tive response to very power- 
THE BEHAVIOR OF PROTOZOA 75 
slight stimulations the first phase of the reaction may result 
in a momentary stopping of the course or even simply a 
slackening of speed, and the aboral turning may manifest 
itself in swinging the anterior side around in a little larger 
spiral than before. In fact this aboral turning may be re- 
garded as but an accentuation of the process which in ordi- 
nary swimming keeps the aboral side facing the outside of the 
spiral. Sometimes, as in a dilute solution of sodium chlor- 
ide, the Paramcecium stops swimming forward and turns 
around aborally, with its posterior end keeping nearly in 
one spot and its anterior end describing a wide circle. 
Paramcecium, as we have described in a previous chapter, 
reacts negatively to both hot and cold water. 
There seems to be no orientation 
here to the heat rays; when Para- 
mcecium swims to a colder or a 
warmer region it gives the motor 
reflex and goes in another direc- 
tion. Ordinary light has little 
effect upon Paramecium but Her- ae err cee 
tel has shown that it gives anega- cium in a drop of dilute 
chemical. 
ful ultra-violet rays. Its reactions to gravity are not deci- 
ded and are influenced by various factors as is mentioned in 
the foregoing section on Geotaxis. 
The behavior of Paramcecium is made up of a very limited 
number of stereotyped modes of response. It reacts to all 
sorts of stimuli by the same motor reflex carried out in 
various degrees of vigor according to the strength of the 
stimulus. But while very machine-like its behavior is at 
the same time highly plastic and adaptive. Conditions 
which are unfavorable act as stimuli, and the animal keeps 
on reacting until it gets into a region which is more favorable 
