RHEOTAXIS IN ISOPODA 



277 



the response may be given, that is, the speed and definiteness of 

 the response. Experiments have shown that these two quahties 

 of the response are closely correlated and when either is stated in 

 this discussion, the other is always implied. 



The possibilities of this method are shown in table 1. The 

 Aselli used in this experiment were carefully selected from a 

 stock of stream isopods that had been reared in known conditions 

 in the laboratory. They were about five months old and were 

 between 9 and 10 mm. in length. All were in the same stage 



TABLE 1 



Table showing possibilities of the method used. Eight stream Aselli, 8 to 9 mm. 

 long; twenty-four hour intervals 



Average positive response 89.4 per cent. 

 Greatest deviation from average 4.9 per cent. 

 *+ indicates a positive reaction. 



— indicates a negative reaction. 



°: indicates an indifferent reaction. 



regarding moulting, and were entirely normal in every way. 

 Throughout their life they had been kept in still water having an 

 average of about 6 cc. of oxygen per liter. 



From table 1 it will be seen that with animals in approximately 

 the same physiological state, the experimental error of the method 

 used is almost 5 per cent. This error is too large for purely quan- 

 titative results, but it will not interfere greatly with the compari- 

 sons that are to be made in this work. In no case however is any 

 importance attached to experimental results that do not show a 

 difference of at least 20 per cent, so that the possible error of 5 

 per cent cannot affect the conclusions drawn. 



