CONTROL OF RHEOTAXIS IN ASELLUS 



167 



minutes elapsing before the isopods lost the power to stand. 

 This turning time may or may not be correlated wdth the sur- 

 vival time of the individual but since it shows the time during 

 which the animal may react it is more important in the study 

 of certain phases of behavior than the death time of the animal. 

 In this curve each space in the ordinates represents twelve min- 

 utes. The dotted line gives the ratio existing between the be- 

 ginning of depression and the turning time reduced to per- 

 centage of the latter. This ratio will be called the toxicif^. 

 depression ratio and is shown here with each ordinate space 

 worth 2 per cent. 



The distribution of the experiments on which these curves 

 are based is shown in table 1. 



TABLE 1 



Showing the number of rheotactic experiments performed with the different strengths 

 of the chlorine salts of the alkali metals 



If one considers the percentage of isopods having their rheotac- 

 tic reaction made more positive the order of effectiveness is 

 Li < Na < K > Rb > Cs. When the amount of increase 

 in positiveness is compared N/5 sodium chloride is less effective 

 than N/10 lithium chloride but if N/5 solutions of both salts 

 are compared the former is more effective and the above order 

 holds for the amount of increase as well as for the number made 

 more positive. If this series be written in the ordinary physio- 

 logical style it is Cs < Li < Na < Rb < K with a sharp division 

 between the sodium and rubidium. This reminds one of the 

 usual physiological order of these cations except that caesium 

 which usually hes between lithium and sodium in its effects 

 on living tissues and colloids (Hober '14, s. 487) here is less 

 effective than either. 



