THE 
AMERICAN NATURALIST 
VoL. XXXIII. May, 1899. No. 389. 
STUDIES ON REACTIONS TO STIMULI IN UNI- 
CELLULAR ORGANISMS. 
H. S. JENNINGS. 
III. Reactions TO LOCALIZED STIMULI IN SPIROSTOMUM 
AND STENTOR. 
In the second of these studies (American Journal of Physi- 
ology, May, 1899) the writer has given an account of the 
mechanism of the reactions of Paramecium that involves an 
entirely different conception of the nature of these activities 
from that which has been generally assumed. It was shown 
that this protozoan has but one motor reaction in response to 
the most varied stimuli, and that it reacts without any relation 
whatever to the position of the stimulating agent, so that it 
cannot be said to be attracted or repelled by any agency or 
condition — its reaction being strictly comparable in all essen- 
tials to that of an isolated muscle. The importance of this, in 
case it should turn out to be the general method of reaction 
for unicellular organisms, is obvious, involving, as it does, the 
rejection of almost all hitherto received theories of the mechan- 
ism of reactions; the question, therefore, immediately arose as 
to whether this method of reaction was to be extended to other 
Protozoa. The following study of the reactions of Spirosto- 
373 : 
