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 l. 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 entosarc. 

 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 



