Jennings, Behavior of Paramecium. 



451 



as I have shown in previous pa- 

 pers, Paramecium responds to heat, 

 cold, mechanical stimuli, chemicals 

 of all sorts, osmotic stimuli — in 

 fact, to stronger stimuli of almost 

 all classes. 



The avoiding reaction is brought 

 about through certain modifica- 

 tions of the three factors in the 

 spiral swimming. The first phase 

 of the reaction is a slowing, suspen- 

 sion or reversal of the forward com- 

 ponent in the spiral course. In a 

 very pronounced reaction, caused 

 by a powerful stimulus, the for- 

 ward course is reversed, while the 

 revolution on the long axis and the 

 swerving toward the aboral side 

 continue as before. The animal 

 therefore swims spirally backward 

 for a distance (Fig. 4). When the 

 stimulus is weaker, the forward 

 course is merely suspended for a 

 moment— the revolution and swerv- 

 ing toward the aboral side contm- 

 uing. Finally, in some cases the 

 forward course is merely made 

 slower. The backward swimming 

 or stoppage is brought about by a 

 reversal of the forward compo- 

 nent in the stroke of the cilia. In 

 a pronounced reaction all the cilia 

 are reversed (Fig. 4); in a less 

 marked reaction the body cilia are 

 reversed while the oral cilia are not 

 (Fig. 5). In the latter case the effect on the currents in the 

 water, as shown by the movements of particles of India ink, is 



Fig. ^. Backward spiral cours 

 of Paramecium in reacting to a 

 stimulus, d, c, b, a, successive po- 

 sitions occupied. The turning is 

 to the left, in the same direction 

 as in Fig. 3. 



