THE BEHAVIOR OF PROTOZOA 85 



lar to the phenomenon of acclimatization to chemicals and 

 other agencies which is apparently a fundamental charac- 

 teristic of living organisms. 



The replacement of reactions by Stentor probably con- 

 sists to a considerable degree in variations in the vigor and 

 completeness of a single reaction. A gentle contraction may, 

 owing to the bodily peculiarities of the animal, involve a 

 turning to the aboral side, as its easiest channel of expres- 

 sion. With stronger contraction this would naturally be 

 obscured, so that an apparently new response may result 

 from the heightened irritability caused by the preceding 

 stimuli. The succession of contractions resulting in the 

 separation of the organism from its attachment naturally 

 falls under the same interpretation, and is analagous to 

 what takes place in an excised heart which, upon being 

 given a single stimulus, may contract once or several times 

 according to its condition of irritability. The successive 

 contractile phenomena in Stentor are more or less analagous to 

 those of summation of stimuli in an excised muscle. The re- 

 versal of the beat of the cilia is a separate reaction, although 

 it may have a certain relation to the phenomenon of con- 

 traction. We are not justified in assuming that Stentor 

 passes through a number of discrete internal states each 

 of which has a correspondingly discrete motor response. 

 The number of physiological states as in every organism 

 is unlimited and the behavior of the animal shows us a 

 series of reactions differing for the most part hi degree of 

 vigor rather than in kind, like the motor reflex in Paramce- 

 cium, which may be carried out in various degrees of com- 

 pleteness from a momentary slowing of speed to a prolonged 

 backward swimming followed by numerous rotations toward 

 the aboral side. 



Schaeffer has carried on a series of careful experiments in 



