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 Paramcecium 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 infusorian 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 
