FACTS AND FACTORS OF DEVELOPMENT 61 



The ciliated infusorian, Paramecium, moves 

 by the beating of ciha which are arranged in 

 such a way that they drive the animal forward 

 in a spiral course. However, when it is 

 strongly irritated, the normal forward move- 

 ment is reversed ; the cilia beat forward instead 

 of backward and the animal is driven back- 

 ward for some distance (Fig. 21, 1, 2, 3) ; it 

 then stands nearly still, merely rolling over 

 and swerving toward the aboral side, and 

 finally it goes ahead again, usually on a new 

 course (Fig. 21, 3, 4, 5, 6). These move- 

 ments seem to be conditioned rather rigidly by 

 the organization of the animal: they are more 

 or less fixed and mechanical in character, 

 though to a certain extent they may be modi- 

 fied by experience or physiological states. 

 Paramecium behaves as it does in virtue of 

 its constitution, just as an egg develops in a 

 particular way because of its particular 

 organization. 



But although limited in its behavior to these 

 relatively simple motor reactions, Paramecium 

 does many things which seem to show intelli- 

 gence and purpose. It avoids many injurious 



