204 BEHAVIOR OF THE LOWER ORGANISMS 



mentally. Wagner tried stimulating one side of the body mechanically, 

 while Mast raised or lowered the temperature of one side. Both authors 

 agree as to the following results : The direction of extension after con- 

 traction bears no definite relation to the side from which the stimulus 

 came; the animal is just as likely to extend toward the source of stimu- 

 lation as in any other direction. In other words, when stimulated, 

 Hydra merely changes its position, without special relation to the locali- 

 zation of the stimulating agent. The direction of bending and exten- 

 sion is determined by internal factors. If the stimulus is repeated, con- 

 traction occurs again, and the animal extends in still another direction. 

 The analogy of these relations with those shown by the infusoria is evi- 

 dent; the latter when stimulated usually merely change the direction 

 of movement, without regard to the direction from which the stimulus 

 came. In the infusoria the internal factors (structural in character) 

 which determine the direction have been determined ; this has not yet 

 been done for Hydra. 



But repeated or strong continued contraction, with extension in a 

 new direction, is not the final recourse of Hydra under strong stimula- 

 tion. If the stimulation continues, the animal finally bends over, places 

 its head against the surface to which it is attached, releases its foot, and 

 moves away from the spot where it has been subjected to such objec- 

 tionable experiences. The locomotion is usually of the sort illustrated 

 in Fig. 117. This reaction has been observed by Wagner (1905) under 

 mechanical stimulation, by Mast (1903) under stimulation by heat, and 

 by the present author under stimulation by chemicals. In all cases it 

 was found that the direction toward which the animal moves bears no 

 definite relation to the direction from which the stimulus comes. Wag- 

 ner stimulated one side repeatedly by striking it with a rod, and found 

 that the animal was as likely to move toward that side as in any other 

 direction. The experiments of Mast are particularly interesting in this 

 connection. Mast placed a considerable number of Hydras in a flat- 

 bottomed trough, and heated one end. At about 31 degrees C. the 

 animals began to release their foothold and move about from place 

 to place. But they were as likely to move toward the heated end as 

 away from it. The results of a series of such experiments are shown 

 in Fig. 127. In this figure are represented not only the movements of 

 locomotion, but also the different directions in which the animal ex- 

 tended after contracting. The diagram shows clearly that both sets of 

 movements are quite -without definite relation to the direction from which 

 the heat comes; their direction evidently depends on internal factors. 

 When it experiences the high temperature, the animal merely changes 

 its position, in a way determined by its structure or other internal fac- 



