98 FLUSTKA FOLIACEA. 



the animal itself ; they are not, like the sheU of the 

 snail, merely a calcareous, or horny deposition, 

 moulded on the body of the animals, but are, in fact, 

 a portion of the tegumentary covering of the polyps, 

 which, by the secretion of calcareous particles in its 

 tissue, assumes a firm consistence, without ceasing 

 to receive nutriment. Thus, then, the cells clustered 

 together, form a vital and organic bond of union 

 between the polyps, which may be regarded as so 

 many seizing and digesting organs, labouring for the 

 good of the general mass, or common skeleton. 



At different ages, the cells undergo certain modi- 

 fications of form ; a proof that they are in vital 

 connexion with the animal, and not extraneous. 

 In species where the young spring from the sides of 

 those first formed, and there remain, examination 

 will satisfy that the configuration of the cells not 

 only changes with age, but that it is upon the external 

 surface that these changes are chiefly effected. If, 

 then, we take up &flustra, (&sF.foliacea,) we may 

 regard the whole, with justice, as a living skeleton, 

 " presenting a long series of generations linked to 

 each other ;" the relative ages of the individuals, 

 composing the general whole, being indicated by the 

 position they occupy; those along the margin, 

 and at the extremity, being the last produced. We 

 sometimes observe parts in calcareous zoophytes, 

 possessing distinct powers of motion, though appa- 

 rently unconnected with the body of the polyp. 

 Thus in the Cellaria avencularia,whosQ polyps have 



