CCELENTERATA : HYDROZOA. 83 



CHAPTER VIII. 



DIVISIONS OF THE HYDROZOA. 



Sub-class Hydroida. 



The Hydrozoa are divided into four sub-classes — viz., the Hy- 

 droida, the Siphonophora, the Lucemarida, and the Discophora. 



Sue-class I. Hydroida. — This sub-class comprises those 

 Hydrozoa which consist of an alimentary region or "polypite," 

 which is provided with an adherent disc or " hydrorhiza," and 

 prehensile tentacles. 



In some few cases the hydrosoma is composed of a single 

 polypite only, as in the Hydrida and in some of the Corynida; 

 but usually' there are several polypites united together by 

 means of a common trunk or " ccenosarc," as in most of the 

 Corynida and in the orders Sertularida and Campanidarida. 

 Further, in the great majority of cases the "hydrorhiza" is 

 permanently attached to some foreign object. 



The Hydroida comprises four orders — viz., the Hydrida, the 

 Corynida, the Sertularida, and the Campanidarida. 



Order I. Hydrida {Gymnochroa, Hincks). — This order 

 comprises those Hydrozoa whose " hydrosoma " consists of a 

 single locomotive polypite, with tentacles and " hydrorhiza^' and 

 with reproductive organs which appear as simple external pro- 

 cesses of the body-wall. The hydrorhiza is discoid, and no hard 

 aiticular layer is at any time developed. 



The order Hydrida comprises a single genus only {Hydra), 

 including the various species of " Fresh-water Polypes," as 

 they are often called. The common Hydra (fig. 12, c) is 

 found abundantly in this country, and consists of a tubular 

 cylindrical body, the " proximal " extremity of which is ex- 

 panded into an adherent disc or foot — the " hydrorhiza " — by 

 means of which the animal can attach itself to some foreign 

 body. It possesses, however, the power of detaching the 

 hydrorhiza at will, and thus of changing its place. At the 

 opposite or " distal " extremity of iJie body is placed the 

 mouth, surrounded by. a circlet of tentacles, which arise a 

 little distance below the margin of the oral aperture. The 

 tentacles vary in number from five to twelve or more, and 

 they vary considerably in length in different species, being 

 much shorter than the body in the Hydra viridis, but being 

 extremely long and filamentous in Hydra fusca. They are 

 highly extensile and contractile, and serve as organs of pre- 



