ACTINIA. 197 



tance from land. In calm weather they float on 

 the surface of the sea, rearing their purple 

 crests, and appearing at first like large air 

 bubbles, but distinguishable by the vivid hues 

 of the tentacula which hang down beneath them. 

 Nothing can exceed the beauty of the spectacle 

 presented by a numerous fleet of these animals, 

 quietly sailing in the tropical seas. Whenever 

 the surface is ruffled by the slightest wind, they 

 suddenly absorb the air from their vesicles, and 

 becoming thus specifically heavier than the 

 water, immediately disappear, by diving into the 

 still depths of the ocean. By what process 

 they effect these changes of absorption and of 

 reproduction of air yet remains to be discovered. 

 Other genera, as the Physsophora, have several 

 of these air-bladders; but in other respects 

 resemble the ordinary Medusae, in having no 

 membranous crest. 



The Actinia are a tribe of Zoophytes, which, 

 from the general resemblance of their forms to 

 those of Polypi, are by most naturalists in- 

 cluded under that order. But they exhibit a 

 much greater developement in their organiza- 

 tion ; having very distinct muscular fibres, en- 

 dowed with strong powers of contraction. Their 

 digestive organs, also, as I shall have afterwards 

 occasion more fully to notice, are constructed 

 upon a more complicated plan than in the poly- 

 pus. Fig. 86 exhibits an Actinia in its con- 



