GENERAL CHARACTERS OF POLYPIFERA. 481 



there are some polypes which do not form associations, and 

 which deposit no hard skeleton, but which are closely allied in 

 general structure to those that do ; such are the Hydra or fresh- 

 water Polype (Fig. 615), and the Actinia or Sea- Anemone (Fig. 

 617). In the classification founded only upon the characters of 

 the compound masses, therefore, the naked Polypes (as they were 

 termed) were associated into a separate order. By this method 

 as by any which depend upon a single set of characters, and 

 are therefore artificial beings the most dissimilar were asso- 

 ciated together ; and those which were really allied in the 

 structure of the individuals, were separated, because there was a 

 dissimilarity in the form of the compound masses. 



1042. Bearing in mind the resemblance formerly pointed 

 out, between one of the compound Polypifera and a Vegetable 

 structure, in regard to the aggregation of distinct individuals into 

 one common life ( 986), we shall at once perceive the necessity 

 of looking for the characters which are to serve for the classifi- 

 cation of this group, not in the degree of density of the stem, or 

 in the arrangement of its branches, but in the structure of the 

 separate Polypes. The Botanist well knows that in the leaves 

 and flowers he must find the materials for his classification ; and 

 not in the degree of firmness of the wood, or the mode of growth 

 exhibited by the whole structure, which are liable to vary so 

 much with age and circumstances, and to differ so completely in 

 species which ar# really in close alliance. In this manner, the 

 class of POLYPIFERA may be divided into four Orders, character- 

 ised by four distinct kinds of structure. Each of these Orders 

 contains Polypes in various degrees of association ; some living 

 solitarily ; some existing in societies ; some united by a con- 

 necting flesh ; and some having a common circulation of fluid 

 amongst them. Each, therefore, contains some which form a 

 solid structure, and others which are destitute of it. The differ- 

 ences between these Orders are such, that it will be desirable to 

 give a detailed account of each separately. 



