Solution Time before 



strength all are on back 



n/5 1 minute 



n/20 8 minutes 



n/50 120 minutes 



n/100 Out of three tried two were dead in 24 hours 



The toxicity was greater than with any other chlorine com- 

 pound tried and by the usual reasoning in such cases this must 

 be ascribed to. the hydrogen ion and the depressing action of 

 acids on rheotaxis must also be ascribed to this ion because 

 each anion tried had the opposite effect on rheotaxis when 

 combined with some other cation. By this I do not mean to 

 maintain that the action of the cation is necessarily direct but 

 that the cation here is the important part of the combination 

 either through its direct action or because it fails to entirely 



THE JOURNAL Ot EXPERIMENTAL ZOOLOGY, VOL. 21, NO. 2 



