PARAMECIUM 185 
the peculiar spiral twist of the body is correlated with this 
motion, but does not necessarily cause it, as the animal may at 
times revolve in the direction opposite to that of the twist. It is 
in consequence of this peculiar revolving motion that the animal 
is able to maintain a course through the water which is practi- 
cally straight. The great majority of swiftly moving animals 
are bilaterally symmetrical, and move in straight lines because of 
that feature of their structure, but Paramecium, together with 
most free infusorians, has an unsymmetrical form and would 
tend to move in circles in consequence, without making progress, 
if it were not for the revolution of its body on its long axis. 
Exercise 1. Draw several simple outlines of the body showing 
its shape as seen in different positions. 
Exercise 2. Draw an outline of an ideal cross section through 
the middle of the body. 
Study the structure of the body, using a high power of the 
microscope when necessary. Study the action of the hair-like 
vibratile cilia which cover the outer surface of the animal and 
by means of which it moves. They are usually difficult to see 
in the live animal because of their very rapid motion, but by 
varying the light and the focus of the microscope they will be 
brought into view, and in the dead animal are plainly visible. 
Determine the direction in which the cilia move. Are they all 
of the same length? Note the delicate transparent cuticula which 
covers the body; it appears as a highly refractive line. 
The body has no internal cavity, and the protoplasm of which 
it is composed is in two distinct layers, the ectosarc and entosare. 
The former is the thick, firm, transparent outer layer which, 
with the cuticula, gives permanent shape to the body; it often 
appears obliquely striated. The entosarc is a semifluid gran- 
ular mass which forms the remainder of the body. From near 
the anterior end the oral groove runs obliquely along the ventral 
side of the body to a point back of the middle, getting deeper 
