CONTROL OF RHEOTAXIS IN ASELLUS ' 165 



of the three summers that the work was in progress with similar 

 results so that weight variations must have been slight. 



The isopods used in these experiments were all Asellus com- 

 munis vSay, collected from the 'dump pond' in Woods Hole. 

 The animals used were mainly immature and were rarel}^ in 

 the laboratory more than three days before experimentation 

 began. When first collected they gave the low rate of positive 

 reactions that is usual for Asellus from such ponds. They 

 were kept in the laboratory under low ot high oxygen condi- 

 tions according to the demands of the experiments for animals 

 with low or high rates of positive rheotaxis. 



II. ELECTROLYTES 



Chlorine salts of the alkali metals 



All the chemicals tested in the concentrations used will cause 

 a decrease in the positive rheotactic reaction if allowed to act 

 for sufficient time; and at one stage the investigation virtuall}^ 

 became a search for reagents that would cause an increase in 

 the positiveness of the reaction. Of the cations tested only 

 rubidium and potassium showed a strong consistent increase. 

 Sodium and barium have similar but less pronounced effects. 



The results of the trials with the chlorine salts of the alkahne 

 metals in N/10 solutions (except sodium chloride which is N/5) 

 are given in figure 1. The cations are arranged in order of 

 their atomic weights which are shown by the solid line with short 

 cross marks. Each space in the ordinates equals 5 units of 

 the atomic weight. As the atomic weight of these metals in- 

 creases the ionic velocity and electronegativeness decreases. 

 The simple broken line gives the percentage that at some time 

 in the treatment showed some increase in the positiveness of 

 their rheotactic reaction. The dotdash line shows the differ- 

 ence in the percentage of positive rheotaxis before treatment 

 and at the most positive period during treatment. In both these 

 lines each space in the ordinates has a value of 5 per cent. 



The unbroken line in the figure represents the relative toxicity 

 of the different salts which is reckoned by the average time in 



