156 Studies in Animal Behavior 



theory of instinct, it is evident that many kinds of 

 adaptive behavior occur in which the stimulus is 

 responded to, not by a direct, appropriate act, but 

 by varied movements which apparently have little re- 

 lation to the source or nature of the stimulating 

 agent. This is well illustrated by the behavior of 

 the protozoan Paramoecium which has been so ex- 

 haustively studied by Jennings. Paramoecium, as it 

 swims through the water, rotates about its long axis 

 and describes more or less of a spiral path. When 

 it encounters an object or receives a sudden stimu- 

 lus of any sort it reverses the beat of its cilia, swims 

 backward, turns to the aboral side and then con- 

 tinues on its way. It matters little on which side 

 Paramoecium is stimulated; its reaction, which Jen- 

 nings has called the "motor reflex," is practically 

 the same. Sometimes the motor reflex brings the 

 animal into closer contact with the stimulating agent. 

 If so, the reaction is repeated, and continued so 

 long as the animal is stimulated by the unfavorable 

 conditions. 



Paramoecia, like many other simple organisms, 

 form aggregations in regions of dilute acids. They 

 are not drawn to these chemicals from a distance, 

 but if a Paramoecium happens to swim into the 

 area of dilute acid it passes through to the boundary 

 between the acid and the surrounding water where 

 it gives the motor reflex and swims in another di- 

 rection. If it again encounters the boundary it 

 gives the motor reflex and swims away, so it is prac- 



