398 Harry Beal Torrey. 



turning inside out, or carrying the mouth with its prehensile 

 tentacles to the bases of the proximal tentacles on all sides. The 

 proximal tentacles are comparatively quiet. For minutes at a 

 time they may be held in full extension, motionless, curved grace- 

 fully back from the proboscis, on a vertical stalk. Now and then 

 one may twitch toward the mouth. Occasionally all may wave 

 inward together, grasping the proboscis tightly. The points of 

 the tentacles may be, but usually are not, carried directly toward 

 the mouth. 



a. Muscular Movements. 



So far as I am aware, the reactions of hydroids (with the ex- 

 ception of Hydra) to different sorts of stimuli have never been 

 studied. Medusae, on the contrary, have been the subjects of 

 extended investigations by Romanes ('76, '']']), Eimer ('78), whose 

 paper I have not seen, and Nagel ('93, '94)- In certain respects, 

 Corymorpha and some of the craspedote medusae (Carmarina, 

 Sarsia) respond similarly to similar stimuli. For instance, the 

 proboscis of each may move toward a point of stimulation not on 

 it; and increasing the stimulation of a tentacle may increase the 

 number of tentacles taking part in the response, and leads finally 

 to the contraction of the body of the animal. Hydra and 

 Corymorpha^ however, resemble each other more closely in their 

 responses than either resembles a medusa. In general, similar 

 structures respond similarly, but the tentacles of neither Hydra 

 nor Corymorpha react to odorous substances, while according to 

 Nagel ('93), the tentacles of Carmarina hastata do. Such excep- 

 tions, coupled with obvious differences in structure and habits 

 between polyp and medusa, make it necessary to treat each case 

 individually. 



The large size of Corymorpha makes it an unusually favorable 

 object for experimentation in this direction. Experiments with 

 mechanical, chemical and thermal stimuli brought out the follow- 

 ing facts : 



Mechanical Stimuli. Each proximal tentacle responds to a 

 touch or pinch from forceps by contracting in the same direction 

 with the same strength, whether the stimulus be applied at the 



