CHAPTER V. 



Divisions of the Hydeozoa. 



The Mydrozoa are divided into four great divisions, each 

 of which requires some notice, as presenting points of special 

 interest. These divisions or sub-classes are known by the names 

 of Sydroida, Siphonophora, Discophora, and Lucernarida. 



SUB-CLASS HYDROIDA. 



This sub-class comprises all the sea-firs and their allies, 

 commonly known to naturalists as the " Hydroid zoophytes," 

 from their resemblance to the fresh-water polype {Hydra), 

 which is also a member of this division. The Sydroida are 

 defined by the fact that tTiey consist of an alimentary region 

 or ^^ polypite" which is furnished with a mouth and prehensile 

 tentacles at one end, and with an adherent disk at its other 

 extremity. In some few cases the hydrosoma consists of but 

 one such polypite (as in the Hydrida and some of the Co- 

 rynida) ; but generally the hydrosoma is composed of a 

 greater or less number of similar polypites all united by a 

 ccenosarc or common trunk (as in the majority of the Corynida, 

 and in the Sertularida and Campanularida). In the great 

 majority of cases, also, the hydrosoma is not unattached, but 

 is fixed to some solid object by one extremity. The Hydroid 

 zoophytes exhibit three principal tj'pes of structure, which 

 constitute so many orders. 



Oedee I. Hydeida. — In the first order we have only the 

 well-known fresh-water polypes or Hydrae, of which we may 

 take the common green Hydra (H. viridis) as the type. This 

 singular little creature may be found with a little trouble in 

 most of our streams and pools, and it is quite visible to the 

 naked eye, though it can only be satisfactorily examined by 



