CONTROL OF RHEOTAXIS IN ASELLUS 



175 



Antagonisms 



When isopods are treated alternately with potassium and cal- 

 cium chlorides the well known antagonistic action of the two 

 salts is plainly shown. The effect of such treatment on eight 

 isopods is graphically exhibited in figures 3, 4, and 5. In mak- 

 ing these trials the isopods were tested as usual in water to which 

 they were accustomed, then dried momentarily on filter paper 

 and introduced into the desired solution. After about a minute 

 they were given five one minute trials and again placed in water. 



Fig. 3 Showing antagonistic action of n/10 potassium chloride and m/5 cal- 

 cium chloride on positive rheotaxis of three isopods. For explanation see text. 



After about two minutes they were placed on filter paper and in- 

 troduced into the other solution. Experiments showed that 

 this handling had a stimulating effect but this was not sufficient 

 to mask either the antagonistic effects here recorded or the 

 depressing effects of most salts tried. 



The method of charting is the same in all three figures. One 

 space in the ordinates is worth 5 per cent, one in the abscissae 

 equals five minutes. The lines show the variation in the per- 

 centage of positive responses given and in all cases at the be- 

 ginning of the treatment, the low points mark the response 

 when treated with calcium, the high ones when treated with 

 potassium chloride. The only exceptions are found in figure 



