REACTIONS OF PLANARIA TO WATER. 121 



the aquarium water was boiled, and in the two tests made with 

 this boiled aquarium water reversal of reaction from positive to 

 negative was obtained. In another experiment cane sugar was 

 dissolved in part of the aquarium water to make a I per cent- 

 solution which induced reversal from positive to negative in the 

 6 tests made. Similarly a well-mixed solution of juices of raw 

 beef in aquarium water induced the same reversal in the 6 tests 

 made. So far as these experiments indicate, therefore, the re- 

 versal of the rheotropic reaction in the new environment seems 

 to be due more to the strangeness of the new conditions than to 

 their fitness or unfitness. In a total of 54 tests with modified 

 aquarium water and fresh water, reversal of reaction from posi- 

 tive to negative was induced in 35 tests, while in 5 tests the re- 

 action was already negative from other causes. These figures 

 may not necessarily represent the proportion of individual worm 

 reactions which can be reversed in this way, since the conditions 

 in different experiments were not entirely identical, but it may 

 be repeated that in each of these tests a large number of worms 

 were tested, in some cases many hundreds of individuals. 



V. REVERSAL OF REACTION INDUCED BY CHANGES 

 OF TEMPERATURE. 



The effect of changes of temperature upon the rheotropism 

 was tested by dividing the water in which worms were living 

 into two portions, preserving one portion at the temperature at 

 which the worms were found at the beginning of the experiment, 

 and cooling or warming the other portion to the desired tempera- 

 ture. The worms were then tested alternately in these two 

 portions of water. It was found that the rheotropism can be 

 reversed from positive to negative by lowering the temperature in 

 the same manner as by changing the composition of the water. 



In 34 individual tests, lowering the temperature by 4-io C. 

 was found to cause a reversal in 17 tests from positive to negative, 

 and reversal from negative to positive in 2 tests. In 4 tests 

 this change in temperature made an indefinite reaction become 

 positive. In the remaining n tests, lowering the temperature 

 produced no observed change in reaction. This remained as 

 before the temperature change; that is, as follows: positive in 7 



