REFLEX ACTION 15 



In the Protozoa this transmission may take place with great 

 readiness, although there are not known to be any specialized 

 pathways which the impulses follow. If the bell-animalcule 

 Vorticella is touched, the long flexible stalk will quickly 

 contract. A Stentor when stimulated contracts with great 

 suddenness, and Paramoecium when it encounters a stimulus 

 of any kind performs a stereotyped motor reflex which con- 

 sists in reversing the stroke of the cilia, swimming backward, 

 and turning toward the aboral side. This response is shown 

 even by small pieces of the inf usorian in much the same way. 

 Most of the protozoa react to stimuli by some form of motor 

 reflex. Even plants show reflexes, as in the drooping of the 

 leaves of the sensitive plant, the curling of the tentacles of the 

 sundew, the movements of the Venus' fly trap, and many 

 other less manifest exhibitions of irritability. 



A conspicuous trait of reflexes is their adaptiveness. The 

 winking of the eye, the withdrawal of the frog's leg, the re- 

 moval of acid from the side of the body, the snapping and 

 swallowing of food are all actions which, in one way or an- 

 other, secure the welfare of the individual. But the pur- 

 posiveness is like that of a machine which is constructed to 

 do a particular thing; it is the result of conformation and 

 arrangement of its parts. By pressing a lever a machine 

 may be made to perform a very simple act or produce an 

 elaborate piece of music, according to the nature of the 

 mechanism. Given the proper organization an animal may 

 likewise perform acts of great complexity and adaptiveness 

 in response to certain kinds of stimulation; the purposive- 

 ness of the behavior results not from choice, but from a sort 

 of mechanical necessity. The explanation of this adap- 

 tiveness, like the explanation of the adaptiveness of a 

 machine to perform a certain kind .of work, lies in the 

 causes that led to the production of a certain kind of 



