76 THE BEHAVIOR OF PROTOZOA 



to its life. While we commonly speak of the positive reac- 

 tions of the organism, strictly speaking there are, with the 

 exception of its thigmotaxis, no positive reactions. If 

 Paramoecia collect in weak acid it is not because the acid 

 attracts them in any way; they are not stimulated even 

 when they accidentally enter the acid; they react only 

 when they pass from the acid into the water. It is not 

 their positive reaction to the acid but their negative reaction 

 to the water that causes what appears to be a positive 

 chemotaxis. The same is of course true of their reactions 

 to various other stimuli. Their life is one of continual 

 avoidance. Only when conditions are favorable is there 

 cessation of movement. If we do not wish to attribute to 

 such creatures the power of choice we must admit that the 

 method of behavior secures it the same advantages that 

 choice affords. 



The behavior of Paramcecium, like that of every other 

 organism, is modified by changes of its internal condition. 

 Strong induction shocks render Paramoecium insensitive to 

 weaker shocks and if individuals are kept for a time at 

 a temperature higher than normal a higher temperature is 

 required to cause the avoiding reaction. Individuals that 

 have been kept without food become restless while well fed 

 ones are more sluggish and more apt to come to rest against 

 solid objects. Reactions to gravity are influenced by food 

 and other conditions and thigmotaxis is markedly affected by 

 temperature. Behavior may be modified by repetition of the 

 same contact stimulus as described in the following quota- 

 tion from Jennings: "If a bit of filter paper is placed in a 

 preparation of Paramoecia, the following behavior may often 

 be observed. An individual swims against it, gives the avoid- 

 ing reaction in a slightly marked way, swimming backward a 

 little; then it swims forward again, jerks back a shorter 



