130 



AN INTRODUCTION TO ZOOLOGY 



tracts (Fig. 60, 3). It then expands (4), bends over in some other 

 direction and attaches its foot (5). The tentacles now loosen 

 their hold and an upright position is regained (6). The whole 

 process has been likened to the looping locomotion of a measur- 

 ing worm. At other times the animal moves from place to place 

 while inverted by using its tentacles as legs. Locomotion may 

 also result from the gliding of the foot along the substratum, and 

 considerable distances are sometimes covered in this way. 



REACTIONS TO EXTERNAL STIMULI. THIGMOTROPISM. 

 Hydra reacts to various kinds of special stimuli. Reaction to con- 

 tact accounts for its temporary fixed condition. The attachment 

 while in the resting attitude is a result of this reaction, and not a 

 response to gravity, since Hydras have the longitudinal axis of the 

 body directed at every possible angle regardless of the force of 

 gravity. Mechanical shocks, such as the jarring of the watch 

 glass containing a specimen, or the agitation of the surface of 

 the water, cause a rapid contraction of a part or all of the animal. 

 This is followed by a gradual expansion until the original condi- 

 tion is regained. 



Mechanical stimuli may be localized or non-localized. That 

 just noted is of the latter type. Local stimulation may be accom- 

 plished by touching the body or tentacles with the end of a fine 

 glass rod. The reactions to local stimuli variously applied have 

 been classified as follows : 



" A. Stimulation of body: 



1. Weak: body partly contracts. 



2. Medium: body completely contracts. 



3. Strong: body and tentacles contract. 

 B. Stimulation of a tentacle: 



1. Weak: tentacle stimulated contracts. 



2. Medium: all tentacles contract. 



3. Strong: tentacles and body contract " (123, p. 594). 



It should be noted that the stimulation of one tentacle may cause 

 the contraction of all the tentacles (B, 2), or even the contrac- 



