THE BEHAVIOR OF PROTOZOA 73 
Paramecium 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 Paramcecia are more apt to 
come to rest, owing to the greater opportunity afforded of 
Fic. 9.—Successive stages of the motor reflex of Paramecium. (After 
Jennings.) 
securing contact stimuli. This trait keeps the Paramecia 
among bacterial scums and in other situations where they 
may obtain their food. 
The principal feature of the behavior of Parameecia 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 
Paramcecium gives in 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 
