HYDRA AND COELENTERATES IN GENERAL 131 



tion of both tentacles and body (B, 3). This shows that there 

 must be some sort of transmission of stimuli from one tentacle 

 to another and to the body. The structure of the nervous sys- 

 tem would make this possible (see p. 125) 



PHOTOTROPISM. There is no definite response to light, al- 

 though the final result is quite decisive. If a dish containing Hy- 

 dras is placed so that the illumination is unequal on different sides, 

 the animals will collect in the brightest region, unless the light 

 is too strong, in which case they will congregate in a place where 

 the light is less intense. Hydra therefore has an optimum with 

 regard to light. The movement into or out of a certain area is 

 accomplished by a method of " trial and error." When put 

 in a dark place Hydra becomes restless and moves about in no 

 definite direction; but if white light is encountered, its locomotion 

 becomes less rapid and finally ceases altogether. The value to 

 the organism of such a reaction is quite important, since the small 

 animals that serve as food for it are attracted to well-lighted areas. 

 Colored lights have the same effect as darkness; blue, however, 

 is preferred by Hydra to white. 



THERMOTROPISM. The reactions of Hydra to changes in 

 temperature are also indefinite, although in many cases they 

 enable the animal to escape from a heated region. No loco- 

 motory change is produced by temperatures below 31 C.; at 

 this temperature, however, the basal disk is released and the 

 animal takes up a new position either away from the heated area 

 or further into it. In the former case the Hydra escapes, in the 

 latter it may escape if subsequent movements take it away from 

 the injurious heat, otherwise it perishes. Hydra does not move 

 from place to place if the temperature is lowered; it contracts 

 less rapidly, and finally ceases all its movements when the freez- 

 ing point is approached (113). 



, ELECTROTROPISM. An attached Hydra, when subjected to a 

 weak constant electric current, bends toward the anode, its body 

 finally becoming oriented with the basal disk toward the cathode 

 and the anterior end toward the anode side. The entire animal 



