382 THE AMERICAN NATURALIST. (VoL. XXXIII. 
toward the outside of the circle), while the animal starts for- 
ward. So long as the anterior end is thus carried into a medium 
which causes the weak stimulus, the forward movement is at 
-once checked and the animal jerks backward again; as soon, 
however, as the anterior end in its circling comes into a direc- 
tion such that swimming forward does not carry the animal 
into a region causing the reaction, the animal continues to 
swim straight forward. All these facts find a precise parallel 
in the reactions of Paramecium. 
When the Spirostomum is swimming backward, the course 
is never changed by the circling about of the posterior end and 
the turning in one way or another, as it is when the anterior 
end is directed forward; the posterior end seems to have no 
power of initiating a turning movement. In this connection 
the reactions of the separate parts of a Spirostomum, cut into 
two or more pieces, is of interest. 
C. Reactions of Separated Parts. 
There is no difficulty in cutting Spirostomum with scissors 
or scalpel transversely into short pieces. Any piece with which 
the anterior end remains in connection, though it be but one- 
tenth of the entire animal, reacts in essentially the same way 
as the entire organism — by contracting, swimming backward, 
turning and swimming forward. Its motion, perhaps, differs a 
little in degree from that of the entire animal in the fact that 
turning is more frequent and pronounced, the piece at times 
swimming in a small circle. The direction of turning is, as in 
the uninjured specimen, toward the aboral side. Any piece 
from which the anterior end is separated, while the posterior 
end is uninjured, reacts as follows. When stimulated, it con- 
tracts and swims backward, does not turn, but soon swims 
forward. It swims but a short distance forward, then starts 
backward again ; after going in this direction once or twice its 
own length, it swims forward about the same distance; then 
again backward. It continues thus to oscillate back and forth 
indefinitely. When the anterior end is removed, therefore, the 
motion takes the form of a rhythmical back and forth move- 
