48 BEHAVIOR OF THE LOWER ORGANISMS 



which add greatly to its effectiveness. After getting rid of the stimulus 

 by swimming backward a distance there must be some way of deter- 



FIG. 36. Diagram of the avoiding reaction of Paramecium. 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.) 



mining the new direction in which the animal is to swim forward. It 

 is evident that some method of testing the conditions in various different 

 directions in advance would be the most effective way of accomplishing 

 this. The infusorian now moves in precisely such a way as to make 

 such tests. It will be recalled that in its usual course the animal is 

 revolving on the long axis and swerving a little toward the aboral side 

 (Fig. 33), so that it swims in a narrow spiral. After swimming back- 

 ward a certain distance in response to stimulation, the revolution on the 

 long axis becomes slower, while the swerving toward the aboral side is 

 increased. As a result the anterior end swings about in a large circle; 

 the animal becomes pointed successively in many different directions, 

 as illustrated in Figs. 37 and 38. From each of these directions it 

 receives in its ciliary vortex a "sample" of the water from immediately 

 in advance, as the figures show. As long as the samples contain the 

 stimulating agent, the hot or cold water, the chemical, or the like, 

 the animal holds back and continues to swing its anterior end in a 

 circle "trying" successively many different directions. When the 

 sample from a certain direction no longer contains the stimulating agent, 

 the animal simply resumes its forward course in that direction. Thus 

 its path has been changed, so that it does not enter the region of the 

 chemical or the hot or cold water. Mechanical obstacles are avoided 

 in precisely the same way, save that of course the ciliary vortex does 

 not bring samples of the stimulating agent, so that the infusorian is 

 compelled to try starting forward repeatedly in various directions, be- 

 fore it finds one in which it can pass freely. 



