146 



THE MECHANICAL FUNCTIONS. 



sophora, have several of these air-bladders; but in other re- 

 spects resemble the ordinary Medusae, in having no mem- 

 branous crest. 



The J^ctinix are a tribe of Zoophytes, v^hich, from the 

 general resemblance of their forms to those of Polypi, are 

 b}^ most naturalists included under that order. But they 

 exhibit a much greater development in their organization; 

 having very distinct muscular fibres, endowed with strong 

 powers of contraction. Their digestive organs, also, as I 

 shall have afterwards occasion more fully to notice, are con- 

 structed upon a more complicated plan than in the polypus. 

 Fig. 86 exhibits an Actinia in its contracted state. When 

 their tentacula, which surround the mouth, and are very 

 numerous, are fully expanded, (as shown in Fig. 87,) these 



animals present a striking analogy of form to many of the 

 compound flowers; and accordingly the particular species 

 are named from these resemblances, the sea-anemone^ the 

 sea-mar y gold, the sea-carnation, the sun-flower, daisy, 

 Sic. Actiniae are seen in great numbers on many shores, 

 adhering by their flat surfaces to rocks, and beinjj generally 

 permanently fixed to their abode. When the weather is 

 fine, and the sea calm, it is very amusing to watch the rapid 

 expansions and retractions of their many coloured tentacula, 

 while they are moving in search of food: to observe the 

 quickness with which they seize on whatever prey comes 

 within their reach, and to notice the suddenness with which 

 they collapse into a round contracted mass, on receiving the 

 slightest injury. 



Yet these animals are not of necessity confined to the par- 

 ticular spots where we see them fixed; for they are capable, 

 when disturbed, of seeking, by a slow progressive motion, 

 a more secure abode. Reaumur has minutely examined the 



