CCELENTERATA : HYDROZOA. 



79 



111 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 Campanularida. 

 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 Campanularida. 



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 

 cuticular layer is at any time developed. 



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

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



Fig. 12. Morphology of Hydrozoa. a Diagrammatic section of Hydra. The dark 

 line is the ectoderm, the fine line and clear space adjacent are the endoderm. 

 b Hydra viridis, showing a single ovum contained in the body-wall near the 

 proximal extremity, and two elevations containing spermatozoa near the bases of 

 the tentacles ; c Hydra vulgaris, with an undetached bud ; d Thread-cell of the 

 Hydra, greatly magnified. 



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 the body is placed the 

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



