122 GEORGE DEL WIN ALLEN. 



tests, negative in 3 tests, positive in stronger current and nega- 

 tive in weaker current in one test. 



The characteristic rheotropic effect, therefore, of lowering 

 the temperature below that to which the worms are accustomed 

 is a reversal of reaction from positive to negative. Return to 

 the normal temperature brings back the positive reaction so 

 that the reactions can be reversed back and forth by alternately 

 lowering the temperature and raising it again to the normal. 

 The reversal from negative to positive on raising the temperature 

 seems to be due to the return to the temperature to which the 

 the worms are accustomed, rather than to the fact that the tem- 

 perature is raised, since in 13 tests of raising the temperature 

 above the normal by 4-io C. reversal from negative to posi- 

 tive was obtained in only one case. In 4 tests the reversal was 

 in the opposite direction ; that is, from positive to negative. In 

 5 tests the negative reaction remained unchanged and in 2 tests 

 the positive reaction remained unchanged, while in one test the 

 reaction became "indefinite." 



VI. VARIATIONS IN RHEOTROPIC REACTIONS. 



The general features of the rheotropic reactions of planarians 

 and their reversibility described above were observed in mass 

 tests of large numbers of individuals. These depend upon con- 

 siderable uniformity in the behavior of the different individuals 

 in the trial, much as do the behavior experiments with cultures 

 of protozoa. The uniformity of the reactions of the different 

 individuals in a single trial in the experiments with planarians 

 was generally very striking, at least with respect to the sign of 

 the reaction in the same velocity of current. The precision of 

 the reactions of different individuals varied greatly in different 

 trials, although the sign of the reaction was definite. When 

 the reaction is definitely either positive or negative, the char- 

 acteristic spiral figure is very striking, and when a spiral could 

 not be seen, the reaction was called indefinite. Only a small 

 number of trials, however, failed to show a general uniformity 

 in the sign of the reaction in the same velocity of current (Table I.). 



The different tests of reversal in the same experiment gener- 

 ally gave fairly consistent results although with some variations 



