RHEOTAXIS IN ISOPODA 



287 



The importance of this factor is emphasized by the two trials 

 where with a low oxygen content the animals tested gave a high 

 positive response. This is the one case in the progress of this 

 work where the laboratory tests have failed to run parallel with 

 the field results, for as will be seen later, keeping young stream 

 Aselli in water having a low oxygen content kept the animals 

 from developing a high positive response, in all the cases tried. 

 These two high results were obtained in the field on July 4, 1911. 

 The stream at this time was reduced to a series of small pools 

 with no running water, and in the case of the higher response, the 

 animals were in a very high temperature, 29°C., which may to 

 some extent account for the difference between the two trials. 

 Reference to this will be made in another part of this paper in 

 connection with experimental data which may tend to clear up 

 the case. 



d. Response to straight current 



Banta ('10, pp. 467-468) described a trough which he devised for 

 testing the response of isopods to a straight current. His appara- 

 tus consisted of a simple straight trough in which the current was 

 equalized by passing through a number of wire screens before it 

 reached the experimental part of the trough. He introduced the 

 isopods to be tested into still water and then turned on the cur- 

 rent. After some crawling back and forth the animals collected 

 at the upper end and stayed there from fifteen minutes to two 

 days, afterward reversing their reaction. 



In order to test the efficiency of the pan response as an index of 

 the rheotactic activity of the isopods, Banta's experiments were 

 repeated. A different type of trough was devised and is shown in 

 figure 2. This trough has a rounded well 10 cm. in diameter. 



Fig. 2 Straight current apparatus 



