PHYLUM PROTOZOA 57 



at its anterior end, it reverses its cilia and swims backward for 

 a short distance out of the region of stimulation; then its rota- 

 tion decreases in rapidity and it swerves toward the aboral side 

 more strongly than under normal conditions. Its posterior 

 end then becomes a sort of pivot upon which the animal swings 

 about in a circle (Fig. 37, 3-5). During this revolution samples 

 of the surrounding medium are brought into the oral groove. 

 When a sample no longer contains the stimulus, the cilia resume 

 their normal beating, and the animal moves forward again. If 

 this once more brings it into the region of 

 the stimulus, the avoiding reaction is re- 

 peated; this goes on as long as the animal 

 receives the stimulus. The repetition of 

 the avoiding reaction is very well shown 

 when Paramecium enters a drop of ^ per 

 cent acetic acid. In attempting to get out 

 of the drop the surrounding water is en- , FIG. 38. Path fol- 



" . lowed by a single Fa- 



countered; to this the avoiding reaction is rametium in a drop 

 given and a new direction is taken within ! aci f- (From Jen - 



nings.) 



the acid, which of course leads to the water 

 and another negative reaction. The accompanying Figure 38 

 shows part of the pathway made by a single Paramecium under 

 these conditions. 



Paramecium responds positively under certain conditions. 

 Often it comes to rest against an object, positive thigmotropism. 

 When subjected to chemical substances or heat, it swims about 

 in all directions, giving the avoiding reaction until it succeeds in 

 getting into a suitable environment. This is the method of trial 

 and error, that is, the animal tries all directions until the one is 

 discovered which allows it to escape from the region of un- 

 favorable stimulation. " For each chemical there is a certain 

 optimum concentration in which the Paramecia are not caused 

 to react." There is also an optimum temperature, which lies, 

 under ordinary conditions, between 24 and 28 C. 



Gravity stimulates Paramecium in some unknown way to 



