No. 389.) STIMULI IN UNICELLULAR ORGANISMS. 375 
appreciated in the figure; while not prominent, they are visible, 
and are of the greatest importance for orientation while study- 
ing the movements of the animal. The difference in the two 
contours of the anterior end possibly does not correspond pre- 
cisely to the distinction between the oral and aboral sides, as 
determined by the position of the mouth; i. 
the exact relation of parts is very difficult to É 
determine on account of the continual twist- 
ing movements of the animal. But the dif- 
erence in contours in any case marks very 
nearly the oral and aboral sides, so that 
when the aboral side is spoken of in the 
following account it signifies that side of 
the anterior end in which the curve is long- 
est, while the oral side is that which presents 
the shortest curve (see Fig. 1). The entire 
surface of the anima] is covered with cilia, 
arranged in somewhat oblique longitudinal 
rows. 
The unstimulated Spirostomum swims for- 
ward by means of the backward stroke of its 
cilia; at the same time it may revolve on 
its long axis. This revolution is usually from 
right to left. The body (as shown in Fig. 1) 
is not quite straight, but a little in front of 
the middle is slightly bent, so that the ante- 
rior part, as the body revolves, describes the 
surface of a cone. Owing to the animal’s 
continual movement and its power of twisting 
into a spiral and of bending sharply, taken in 
connection with the only slight differentiation on the surface of 
the cylindrical body, the orientation of the body in relation to the 
direction of movement is very difficult to determine. But by 
keeping the attention fixed upon the form of the anterior end, 
as described above, it is possible under favorable conditions to 
observe that the aboral side of the anterior end is always on 
the outside of the cone, the oral side looking within. As the 
animal moves forward in connection with this revolving motion, 
prr A eae 
