132 AN INTRODUCTION TO ZOOLOGY 



then contracts. In an animal attached by the tentacles a similai 

 bending occurs, but the basal disk in this case is directed toward 

 the anode. These reactions are caused by local contractions on 

 the anode side for which the electric current is directly respon- 

 sible (115). 



Hydra shows no rheotropic reactions. When placed in a cur- 

 rent of water it neither orients itself in a definite way nor moves 

 either up or down stream (123). 



GENERAL REMARKS ON THE BEHAVIOR OF HYDRA. It is evi- 

 dent from the above outline of the reactions of Hydra to stimuli 

 that the only movements involved are produced by contraction 

 and expansion of the body when attached, and by undirected 

 changes of position. Being radially symmetrical, the body may 

 be flexed in any direction. 



Local stimuli, such as the application of heat or a chemical to a 

 limited area of the body, causes a contraction of the part affected 

 and a bending in that direction. This results in the movement 

 of the tentacular region toward the stimuli, and the contraction 

 of the entire animal follows, thus carrying it out of the influence 

 of the stimulus. 



Non-localized stimuli, such as the jarring of the vessel contain- 

 ing the animals, produces, immediately, a contraction of the 

 entire body, which, in most cases, is beneficial, since it removes it 

 from an injurious agent. If, however, this simple contraction 

 is not effective, as in the case of a constant application of heat, 

 the Hydra usually resorts to some other reaction, e.g. locomotion, 

 which often enables it to escape from the injurious stimulus. 



Finally, it should be remembered that the physiological condi- 

 tion of the animal determines to a large extent the kind of reac- 

 tions produced, not only spontaneously, but also by external 

 stimuli. " It decides whether Hydra shall creep upward to the 

 surface and toward the light, or shall sink to the bottom; how it 

 shall react to chemicals and to solid objects; whether it shall 

 remain quiet in a certain position, or shall reverse this position 

 and undertake a laborious tour of exploration " (no, p. 231). 



