CONTROL OF RHEOTAXIS IN ASELLUS 



173 



the bromide-iodide relation just mentioned. At the weakest 

 concentration the bromide became more effective than either 

 the nitrate or the iodide and took its usual place in the anion 

 series viz: NO3 < I < Br < Ac. 



It is noteworthy that the toxicity-depression ratio is uniformly 

 high which means that the potassium cation, regardless of the 

 anion, tends to cause the isopods to give a positive reaction well 

 after the amount of activity is markedly depressed. The varia- 

 tion in effect of these anions on rheotaxis must be due in part to 

 some other factor than relative toxicity because the toxicity- 

 depression ratio does not vary sufficiently to account for the 

 rheotactic results obtained. 



Chlorine salts of the alkali earths 



Although any chemical at the concentrations used will ulti- 

 mately cause a decrease in rheotactic positiveness, calcium and 

 strontium chlorides were found to be particularly effective in 

 causing this depression without prehminary stimulation. Ba- 

 rium and magnesium chlorides also cause an ultimate depression 

 but they often give a preliminary period of stimulation. 



Some of the experimental data on which these conclusions 

 are based are shown in table 3 which shows the toxicitv of M '5 



TABLE 3 



Showing the toxicity and effect on rheotaxis of chlorine salts of alkali earths. Stron- 

 tium chloride experiments were run in n/5 and the others in m/5 solutions. Iso- 

 pods with an initial response of more than 71 per cent positive were not consid- 

 ered 



Active after. 



