This order comprises those Hydrozoa whose "hydro-soma" consists of a single locomotive polypite, with tentacles and "hydrorhiza" and with reproductive organs which appear as simple external processes 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 they are often called. The common Hydra (fig. 37, A) is found abundantly in this country, and consists of a tubular cylindrical body, the "proximal" extremity of which is expanded 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 (fig. 38), but being extremely long and filamentous in Hydra fusca. They are highly extensile and contractile, and serve as organs of prehension, 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 consists of a prolongation of both ectoderm and endoderm, enclosing a diverticulum of the somatic cavity, and they are abundantly furnished with thread-cells. The cylindrical hydrosoma (fig. 37, B) is excavated into a single large cavity, lined by the endoderm, 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.
Fig. 38. - The Green Fresh-water Polype (Hydra 7>iridis), suspended head-downwards from a piece of astern of an aquatic plant, enlarged. a One of the tentacles; b Testis or spermarium, with spermatozoa in its interior; c A single large ovum, protruding from the side of the body; d Disc of attachment ("hydrorhiza").
* If the Protohydra of Greeff be a mature form, it also belongs to this order. It differs from Hydra in having no tentacles, but it seems more probably to be the larva of some other Hydroid.
The Hydra possesses a most extraordinary power of resisting mutilation, and of multiplying artificially when mechanically divided. Into however many pieces a Hydra may be divided, each and all of these will be developed gradually into a new and perfect polypite. The remarkable experiments of Trembley upon this subject are well known, and have been often repeated, but space will not permit further notice of them here. Reproduction is effected in the Hydra both asexually by gemmation, and sexually - the former process being followed in summer, and the latter towards the commencement of winter, few individuals surviving this season. In the first method the Hydra (fig. 37, A) throws out one or more buds, generally from near its proximal extremity. These buds at first consist simply of a tubular prolongation of the ectoderm and endoderm, enclosing a caecal diverticulum of the body-cavity; but a mouth and tentacles are soon developed, when the new being is usually detached as a perfect independent Hydra. The Hydra thus produced throw out fresh buds, often before they are detached from the parent organism, and in this way reproduction is rapidly carried on.
In the second or sexual mode of reproduction, ova and spermatozoa are produced in outward processes of the body-wall (fig. 38). The spermatozoa are developed in little conical elevations, which are produced near the bases of the tentacles, and the ova are enclosed in sacs of much greater size, situated nearer the fixed or proximal extremity of the animal. Ordinarily there is but one of these sacs, containing a single ovum, but sometimes there are two. When mature, the ovum is expelled through the body-wall, and is fecundated by the spermatozoa, which are simultaneously liberated. The primitive body-cavity of the non-ciliated embryo is ultimately placed in communication with the outer world by the formation of the mouth, which is produced directly as an opening in the walls of the body, and not by invagination of the ectoderm.