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COLLEGE ZOOLOGY 



the animal bends over (Fig. 70, 1) and attaches itself to the sub- 

 stratum by its tentacles (2) ; the basal disc is then released and 

 the animal contracts (3) ; the body then expands (4), bends over 

 in some other direction and becomes attached (5); finally the 

 tentacles are released and an upright position is regained (6). 



Fig. 6g. — Spontaneous changes of positions in an undisturbed Hydra. 

 Side view. The extended animal (i), contracts (2), bends to a new position (3), 

 and then extends (4). (From Jennings.) 



This method of locomotion has been compared to that of the 

 measuring-worm. At other times the animal uses its tentacles 

 as legs, or glides along on its basal disc. 



Hydras react to mechanical stimulation, to light, temperature, 

 and electricity. If a watch-glass containing a specimen is jarred, 

 or the surface of the water agitated, a part or all of the body and 

 tentacles contract; this is the result of a non-localized mechanical 

 stimulus. If the body or a tentacle is touched with a glass rod, 

 the body or tentacles contract, depending on the strength of the 

 stimulus. 



Changes in the intensity of the light cause Hydras to move 



