74 AN INTRODUCTION TO ZOOLOGY 



cilia and swims backward for a short distance out of the region oi 

 stimulation; then its rotation decreases in rapidity and it swerves 

 toward the aboral side more strongly than under normal condi- 

 tions. Its posterior end then becomes a sort of pivot upon which 

 the animal swings about in a circle (Fig. 33). During this revo- 

 lution 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 



I. *- 



c?. 



FIG. 33. Diagram of the avoiding reaction of Parameclum. A is a solid 

 object or other source of stimulation. 1-6, successive positions occupied 

 by the animal. (The rotation on the long axis is not shown.) (From 

 Jennings.) 



forward again. If this once more brings it into the region of the 

 stimulus, the avoiding reaction is repeated; this goes on as long 

 as the animal receives the stimulus. The repetition of the avoid- 

 ing reaction is very well shown when Parameclum enters a drop 

 of ^V P er cent acetic acid. In attempting to get out of the drop 

 the surrounding water is encountered; to this the avoiding reac- 

 tion is given and a new direction is taken within the acid, which 

 of course leads to the water and another negative reaction. The 

 accompanying Figure 34 shows part of the pathway made by a 

 single Paramecium under these conditions. 



Positive Reaction. If a little acid is placed in the center of a 

 large drop of water containing a number of Paramecia, all of 



