CCELENTERATA : HYDROZOA. 9 I 



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

 Corynida and in the orders Sertularida and Campcnmlarida. 

 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 ?io 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 

 they are often called. The common Hydra (fig. 18, 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 

 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- 

 hension, being capable of retraction till they appear as nothing 

 more than so many warts or tubercles, and of being extended 

 to a length which is in some species many times longer than 

 the body itself. (In the Hydra fusca the tentacles can be 

 protruded to a length of more than eight inches.) Each con- 

 sists of a prolongation of both ectoderm and endoderm, en- 

 closing a diverticulum of the somatic cavity, and they are 

 abundantly furnished with thread-cells. The cylindrical hydro- 

 soma is excavated into a single large cavity, lined by the en- 

 doderm, and communicating with the exterior by the mouth. 

 This the " somatic cavity ;? is the sole digestive cavity with 

 which the Hydra is provided, the indigestible portions of the 

 food being rejected by the mouth. 



The Hydra possesses a most extraordinary power of resist- 

 ing mutilation, and of multiplying artificially when mechani- 

 cally divided. Into however many pieces a Hydra may be 



