INVEKTEBKATE ANIMALS. 



CHAPTEE Yl. 

 DIVISIONS OF THE HYDROZOA. 



The Hydrozoa are divided into five 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 Hydroida, Sipho- 

 nophora, Zucernarida, G raptoUtidce, and llydrocundlince. 



SUB-CLASS HYDEOIDA. 



This sub-class comjirises all the Sea-firs and their allies, commonly 

 known to naturalists as the " Hydroid Zoophytes," from their resem- 

 blance to the Fresh-water Polype {Hydra), which is also a member of 

 this division. The Hydroida are defined by the fact that they consist 

 uf an alimentary region or " polypite" whivh is furnished with a nioiith 

 and prehensile tentacles at its distal end, and with an adherent disc at 

 its proximal e.rtremity. In some few cases the hydrosoma consists of 

 but one such polypite (as in the Hydrida and some of the Corynida) ; 

 but generally the hydrosoma is composed of a greater or less number 

 of similar polypites all united by a crenosarc or common trunk (as in 

 the majority of the Gorynida, and in the Sertidarida and Campanu- 

 larida). In the great majority of cases, also, the hydrosoma is not 

 unattached, but is fixed to some solid object by its proximal extrem- 

 ity or " hydrorhiza." The Hydroid Zoophytes exhibit four principal 

 types of structure, which constitute so many orders. 



Order I. Hydrida. 



In the first order we have only the well-known Fresh -water 

 Polypes or Hydra, of which we may take the common green 

 Hydra (H. viridis) as the type. When uncontracted, the body of 

 the Hydra is in the f(ii-m of a cylindrical tulie (fig. 30), composed 

 of the two fundamental layers, the ectoderm and endoderm, of 

 which the former contains many thread-cells. The integument 



