374 THE AMERICAN NATURALIST. (VoL. XXXIII. 
mum and Stentor is presented as a contribution toward answer- 
ing this question. 
The minute size of Paramecium brought with it the great 
disadvantage that it was not possible under experimental condi- 
tions to apply localized stimuli to definite parts of the body, so 
that recourse was necessary to observation of chance contacts 
of sources of stimuli with one or another part of the body; 
an unsatisfactory method, and one requiring much time and 
patience. This difficulty is obviated in Spirostomum ambiguum 
and Stentor polymorphus, which are both so large that there is 
no difficulty in applying stimuli to any desired region on the 
surface of the body. The simplest means of doing this is to 
touch any point on the surface of the body with a proper instru- 
ment, thus giving the animal a sharply localized mechanical 
stimulus. Other methods are given in the following account 
of observations. 
Spirostomum ambiguum (Fig. 1). 
Spirostomum ambiguum is one of the largest of unicellular 
animals, reaching a length of two or three millimeters. The 
average length of those on which the following observations 
were made was about one and one-half millimeters. In form 
Spirostomum is a long, slender cylinder of nearly equal diam- 
eter throughout, but slightly smaller at the ends. The mouth 
lies behind the middle of the body, and from this a band of 
large cilia (the adoral zone) runs to the anterior end of the 
body. The large contractile vacuole lies at the posterior end, 
and from this a canal runs almost the entire length of the body 
near its aboral side, or curving a little onto the right side. The 
adoral zone and this canal form important landmarks for deter- 
mining directive relations in studying the movements of the ani- 
mals. The posterior end is truncate, while the anterior end is 
rounded and shows a difference in its contour on the two sides 
of the animal. The tip is curved slightly toward the side on ' 
which the mouth is situated (the oral side), so that on the oppo- 
site or aboral side the contour is a long convex curve, while on 
the oral side there is almost an angle. These facts are best 
