THE BEHAVIOR OF INFUSORIA; PARAMECIUM 



49 



This method of behaving is perhaps as effective a plan for meeting 

 all sorts of conditions as could be devised for so simple a creature. On 

 getting into difficulties the animal retraces its course for a distance, 

 then tries going ahead in various directions, till it finds one in which 

 there is no further obstacle to its progress. In this direction it continues. 

 Through systematically testing the surroundings, by swinging the an- 

 terior end in a circle, and through performing the entire reaction re- 

 peatedly, the infusorian is bound in time to find any existing egress from 

 the difficulties, even though it be but a narrow and tortuous passageway. 



The different phases of this avoiding reaction are evidently due to modifications 

 of the three factors in the spiral course. The swimming backward is due of course 

 to a reversal of the forward stroke of the cilia. The turning toward the aboral side 

 is an accentuation of the swerving that takes place always ; it is due to the fact that 

 the cilia at the left side of the body strike during the reaction toward the oral groove 

 instead of away from it. Thus the ciha of both right and left sides now tend to turn 

 the animal toward the aboral side. The difference between the usual condition and 

 that found during the reaction is illustrated in Fig. 34. Finally, the decrease or 

 cessation in the revolution on the long axis is due to the same factor as the increase 

 in swerving toward the aboral side. During the reaction the cilia of the left side 

 oppose the usual revolution on the long axis to the left (as shown in Fig. 34), 

 through the same change which causes them to assist in turning the body toward the 

 aboral side. 



The avoiding reaction varies greatly under different conditions, 

 though its characteristic features are maintained throughout. But its 

 different phases vary in intensity depending 

 on circumstances. The backward movement 

 may be long continued, or may last but a 

 short time; or there may be merely a stop- 

 page or slowing of the forward movement. 

 The swerving toward the aboral side may be 

 only shghtly increased, while the revolution 

 on the long axis becomes a Kttle slower. ' In 

 this case the anterior end swings about in 

 a small circle, as in Fig. 37, so that the ani- 

 mal is pointed successively in a number of 

 directions varying only a httle from the origi- 

 nal one. With a stronger stimulus the swerv- 

 ing toward the aboral side is more decided, 

 while the rotation on the long axis is slower ; 

 then the anterior end swings about a larger 

 circle, as in Fig. 38. The Paramecium thus 

 becomes pointed successively in many directions differing much from 

 the original one. Finally, the rotation on the long axis may com- 



FlG. 37. — Paramecium 

 swinging its anterior end about in 

 a small circle, in a weak avoiding 

 reaction, i, 2, 3, 4, successive 

 positions occupied. 



