PARAMECIUM 149 



to maintain a course through the water which is practically 

 straight. The great majority of swiftly moving animals are bi- 

 laterally 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 consequently 

 would tend to move in circles 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 hairlike, 

 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 vary- 

 ing 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 the semifluid, granular 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 as it goes. At 

 its inner end the groove becomes a closed tube, which extends into 

 the entosarc and ends with the mouth. Notice the trichocysts, 

 slender, radially arranged bodies which fill the ectosarc. They 

 are organs of defense, which remind one of the nematocysts of the 

 Coelenterata ; when the animal is attacked by an enemy or irri- 

 tated by a sharp reagent it discharges long, delicate bristles from 

 the trichocysts, which project beyond the cilia and protect them. 



