74 THE BEHAVIOR OF PROTOZOA 



when the posterior end is stimulated the Paramcecium may 

 swim back directly against the needle. A slight stimulus 

 at the posterior end often causes the infusorian to accelerate 

 its swimming, but, with this exception, the nature of the 

 response seems to bear no relation to the part of the body 

 to which the stimulus is applied. The anterior end of the 

 body is the most sensitive region, although stimuli at that 

 point do not call forth any different kind of response, but 

 only evoke the usual motor reflex with greater readiness. 



The motor reflex or avoiding reaction has the effect of 

 getting the animal away from inj urious stimulations. Often, 

 however, at first it may bring it into a worse situation than 

 before; the Paramoacium may back into an injurious chem- 

 ical, or turn toward a mechanical or other stimulus which 

 affects its aboral side; if so, the motor reflex is repeated until 

 the organism makes its escape from the unfavorable situa- 

 tion. The method of adjustment may be clumsy and in- 

 direct; had Paramoecium the power as higher organisms 

 have, of turning directly away from the stimulus, its reac- 

 tions would probably be more effective; but its restless 

 activity and quick movements are a partial compensation 

 for defects in precision of response. 



The motor reflex may be carried out in varying degrees of 

 completeness. The duration of the backward swimming 

 and the amount of turning to the aboral side are subject 

 to much variation. The two phases of the response may 

 be greatly prolonged in a solution of potassium iodide. The 

 Paramoecium swims backward for several minutes; then 

 spins around toward the aboral side for a considerable time 

 and finally swims forward. In ordinary water the distance 

 the Paramcecium swims backward is increased with a 

 stronger stimulus and the aboral rotation may continue until 

 the infusorian describes several complete circles. With 



