182 



BIOLOGY: GENERAL AND MEDICAL 



movement, becomes encysted and remains dormant for 

 several weeks after falling from the parent. 



The hydra is thus seen to develop in itself both male 

 and female sexual elements, and is, therefore, a her- 

 maphrodite. 



In other coelenterates a somewhat different sequence 

 of events is observed. Thus in the small marine hy- 

 droid polyp, known as Obelia, we find near the base of 

 the stalk certain members somewhat resembling the 

 polyps, but having no tentacles and containing rounded 

 bodies of small size the sporosacs. When ripe they 



Stomach 



FIG. 78. Lateral view of a medusa of Obelia. Magnified. (Ad. nat.) (After 



Masterman.) 



rupture and permit the escape of small modified polyps, 

 not unlike the jelly-fishes in general appearance, though 

 extremely minute, and known as medusce. They are 

 umbrella-shaped, the opening below. From the centre 

 there hangs a central member, known as the manubrium, 

 upon which the mouth opens into a little bag, the coelen- 

 teron or general digestive cavity, which communicates 

 with four radiating canals which run to the rim of the 

 umbrella where they connect with a ring canal, opposite 

 to which there is a sense organ. The structure is highly 

 specialized, as contrasted with the hydroid parents. The 



