196 



IRRITABILITY 



of the individuals which are freely swimming and turning in 

 a screw-like manner around their axis, steer immediately toward 

 the cathode, exactly as in the case of the Paramecia. On the 

 other hand, those which are fixed to the bottom of the slide as 

 a result of thigmotaxis, upon closing of the current, make a short 

 turn and assume a position wherein the long axis is at right 

 angles to the direction of the current, and the perioral rim is 

 directed toward the cathode. In this position they move through 

 the field. (Figure 44.) When the current is broken the indi- 



Fig. 44. 



Urostyla grandis. Interference of galvanotaxis and thigmotaxis. The 

 freely swimming individuals move towards the cathode (left side). 

 The creeping individuals move in tranverse direction. 



viduals draw backwards, distribute themselves and creep and 

 swim in all directions in the water. If during the course of the 

 passage of the current, an individual which has been swimming 

 begins to creep, the axis immediately assumes the position above 

 described in the case of the organisms which are in contact with 

 the bottom and vice versa. The thigmotaxis, therefore, influences 

 galvanotactically swimming organisms in a most characteristic 

 manner. As a consequence of the interference of thigmotaxis 

 and galvanotaxis, the organisms move in a direction transversely 

 to the direction of the current. This most striking reaction has 

 been cleared up by Piitter,^ the explanation being based upon an 



1 Putter: 1. c. 



