384 THE AMERICAN NATURALIST.  [Vol. XXXIII. 
left. (In the figure the oral surface is above, so that right 
and left sides are reversed.) The body of the animal is usually 
curved, being bent from the direction of the anterior end 
toward the left side; sometimes there is near the posterior 
end a second short curve to the right. 
Stentor polymorphus is often found attached by its posterior 
end; at other times it swims freely in the water. The motion 
in the free-swimming individuals is as follows: The animals 
swim slowly forward; at the same time they may or may not 
revolve on ‘the long axis. The revolution when it occurs is 
usually, if not invariably, to the left. When the animal swims 
forward without revolving on its long axis, the path is usually 
a curved one, the animal continually swerving toward its right 
side. In this way the Stentors usually describe circles of 
greater or less radius. If they revolve as they swim forward, 
they continually swerve to the right also; but owing to the 
revolution, the right side continually changes its position, so 
that the path becomes a spiral one, as in Paramecium and 
Spirostomum. The motion of Stentor polymorphus is usually 
very slow, so that all these relations are observable without the 
slightest difficulty. 
A. Reactions to Mechanical Stimuli. 
The animals were stimulated in the same way as Spirosto- 
mum, by touching them at any desired point with a capillary 
glass rod. 
1. Stimulus at Anterior End, on Peristome,— The animal con- 
tracts and swims backward a short distance (its own length or 
a little more). As it swims backward it revolves on its long 
axis to the left— in the same direction as when swimming 
forward. Then it turns on its short axis to its right and 
swims forward. 
2. Stimulus at Side.—Identically the same reaction is given 
as when stimulated at the anterior end; the animal contracts, 
swims backward, turns to right, and swims forward. The 
turning is not with reference to the position of the source 
of stimulus, but is always toward the right side. Therefore, if 
