264 THE HYDRA 



for a very small animal to be caught by one tentacle alone and 

 passed to the mouth without a disturbance of the other tentacles. 

 In either case the prey is drawn toward the mouth, which may 

 open in advance of the actual contact. When food begins to enter 

 the mouth, the tentacles cease to function. In the case of large 

 food masses, they may even become folded back away from the 

 hypostome (Fig. 119), the final stages of ingestion being accom- 

 plished by a lip-like creeping of the mouth around the food. Hun- 

 gry hydras will sometimes ingest the bodies of animals or bits of 

 meat many times larger than themselves, or they may attempt the 

 impossible with large masses of food, even turning the body wrong 

 side out for a short distance. The process of feeding is so remark- 

 able that it must be seen many times to be fully appreciated. 

 The fact that the tentacles and the entire oral region of the 

 body behave in a coordinated fashion can be accounted for by 

 the presence of the nervous mechanism described in a later section. 

 Another point that is of interest here is that hydras kept without 

 food, and therefore " hungry," are much more responsive in their 

 feeding reaction than individuals that have recently fed. This 

 indicates that the internal physiological state of the animal may 

 influence its responses to external stimuli. 



Tropisms. — The complexities in the hydra's behavior show 

 that it must respond with some delicacy to a considerable range of 

 stimulation. This is found to be true upon observation and exper- 

 imentation. Siereotropism, or reaction to contacts with external 

 objects (cf. p. 181), is one of the most important forms of response, 

 since the stimuli of contact with a substratum are recurrent 

 whether the hydra is in locomotion or at rest. The animals are 

 also very sensitive to touch, as may be demonstrated in the labora- 

 tory by bringing a glass rod into contact with the tentacles or the 

 side of the body. They also respond to mechanical shocks, as 

 when one jars the table on which an aquarium is standing. Pho- 

 iotropism, or the response to light, is particularly in evidence in 

 green hydras, which will collect in the parts of an aquarium where 

 the illumination is at an optimum. This response is sometimes 

 complicated by the fact that water-fleas and similar animals may 

 collect in such places in response to light or contact stimulation 

 and the hydras may come to occupy the same territory in response 

 to the presence of this food supply. The response to heat, or ther- 

 motropism, may be shown by heating one end and chilling the other 



