192 



IRRITABILITY 



of the whole body suddenly cease their movement. (Figure 41, 

 A.) At other times, this cessation is limited to the cilia in the 

 anterior portion of the body (Figure 41, B), while the movements 

 of those on the posterior portion of the body are of less amplitude 

 or are irregular and weak. In all cases the infusorium remains 

 quiescent in the water in contact with the resistance, and it is not 

 uncommon to find numerous individuals in apposition with 

 particles of ground, slimy detritus, plant fibers and so forth. 

 (Figure 41, C.) In short, the rhythmic activity of the cilia of 



Fig. 41. 

 Thigmotaxis of Paiamaecium aurelia. 



the Paramecia receiving their normal impulses of excitation from 

 the ectoplasm of the cell body interfere with strong mechanical 

 stimuli in such a manner that a negative thigmotaxis develops; 

 following weak mechanical stimuli a positive thigmotaxis results. 

 Here is an instance of the relation between the intensity of the 

 stimulus and the manner in which its effects interfere with an 

 already existing excitation. 



However, the strength of the inhibitory effect of a weak con- 

 tact stimulus upon another excitation is best appreciated when 



