114 



COLLEGE ZOOLOGY 



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

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

 the animal contracts (j); 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 



