CONTROL OF RHEOTAXIS IN ASELLUS 



177 



Tests were made for similar antagonisms between sodium 

 and magnesium chlorides which are less effective in influencing 

 the rheotactic response than the ones just given. No evidence 

 of marked antagonism was found until the isopods were left 

 in the different solutions for several hours when a definite an- 

 tagonism was exhibited as is recorded in figure 6 where each 

 space in the abscissae represents an hour. 



Acids {H ions) 



It is particularly hard to summarize in brief form detailed 

 results of the experiments with acids. Prehminary tests were 

 run with acetic, N/100, hydrochloric; N/50, N/100, N/500 



Fig. 6 Showing antagonistic action of n/5 sodium and magnesium chlorides 

 on positive rheotaxis of one isopod. 



and N/1000; sulphuric, N/100, N/500, and N/1000; and with 

 nitric acid N/100. There was some slight evidence of stimula- 

 tion with N/1000 hydrochloric and with N/100 acetic acids. 

 These reagents accordingly were tried again the following season 

 and 3 isopods with N/100 acetic acid showed complete depression 

 while 15 trials with 6 isopods using N/1000 solution of the same 

 acid showed depression in all cases save one. A similar repeti- 

 tion with N/1000 hydrochloric showed only one stimulation. 

 To test the matter further a fairly complete series was run 

 with N/4000 hydrochloric acid the results of which are shown 

 in figure 7. In this series 36 trials were made with 14 isopods 



