66 AN INTRODUCTION TO ZOOLOGY 



oral side is to the left, is to the right; when the oral side is above, 

 the body swerves downward ; when the oral side is to the right, 

 the body swerves to the left, etc. Hence the swerving in any 

 given direction is compensated by an equal swerving in the 

 opposite direction; the resultant is a spiral path having a straight 

 axis" (62, p. 44; Fig. 28). 



Rotation is thus effective in enabling an unsymmetrical animal 

 to swim in a straight course through a medium which allows 

 deviations to right or left, and up or down. It is well known that 

 a human being cannot keep a straight course when lost in the 

 woods, although he has a chance to err only to the right or left. 



Nutrition. The food of Paramecium consists principally of 

 bacteria and minute Protozoa. The animal does not wait for 

 the food to come within its reach, but by continually swimming 

 from place to place is able to enter regions where favorable food 

 conditions prevail. The cilia also aid in bringing in food particles, 

 since a sort of vortex is formed by their arrangement about the 

 oral groove which directs a steady stream of water toward the 

 mouth. 



Figure 29 illustrates the formation of a food vacuole (Ne). 

 Food particles that are swept into the mouth (Mu) are carried 

 down into the cytopharynx (s) by the undulating membrane 

 (Mb) ; they are then moved onward by the cilia lining the cyto- 

 pharynx and are finally gathered together at the end of the 

 passageway into a vacuole which gradually forms in the endo- 

 plasm ( Ne). When this vacuole has reached a certain size, it is 

 pinched off from the extremity of the cytopharynx by a contrac- 

 tion of the surrounding protoplasm, and the formation of another 

 vacuole is begun. A food vacuole ( N) is a droplet of water with 

 food particles suspended within it. As soon as one is separated 

 from the cytopharynx, it is swept away by the rotary streaming 

 movement of the endoplasm known as cyclosis. This carries 

 the food vacuole around a definite course which begins just above 

 and behind the cytopharynx, passes backward to the posterior 

 end, then forward near the dorsal surface to the anterior end, 



