74 THE BEHAVIOR OF PROTOZOA 
when the posterior end is stimulated the Paramecium 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 injurious stimulations. Often, 
however, at first it may bring it into a worse situation than 
before; the Paramcecium 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 Paramcecium 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 
Paramecium 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 
