274 * W. C. ALLEE 



atmospheric pressure was used throughout. The vessels were 

 placed in running water to keep the temperature more uniform. 

 During the period of experimentation which lasted seventy-five 

 days the temperature ranged from 15 to 22°. These variations 

 in temperature usually took place slowly and according to pre- 

 vious experiments ('12, pp. 316, 326) probably did not affect 

 the rheotactic response and certainly cannot be held to account 

 for all the variations. 



The oxygen varied from 5.5 to 7.0 cc. per liter but this does 

 not affect the rheotactic reaction (Allee '12, p. 281). The amount 

 of carbon dioxide present was practically constant. Food was 

 furnished by placing a few green leaves of Elodea with each 

 isopod; these were renewed often. The records of these tests 

 are shown in charts 1 to 6 pp. 275-282. 



Molting was found to be responsible for part of the variation, 

 but there was some variation other than that caused by the 

 molting period in the response of the most uniformly positive iso- 

 pods. Thus isopod no. I, chart 1, gave a normal positive response 

 of 88 per cent of the total number of trials and yet repeated the 

 response of the preceding day only 14 times in 60 trials. Isopod 

 no. II, chart 2, with an average positive Response of 86 per cent 

 repeated the preceding reaction 16 times in 53 trials. Isopod 

 no. VIII, chart 4, with an average of 90 per cent positive reac- 



Chart 1 The rheotactic and concussion responses of isopod no. I. 



Chart 2 The rheotactic and concussion responses of isopod no. II. 



In the graphs the abscissae represent time; the ordinates give the response. 

 In all instances unless otherwise mentioned the intervals marked on the base 

 line indicate days. The solid line gives the positive rheotactic responses of the 

 isopods expressed in per cent of the total number of trials. The slightly broken 

 line shows the efficiency of the rheotactic movement, while the dotted line gives 

 the concussion reactions. In each case the straight line shows* the average 

 response given by the individual under consideration under normal conditions. 

 In the plotting the scale for the percentage of positive responses is one-twentieth 

 of that for either of the other responses. The letters on the charts have the 

 following meanings: M.a., molted anterior part; M.p., molted posterior part; 

 M., molted whole covering; KCN, put in potassium cyanide; Normal water, 

 changed from potassium cyanide to the water normally used; Low oxygen, put in 

 water having an oxygen content of 0.5 to 1.0 cc. per liter; High oxygen, returned 

 to the water normally used (O2 5.5 to 7.0 cc. per liter). 



