I2O GEORGE DELWIN ALLEN. 



trials 2 and 4. In the control tank the worms remained in the 

 aquarium water in all the trials. The temperature was 21 C. 

 The reactions in each trial are described in Table III. 



From the table it can be seen that placing the worms in fresh 

 water reversed the reaction from positive to negative while 

 returning them to their former aquarium water brought back 

 the former positive reaction. These reactions were reversed 

 back and forth by changing the water. The reversal of reaction 

 cannot be attributed to a difference in the velocity of the current. 

 The negative reactions in the fresh water were given in fairly 

 strong current while the positive reactions in aquarium water 

 were given in very weak as well as stronger current; and in the 

 control, in which the velocity of the current was approximately 

 the same at each moment as that in the experiment, a positive 

 reaction was given at the same time that a negative reaction was 

 observed in the experiment. In the two tests in this experiment 

 with fresh water (trials 2 and 4 ), therefore, a change to fresh 

 water induced a reversal of reaction from positive to negative 

 while a return to the normal environment brought back the 

 positive reaction. In 40 such tests in experiments at different 

 times, a reversal from positive to negative was given in 21 tests 

 and a reversal from negative to positive in 5 tests. In 7 tests 

 a positive reaction remained unchanged while in 5 tests a nega- 

 tive reaction remained unchanged. In 2 tests the reaction was 

 "indefinite." 



Fresh water differs from aquarium water in oxygen content 

 and in the absence of the metabolic products of the worms and 

 the decomposition products that accumulate in the aquarium. 

 The aquarium water in experiments at different times varied 

 greatly in composition, while the fresh drawn tap-water probably 

 varied little. It would seem that fresh water would make a 

 more suitable environment than stale aquarium water where 

 many worms have lived together without green vegetation. 

 In the experiment described above the aquarium water was 

 quite foul from the decomposition products of juices of meat 

 that had stood in the aquarium for nearly 24 hours. When 

 modified aquarium water was used instead of fresh water, re- 

 versal of the same sort was induced. In one experiment part of 



