ORDER II. ASTEROIDA. 



1149. In many of the Polypes of this Order, the polypidom, 

 or solid framework, so closely resembles Sponge, that, in the dry 

 state, the former can only be distinguished by the regular dis- 

 position of cells upon its surface ; and, in the immature con- 

 dition, 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. There is a great ap.- 

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

 Whilst one family approaches the Sponges so closely, that it 

 might almost be 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 dif- 

 ferent in the character of their aggregated masses, they all bear 

 a general resemblance, as to the structure of the individual Po- 

 lypes ; and to these we shall first, therefore, direct our attention. 



1150. None of the Alcyonian Polypes live so entirely isolated 

 as the Actinia. On comparing the external appearance of one 

 of them with that of a Sea- Anemone, we perceive that, whilst 

 the mouth is surrounded by tentacula in the same way, there is 

 a marked difference in the form and number of these prolonga- 

 tions. Instead of numerous short cylindrical tubes like those of 

 most of the Actinice, we find six or eight broad leaf-like expan- 

 sions, disposed with great regularity around the mouth, so as 

 very much to resemble a Star-fish (Asterias). Hence the term 

 Aster oida has been proposed as a designation for the group ; 

 and it represents sufficiently well its chief external character. 

 These tentacula are unprovided with cilia; but a number of 

 little projections may be seen along their margins, which pro- 

 bably increase their prehensile power. The mouth leads into a 

 stomach, which resembles that of the Actinia, in being suspend- 

 ed in the midst of the general cavity of the body, by partitions 



