No. 389.] STIMULI IN UNICELLULAR ORGANISMS. 383 
ment. This is true when the posterior piece comprises as much 
as nine-tenths of the entire animal. 
It seems; therefore, that the power of initiating a turn, and 
the power of continuing a course once begun, are localized in 
some way in the anterior end. 
` Stentor polymorphus (Fig. 3). 
Stentor polymorphus is smaller than Spirostomum ambiguum, 
but is still of sufficient size to make the application of localized 
stimuli a matter of no difficulty. It is a trumpet-shaped animal, 
exceedingly changeable in A 
exact form and proportions. 5 
Fig. 3 shows a usual form 
of the animal when swim- 
ming freely ; when anchored 
by its base the form is more 
extended and slender. The 
surface of the animal is 
covered with cilia in longi- 
tudinal rows, whilethe broad R._....% 
anterior end, known as the 
peristome, is surrounded by 
a circle of larger cilia form- 
ing the adoral zone. At 
one side of the disk is a 
funnel-like depression which 
leads into the mouth. That 
surface of the body nearést 
to which the mouth lies may 
be called the oral surface; 
the o ite one, the aboral one 
Laie te ye . 5 Fic. 3. — Stentor polymorphus, partially contracted 
surface. Considering the free-swimming individual, pict amend rT 
° ior end; Z, posterior end; zi left side; 2, right 
Oral as equivdlent: to vor 2a. muh; J prit 
tral surface, we may define 
right and left sides as follows: When the oral surface is below, 
and the anterior end is away from the observer, the right and 
left sides of the animal correspond to the observer's right and 
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