THE BEHAVIOR OF INFUSORIA; PARAMECIUM 



45 



axis, and the swerving toward the aboral side. Each of these factors is due to a 

 certain peculiarity in the stroke of the cilia. The first results from the fact that the 

 cilia strike chiefly backward. The second is due to the fact that the cilia strike, not 

 directly backward, but obliquely to the right, causing the animal to roll over to the 

 left. The third factor the swerving toward the aboral side is due largely to the 

 greater power of the stroke of the oral cilia, and the fact that they strike more nearly 

 directly backward. It seems partly due however to a peculiarity in the stroke of the 

 body cilia, by which on the whole they strike more strongly toward the oral groove 

 than away from it, thus driving the body in the opposite direction. 



Each of these factors may vary in effectiveness, and the result is a change in the 

 movements. The forward course may cease completely, or be transformed into a 

 backward course, while the rotation and the swerving continue. Or the rotation 

 may become slower, while the 

 swerving to the aboral side 

 continues or increases; then 

 the spiral becomes much 

 wider. This result is brought 

 about by a change in the 

 direction of the beating of the 

 cilia to the left of the oral 

 groove; they beat now to the 



left (toward the oral groove) FIG. 34. Diagrams of transverse sections of Parame- 



cium, viewed from the posterior end, showing the change 

 in the beat of the cilia of the left side. a, Stroke of the 

 cilia in the usual forward movement. All the cilia strike 

 toward the right side (r), rotating the organism to the left 

 (/), as shown by the arrows. b, Stroke of the cilia after 

 stimulation. The cilia of the left side strike to the left, 

 opposing the lateral effect of the cilia of the right side. 

 This causes the animal to cease revolving, and to swerve 

 toward the aboral side (ab). o, Oral groove. 



instead of to the right (Fig. 

 34). The result of this is, as 

 the figure shows, to oppose 

 the rotation to the left, but 

 to increase the swerving 

 toward the aboral side. The 

 width of the spiral, or the final 



complete cessation of the rota- 

 tion on the long axis which sometimes occurs, depends on the number and effective- 

 ness of these cilia of the left side that beat toward the oral groove instead of away 

 from it. A large part of the behavior of Paramecium depends, as we shall see, on 

 the variations in the three factors which produce the spiral course. 



3. ADAPTIVENESS OF THE MOVEMENTS 



How does Paramecium meet the conditions of the environment? 

 Under the answer to this question must be included certain aspects of 

 the spiral movement, described in the foregoing paragraphs. The 

 problem solved by the spiral path is as follows: How is an unsym- 

 metrical organism, without eyes or other sense organs that may guide it 

 by the position of objects at a distance, to maintain a definite course 

 through the trackless water, where it may vary from the path to the right 

 or to the left, or up or down, or in any intermediate direction? It is 

 well known that man does not succeed in maintaining a course under 



