CLASS HYDEOZOA. 269 



The Sea Anemone is a soft-bodied Polyp, ttat can. change 

 its locality at will, adhering to the rock by a fleshy disk that 

 adapts itself to all inequalities. Its stomach is simply a sac 

 suspended in the cavity of the body, into which it opens at 

 the bottom by a wide aperture. Between the stomach and 



Fig. kB9. 



MetricRum marginatum. Sea-anemone or Actinia. 

 A. Opened ; B. Closed ; C. Opening. 



the body-walls are mesenteric spaces, which communicate 

 freely with the numerous hollow tentacles. By muscular 

 contraction, water is forced from these chambers into the 

 tentacles, to prolong them. The animal, plastered, as it 

 were, to a rock, with its tentacles bloomed out like the rays 

 of its flowery namesake, awaits its prey ; and woe to the luck- 

 less victim that walks over the trap and springs it into 

 activity. (See Fig. 393.) , 



CLASS III. HYDROZOA. 



General Characteristics. — The Hydrozoa (water-dragon 

 animals), or Jelly-fishes,* have no mesenteric spaces, and 

 the eggs are developed on the external instead of the internal 



* On account of the stinging sensation produced when handled, the animals 

 belonging to this class have been called Acalephae (Nettles) ; Medasse from the 

 numerous tentacles resembling the hair of Medusa ; and Jelly-fishes &om the gelati- 

 nous nature of their substance. 



