No. 389.] STIMULI IN UNICELLULAR ORGANISMS. 379 
tion. He then “heads it off” again by touching the other 
end toward which it is now swimming, with the result that it 
reverses again, This alternation on the part of experimenter 
and infusorian may continue for a number of times, giving the 
impression that the animal is clearly reacting with reference to 
the position of the source of stimulus, fleeing from it in each 
case. But this alternation is really an independent phenome- 
non in each of the two organisms concerned in the experiment, 
as is proved by the following. If the experimenter continues 
to stimulate the animal at the same end, regardless of the 
direction in which it is moving, the animal’s direction of motion 
will alternate as before. Thus, after the second stimulation, 
while the Spirostomum is swimming backward, if stimulated 
at the anterior end, it will now swim forward or toward the 
source of stimulus ; if stimulated again at the anterior end, it will 
reverse and swim backward; again, and it swims forward once 
more. In these cases, as in the others, therefore, the direction 
of motion has no relation to the localization of the stimulus. 
If the stimulations are continued after five or six times, the 
animal will continue to swim violently in one direction or the 
other, without regard to the repetition of the stimulus or its 
localization. It is thus possible to stimulate the animal repeat- 
edly at its posterior end while it is swimming violently to the 
rear, and thus toward the source of stimulus; in other cases it 
swims as violently away from the stimulus. In no case does 
the position of the stimulus have any effect on the direction of 
motion. 
B. Chemical Stimuli. 
It is easy to localize the action of chemical stimuli in the 
following manner. A capillary glass rod is coated with paraffin. 
A crystal of NaCl or other salt is then attached to the rod by 
means of the paraffin coating, and can then be held near to any 
part of the animal’s body. 
1. Stimulus at Anterior End.— The crystal of NaCl is held 
close to the anterior end of the Spirostomum, but without 
touching it, so that only the diffusing salt in solution comes in 
contact with the animal. The Spirostomum contracts, swims 
