378 THE AMERICAN NATURALIST. (VoL. XXXIII. 
lus can be given by jarring the dish or slide containing the 
animals. They contract, swim backward, turn toward the 
aboral side, and swim forward, exactly as in the other cases. 
Thus the reaction given by Spirostomum to a mechanical 
stimulus is identically the same, whatever part of the body is 
stimulated, or even if the stimulus is not localized at all. A 
diagram of the reaction of Spirostomum to a stimulus is given 
in Fig. 2. 
5. Repeated Stimuli. — When the same animal is repeatedly 
stimulated, certain features in the reaction are especially worthy 
of notice. At the first stimulus the animal contracts, then 
8 
Fic. 2.— Reaction of Spirostomum toa stimulus. The numbers give the successive 
positions of the animal, while the arrows show the direction of motion. The 
position of the adoral is sh pt where it lies ab below. No. 
shows the motion before stimulation occurs. 
swims backward. Now if, after recovering from the contrac- 
tion, but while still swimming backward, it is again stimulated, 
it again contracts and continues to swim backward. On a third 
stimulation, while still swimming backward, it usually reverses 
its course and swims forward. A reversal of the direction of 
motion now usually occurs at each new stimulus up to four or 
five. This reversal at each new stimulus may easily give the 
impression that the animal is swimming each time away from 
the source of stimulation, and hence that it is reacting with 
relation to the localization of the stimulus; but this appearance 
is due merely to a psychological peculiarity of the experimenter. 
It is natural for the experimenter to touch the animal at the 
end toward which it is moving —to “head it off” as it were. 
On the third trial he will, as above stated, usually succeed in 
getting it to reverse its motion and swim in the opposite direc- 
