CONTROL OF RHEOTAXIS IN ASELLUS 169 



exposure even to these stimulating salts results in a decrease in 

 the positiveness of the reaction but at times, particularly in potas- 

 sium chloride, this decrease comes only when the isopods lose 

 coordination through the toxic action of the solution. They 

 may either lose power of positive orientation while still strongly 

 stimulated or may orient repeatedly but be unable to hold the 

 position. While in this state the isopods have a tendency to 

 run in small circles of about a centimeter in diameter. Similar 

 circular reactions have been observed under natural conditions 

 but not to the extent produced in these solutions. Sodium chlor- 

 ide, the next most stimulating salt, gives none of these phenomena 

 and its stimulating action is slower as well as less pronounced. 



It has been shown that a relationship exists between the posi- 

 tiveness and the efficiency (the distance covered during a minute 

 reaction period) of the rheotactic reaction ('13). This has 

 since been amply confirmed in the study of reactions under 

 natural conditions or under such depression as may be caused 

 by calcium chloride or cane sugar. That the two phases of 

 the reaction are not correlated at all times is shown by the ex- 

 periments with rubidium and potassium chlorides just mentioned 

 and is typically illustrated by the results shown in table 2. 

 In the trials listed there the positiveness of the isopods was in- 

 creased markedly in all cases but in all save one the efficiency 

 of the response decreased, and while the positive rheotactic 

 reaction was increased 76 per cent the efficiency was cut al- 

 most in half. In the very beginning of the treatment the activity 

 increased as has been noted but the subsequent depression often 

 came before the isopods lost the power of positive orientation. 



This toxicity -depression ratio (dotted line figure 1) which was 

 high in the case of potassium and relatively so with rubidium 

 was quite low with lithium and caesium both of which depress 

 the rheotactic positiveness long before the toxic effect is appar-. 

 ent. The ratio was not found for sodium because the toxicity 

 was not accurately determined. 



In considering the relative toxicity of the different alkali 

 metal cations it should again be noted that the comparison in 

 the figure (unbroken line) is between N/5 sodium as compared 



