THE BEHAVIOR OF PROTOZOA 73 



Paramoecium is a very restless organism, swimming 

 actively much of the time. Occasionally it remains quiet 

 when in contact with solid objects. When manifesting its 

 so-called thigmotactic response the cilia in contact with 

 the solid remain stiff and immobile as if anchoring the animal 

 to the spot, while the cilia over the rest of the body keep 

 moving, although with diminished vigor. If bits of cotton 

 wool are placed in the water Paramoecia are more apt to 

 come to rest, owing to the greater opportunity afforded of 



FIG. 9. Successive stages of the motor reflex of Paramoecium. (After 



Jennings.) 



securing contact stimuli. This trait keeps the Paramoecia 

 among bacterial scums and in other situations where they 

 may obtain their food. 



The principal feature of the behavior of Paramoecia is 

 what Jennings has called the "motor-reflex" or "avoiding 

 reaction." It consists of swimming backward by reversal 

 of the action of the cilia, turning to the aboral side and then 

 going ahead again. This is the stereotyped response which 

 Paramoecium gives hi essentially the same way when en- 

 countering almost any kind of stimulus. If stimulated by 

 a fine needle on the aboral side it will back off and turn to- 

 ward instead of away from the stimulating object. Even 



