JELLY-FISHES. 373 



below it. This structure is developed first in those at 

 the free extremity of the polype, and progressively 

 downwards ; and the terminal cups are nearly free, 

 rocking in their successors with every wave, while 

 the lowest segments are scarcely visible as such. 



At length the extreme cup rocks and oscillates 

 until the slender thread of connexion is snapped, and 

 it is free. It at once turns itself over, so as to present 

 its concavity downwards, and contracting its margin 

 with the well-known pulmonic spasm, shoots away 

 with the movement as well as the form of a veritable 

 Medusa. The little progeny has, at length, after 

 passing through so many changes, returned to the 

 image of its parent. 



Such are, in brief, the phenomena of one of the 

 most remarkable series of facts that modern zoology 

 has discovered, and which have been propounded 

 under the title of the Law of Alternation of Genera- 

 , tions. 



