No. 389.] STIMULI IN UNICELLULAR ORGANISMS. 381 
but swim straight on, crossing the area and coming out on the 
other side, unless plasmolyzed and killed during the passage. 
It is evident that the Spirostoma are neither attracted nor 
repelled by the NaCl; it merely sets in operation their one 
reaction, and this takes them into danger or safety as chance 
may direct. Under normal conditions, of course, the anterior 
end will usually be directed toward the stimulating agent, since 
the animal generally swims forward, and masses of dangerous 
chemicals are not often dropped suddenly into the midst of a 
group of the Infusoria ; hence the device of swimming backward 
usually saves the animal. The following curious experiment 
shows how possibly a combination of circumstances might arise 
even under normal conditions such that the reaction would 
result in the destruction of the animal. A small mass of NaCl 
was slowly dissolving in the center of the slide. A Spirostomum 
was swimming forward directly away from the diffusing salt, 
not being in the region of its influence at all. Its posterior end 
was thus pointed toward the salt, but as it was swimming away 
it was in no danger. Nowa slight jar was given to the prepa- 
ration — such as might easily occur in nature. Thereupon the 
Spirostomum reacted in the usual manner, by contracting and 
swimming backward. It thus swam toward the NaCl, until 
finally its posterior end came in contact with the advancing 
flood of salt solution. Thereupon the customary reaction was 
again induced still more powerfully ; the animal contracted and 
swam still more swiftly backward; thus entering the salt solu- 
tion, it was plasmolyzed and killed. 
When a Spirostomum swimming forward comes in contact 
with a diffusing chemical, it contracts, darts backward, then 
swings its anterior end about, finally turning toward the aboral 
side and swimming straight forward — so long as this does not 
take it again into the region of the stimutating agent. If it 
does, the reaction is repeated until by the laws of chance the 
Spirostomum is directed into a region which does not cause 
stimulation. If the stimulus with which the anterior end comes 
in contact is very weak, the animal may omit the contraction 
and move a little backward without contracting ; then the ante. 
rior end is swung about in a circle (the aboral side, of course, 
