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 in degree of 
vigor rather than in kind, like the motor reflex in Paramee- 
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 
