No. 389.] STIMULI IN UNICELLULAR ORGANISMS. 385 
stimulated on the right side, the animal turns toward the side 
stimulated; if on -the left side, it turns away from the side 
stimulated. Owing to the revolution on the long axis while 
swimming backward, the position of the right side at the time 
of turning on the short axis bears no definite relation to its 
position at the time of stimulation ; so we find that the absolute 
direction toward which the animal turns has no constant or 
prevailing relation to the absolute direction from which the 
stimulus came. 
3. Stimulus at the Posterior End. — The posterior end of 
Stentor, narrowing to a point of attachment, is very little 
sensitive, so that touching it with the rod of glass usually 
causes no reaction whatever. By giving it a smart blow, 
however, a reaction can be induced, and this is then identical 
with the reaction already described. The animal thus, of 
course, swims at first toward the source of stimulus. 
4. Stimulus not Localized. — If the vessel containing the 
Stentors is jarred, they react in the same manner as to 
localized stimuli. 
B. Chemical Stimult. 
Stentor gives the same reaction to chemical as to mechanical 
stimuli. If when swimming forward through the water it comes 
TAS 
Eeg e AN 
d hu seu Races 
Fic. 4. — Diagram of the reaction of Stentor. The arrows show the direction of motion, while 
the numbers indicate the successive positions of the animal; No. 1 showing t 
before stimulation occurs. 
~*~ . 
in contact with a chemical substance sufficiently powerful to 
act as a stimulus, it contracts, swims backward, turns to the 
