324 



EVENINGS AT THE MICROSCOPE. 



to be a shallow cup, notched at its margin, and sitting in 

 the concavity of the one next below it. This structure 

 is developed first in those at the free extremity of the 

 polype, and progressively downward; 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 



TURBIS AND ITS YOUNG. 



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 dis- 

 covered, and which have been propounded under the 

 title of the Law of Alternation of Generations.* 



See pages 333, 334, infra. 



