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ORDER III. ASTEROIDA. 



1085. Having thus considered the two Orders, in which 

 the structure of the single and associated Polypes is most cha- 

 racteristically presented, we may pass on to the two others ; 

 which exhibit such modifications of this structure, as connect the 

 Polypifera with the classes below and above them. 



1086. We shall first consider (as being most nearly allied to 

 the preceding) the Order, which, from one of its principal genera, 

 we may term that of Alcyonian Polypes. In many of these, 

 the polypidom, or solid framework, so closely resembles Sponge, 

 that, in the dry state, the former can only be distinguished by 

 the regular disposition of cells upon its surface ; and, in the 

 immature condition, the resemblance is still more close, the 

 young branches of the polypidom, on which the polypes have 

 not yet appeared, presenting all the characters of sponge, as will 

 be presently shown ( 1088). There is a greater apparent 

 diversity in the character of the members of this Order. Whilst 

 one family approaches the Sponges so closely, that it may be al- 

 most regarded as formed of Sponges provided with polype-mouths, 

 another has a dense horny, or even calcareous, arborescent stem ; 

 whilst in another, the soft parts of the body are entirely inclosed 

 in a firm tubular sheath. But, however different in the cha- 

 racter of their aggregated masses, they all bear a general 

 resemblance, as to the structure of the individual polypes ; and to 

 these we shall first, therefore, direct our attention. 



1087. None of the Alcyonian Polypes live so entirely isolated 

 as the Hydra or Actinia. The closeness of the connection 

 between the individuals, differs extremely in the various groups ; 

 for whilst, in the lower forms which assimilate with the Sponges, 

 the Polypes seem quite subordinate to the general mass, from 

 which they are developed, and to the support of which they 

 minister in common, there is another group, in which each 

 seems to live for itself alone, although united externally with its 

 fellows. In all, however, the essential characters of the Polypes 



