PHYLUM COELENTERATA 



145 



ing movements when only beef extract is in solution in tlie water. 

 Thus it responds to a chemical stimulus alone, but it will not respond 

 to a mechanical stimulus only. 



These animals show response to a number of environmental con- 

 ditions. Any sudden change is likely to bring about a negative 

 response. If the stimulus is of a general nature and of considerable 



Fig-. 68. — Locomotion in hydra. Successive positions taken when progressing by 

 somersaults. (From Jennings, Behavior of the Loiuer Organisms, published by The 

 Columbia University Press.) 



intensity, the animal will contract all of the tentacles and the body 

 also. If the stimulus is restricted to one locality and is not too 

 strong, the animal will contract in the affected area, by the with- 

 drawal of one tentacle. The movements of the animal are per- 

 formed by contraction and relaxation of the contractile fibers con- 

 nected with certain of the cells. The activities come in response 

 to internal as well as external stimuli. 



