RHEOTAXIS IN ISOPODA 289 



9, chart 4) but usually there was a more or less decided decrease 

 in the percentage of positiveness (table 9 and charts). When 

 this occurred there was nearly always a decrease in the efficiency 

 of the reaction and an increase in the negative response. 



Sometimes there was an increase in the vigor of the concussion 

 response at this period but this is too infrequent and the data 

 on this point too meager to warrant a definite statement on 

 this relationship. 



SUMMARY OF RESULTS 



I. Relation between efficiency and the sign of rheo tactic 

 response of stream isopods: 



1. In the daily variations to the current the efficiency of 

 movement tended to vary with the percentage of positive 

 reactions (p. 268). 



2. The larger variations of the positive reactions are usually 

 accompanied by similar variations in efficiency (charts 1 to 8) . 



3. Low efficiency is accompanied by a low percentage of 

 positive and negative and by a high percentage of indefinite 

 rheotactic reactions (p. 270). 



4. High efficiency is always accompanied by a low percentage 

 of indefinite responses; usually by a low percentage of negative 

 and a high percentage of positive reactions (p. 270). 



5. In exceptional individuals high efficiency may be accom- 

 panied by either low or high percentage of positive responses 

 (p. 272). 



6. Under the influence of potassium cyanide and low oxygen 

 both the positive responses and the rate of efficiency is decreased. 

 The rheotactic reactions may go directly from a preponderance 

 of positive to one of indefinite reactions, or there may be an 

 interval of increased negative response. In the latter case the 

 increased negative response always accompanies an intermediate 

 rate of efficiency (p. 280). 



7. At the time of molting, sensitiveness to currents of water 

 is reduced to zero, but recovery is rapid. The rheotactic reac- 

 tions may be affected for a ten-hour interval at each molt. Dur- 

 ing this time both the number of positive reactions and the 



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



