EHEOTAXIS IN ISOPODA 



273 



stronger positive rheotactic reaction with a low as with a high 

 vigor of shock reaction. This gives some indication of relation- 

 ship between the shock and rheotactic reactions but the evidence 

 is too slight to warrant the conclusion that such a relationship 

 exists, particularly in the face of the conflicting evidence given 

 in other instances when this possible relationship has been con- 

 sidered. 2 



DAILY VARIATIONS IN THE RHEOTACTIC REACTION 



In my earlier work on rheotaxis with groups of isopods ('12, 

 p. 340) irregularities were observed in the reactions of different 

 members of a group kept under constant experimental conditions. 

 It was also found (p. 277) that the reaction of a group of eight 

 highly positive isopods varied from day to day. From these 

 observations the question arose as to the difference in the rheo- 

 tactic response of the indi\ddual isopods and the degree of daily 

 variation of this response when kept under similar experimental 

 conditions. In other words, to what extent is the internal mech- 

 anism of different isopods from the same habitat unlike and how 

 great a variation of this internal mechanism takes place automat- 

 ically from day to day? 



In testing for these points isopods selected at random were 

 isolated in glass jars in a liter of water which was changed 

 daily. University of Illinois tap water saturated with oxygen at 



2 Experiments on a different phase of this subject performed since the above 

 was written, incidentally confirm these results. 150 isopods were tested for 

 from one to twenty-five days with the following results: 



These later concussion tests were made with the isopods in a hemispherical 

 container and show but little more correlation with the rheotactic factors than 

 did the response in petri dishes. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGT, VOL. 15, NO. 3 



