452 Journal of Comparative Neurology and Psychology. 



peculiar. The currents pass forward everywhere, save in the 

 oral groove, where they pass backward. Since the animal at 

 the same time revolves on its long axis, the particles in a given 

 region close to the Paramecium at first dart forward, then later 

 backward, depending on whether the body surface or the oral 

 groove is directed toward the region in question. 



The second feature in the 

 avoiding reaction is the increased 

 turning toward the aboral side. 

 This is due to two changes in 

 the stroke of the cilia. The first 

 and less important is the fact, 

 mentioned above, that after a 

 stimulus of not very great inten- 

 sity the body cilia are reversed, 

 while the oral cilia continue to 

 beat backward. This of neces- 

 sity turns the anterior end toward 

 the aboral side. The second and 

 more important factor is a change 

 in the stroke in the body cilia of 

 the left side, in the anterior por- 

 tion of the animal. In the ordi- 

 nary swimming, as we have seen, 

 the cilia of the right side strike 

 toward the oral groove (Fig. i, 

 b), those of the left side away 

 from the oral groove (Fig. i, c). But in'^the avoiding reaction, 

 both while the swimming backward continues and after it has 

 ceased, the cilia of both right and left sides strike toward the 

 oral side. This of course drives the body of the animal toward 

 the aboral side. The difference between the stroke of the cilia 

 in the usual course, and in the avoiding reaction is shown in 

 sectional views in Fig. 6. 



Thus the cilia to the left of the oral groove play a most 

 important part in the avoiding reaction, reacting by a reversal 

 of the direction of the usual stroke — at least by a reversal of 



Fig. 5. Currents in the reaction 

 to a weak stimulus, or near the end 

 of a reaction to a strong stimulus. 

 The animal moves backward : the 

 body cilia are reversed, the oral cilia 

 are not. The arrows show the direc- 

 tion of the water currents. 



