REACTIONS OF PLANARIA TO WATER. 115 



actions at the same time, that is, an intermingling of the two 

 systems of spirals, was not common. The reactions varied, 

 however, in uniformity at different times; that is, in the pre- 

 cision of orientation, and in some cases the worms scattered from 

 the center without forming any very definite spiral in either direc- 

 tion. Such "reactions" were designated "indefinite." 



III. DIFFERENCE OF REACTION TO CURRENTS OF 

 DIFFERENT VELOCITIES. 



In studying the reversal of rheotropic reactions induced by 

 changing the chemical and thermal conditions, it was found 

 that it is important to consider the velocity of the current. A 

 change in the velocity of the current, or, in other words, in the 

 strength of the rheotropic stimulus, may itself cause reversal of 

 the sign of the reaction. 



Since a current in a small vessel diminishes rapidly in velocity 

 after the stirring is discontinued, the different worms in a 

 single trial are subject to different velocities of current, the 

 first worms to leave the center entering the strongest current 

 and later worms finding successively weaker current. It was 

 frequently observed that the positive reaction was given by 

 the earlier worms; that is, those in the strongest current, and 

 the negative reaction by the later worms; that is, those in the 

 weaker currrent. The proportion of individuals giving each 

 reaction varied in different observations while in some cases 

 no negative reaction was given and in other cases no positive 

 reaction was given. 



A positive reaction in stronger current and a negative reaction 

 in weaker current can be seen at the same time in a broad- 

 bottomed pan where the negative spiral may begin about the 

 center before the last individuals belonging to the positive spiral 

 have reached the sides. This is possible because the current 

 is swifter about the outside where the positive reaction is still 

 being given than about the center where the negative reaction 

 is beginning. Fig. 2 shows such a combination of positive 

 spiral about the outside and negative spiral about the center. 

 In the case which is photographed there is considerable irregu- 

 larity in the positive spiral. In many cases the positive spiral 

 about the outside was as regular as that in Fig. I. 



