Jennings, Behavior of Parauicciuni. 



45 3 



the transverse or oblique component of the stroke. They thus 

 play a part similar to the large cilia at the left of the peristome 

 in the Hypotricha, and to the cilia which Wallengren (1902) 

 designates as the ' ' Drehungswimpeni' in Opalina. It is to their 

 reversal that the most characteristic features of the reaction 

 are due. 



The change in the stroke of these cilia 

 of the left side explains the third feature 

 of the avoiding reaction ; namely, the 

 modification of the revolution on the long 

 axis. The turning toward the aboral side 

 in the reaction involves an increase in the 

 swerving found in the normal swimming, 

 in proportion to the rate of revolution. 

 The change in the stroke of the cilia of the 

 left side causes, as we have seen, the in- 

 creased swerving ; it likewise causes a de- 

 crease or stoppage in the revolution on the 

 long axis. In the usual swimming, the cilia 

 of both right and left sides tend to turn the 

 body over to the left (Fig. 6, «); in the 



Fig. 6. Diagram of cross sections of Paramecium (viewed trom the anterior 

 end), showing the obliquity of the ciliary stroke, a, condition in the usual for- 

 ward progression : the body cilia all strike toward the right side ; b, condition 

 while turning toward the aboral side, in reacting to a stimulus : the cilia of the 

 left side have changed the direction of their stroke ; /, lett side ; r, right side ; 

 0, oral grcJove. The arrows show the direction in which the cilia tend to turn 

 the body. 



avoiding reaction the cilia of the left side tend to turn the body 

 to the right, those of the right side to turn it to the left (Fig. 

 6, b). Thus the cilia of the two sides oppose each other so far 

 as revolution is concerned, but co-operate in causing the body 

 to swerve toward the aboral side. 



The effectiveness of the change of beat of the cilia of the 

 left side varies much, apparently as a result of the fact that the 

 number of cilia having the changed stroke varies. On this 

 point it is exceedingly difficult to determine numerical or pre- 

 cise quantitative relations. But if the stimulus is weak, appar- 



