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 Paramoscium stops swimming forward and turns 

 around aborally, with its posterior end keeping nearly in 

 one spot and its anterior end describing a wide circle. 



Paramoecium, 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- 

 moecium swims to a colder or a 

 warmer region it gives the motor 

 reflex and goes hi another direc- 

 tion. Ordinary light has little 

 effect upon Paramcecium but Her- 



FIG. 10. Course of Paramce- 



t el has shown that it gives a nega- cium in a drop of dilute 



chemical. 



tive response to very power- 

 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 hi 

 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 



