THE HYDRA AS A SIMPLE METAZOAN 



265 



end of an aquarium and observing that the hydras will begin 

 moving at a temperature of 31° C. There is, however, no definite 

 movement toward or away from the source of heat, only a vague 

 wandering that continues until a region of optimum temperature 

 is reached in a wholly accidental fashion, unless the animal is 

 killed by blundering into a region that is too hot. What evidently 

 happens is that the rising temperature induces a changed internal 

 state of the hydra which leads it to wander until this internal regu- 

 ulation is modified by new external conditions. At temperatures 

 that are too high or too low the animal contracts and remains 

 (luiescent until it dies or is relieved by the restoration of a suita])le 

 temperature. Response to chemicals in solution, or chemotropism, 



Fig. 127. — A young jellyfish, Gonionemus murbachi, resting upon the bottom 



with oral side uppermost. 



(After Perkins.) 



is also demonstrable, as when meat juice is squirted from a capil- 

 lary pipette against the tentacles or hypcstome, or when various 

 chemicals like acetic acid, to which the hydra is probably not 

 subjected in nature, are similarly used. Although it has no special 

 sense-organs, but only sensory cells (cf. Fig. 129), the animal thus 

 responds to as many fundamental types of stimuli as do many 

 of the higher animals. 



The more dehcate responses are perhaps the result of a combina- 

 tion of stimuh, such as probably occurs in feeding. If the tenta- 

 cles are touched gently with the end of a capillary tube there is 

 not much reaction, nor is there a very marked response when 

 meat juice is brought against them with the minimum of mechan- 

 ical stimulation. When, however, these two stimuli are combined, 

 the maximum reaction is obtained. The small size of hydra makes 



