46 BIOLOGY: GENERAL AND MEDICAL 



movements of prehension, associated with discharge of 

 the nettle threads may be provoked so that the object 

 may be paralyzed, caught, and forced into the mouth 

 of the animal. If, however, the disturbance be greater, 

 the tentacles may be withdrawn and the animal retracted 

 to a rounded mass scarcely recognizable as a hydra. 

 If frequently disturbed, the animal may let go and move 

 off to a position of greater security. 



As these primitive reactions are observed, we may 

 interpret their purposes to be primarily defensive as the 

 animals at rest are less conspicuous and may escape the 

 observation of their enemies. Soon, however, we come 

 to realize that even in the pseudopod of the amceba we 

 see the foreshadowing of a discriminating power, based 

 upon the intensity and quality of the impression received, 

 which becomes developed more and more perfectly until 

 the tactile sense of the higher animals develops. 



So soon as animals evolve a complex nervous system 

 the importance of thigmotropic irritability increases, 

 special organs being developed to receive and transmit 

 it as the sense of touch, and with it probably comes 

 the subjective appreciation of pain as well as the peculiar 

 coordination of nervous and muscular stimuli known as 

 reflex action by which involuntary escape from injurious 

 stimulations is effected. 



CHEMOTROPISM OR RESPONSE TO CHEMICAL STIMULATION. 



Substances capable of exciting a deleterious action 

 upon living substance are known as poisons. 



Certain of them act by virtue of their ability to effect an 

 immediate destructive combination with protoplasm and 

 are known as caustics and may be subdivided into co- 

 agulating caustics by which the protoplasm is coagulated, 

 and liquefying caustics by which it is liquefied. 



Among the former may be mentioned the metallic 

 salts, acids, and some of the essential oils; among the 

 latter, bases such as potash, soda, ammonia, and arsen- 

 ious acid. 



