THE TROPISM THEORY 



267 



be illustrated most simply by considering an organism covered with 

 cilia. For this purpose we may employ the accompanying diagrams, 

 based on those given by Verworn (1895, p. 484), but modified to make 

 them clearer. In Fig. 142 a stimulus is supposed to act from the right 



side on the organism, as indicated by the arrows, and to cause the cilia 

 of that side to contract more strongly, as is indicated by the heavier 

 shade and greater curving. This must, of course, turn the body to the 

 left, as a boat is turned to the left when the right oar is more strongly 



