366 COELENTERATA ANTHOZOA chai 



the one hand, and the organisms, animals or plants, which fori 

 large, massive skeletons of carbonate of lime, on the other. 



The form of the solitary zooids and of the colony of zooids in 

 the Zoantharia, then, may be very divergent. In the Actiniaria 

 we find single soft gelatinous zooids of considerable size adherent to 

 rocks or half-buried in the sand. Among the Madreporaria we 

 find great branching colonies of thousands of zooids supported by 

 the copious skeleton of carbonate of lime that they have secreted. 

 Among the Antipathidea, again, we find a dendritic skeleton of a 

 dark horny substance, formed by a colony of small zooids that 

 cover it like a thin bark. The majority of the Zoantharia are, 

 like other zoophytes, permanently fixed to the floor of the ocean. 

 Where the embryo settles, there must the adult or colony of 

 adults remain until death. Some of the common Sea-anemones 

 can, however, glide slowly over the surface on which they rest, and 

 thus change their position according to the conditions of their 

 surroundings. Others (the Minyadidae) float upside down in the 

 sea, and are carriei hither and thither by the currents. Others, 

 again (Cerianthus, Edwardsia, Peachia), burrow in the sand or 

 mud at the sea-bottom. 



The structure of the zooid varies considerably, but in the 

 following characters differs from the zooid of the Alcyonaria. 



The tentacles are usually simple finger- 

 like processes, and when they bear 

 secondary pinnae these can readily 

 be distinguished from the rows of 

 secondary pinnules of the Alcyonarian 

 tentacle. The number of tentacles 

 is very rarely eight (young Halcampa), 

 and in these cases they are not 

 pinnate. The number of tentacles 

 may be six (many Antipathidea and 

 c D some zooids of Madrepora), twelve 



Fig. 162. Large (A) and small (Madrepora), some multiple of six, or 



(B) plumose tentacles of Acti- . . 



nodendron piumomm. Large an indefinite number. In the Ihalas- 



(C) and small (D) plumose s i an thidae and some other families of 



tentacles of A. glomeratum. 



(After Haddon. ) Actiniaria the tentacles are plumose, 



but do not exhibit the regular 

 pinnate form of the tentacles of Alcyonaria. 



As regards the number of mesenteries, the Zoantharia exhibit 



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