Freshwater polyps which produce eggs with hard chitinous shells. Although habitually anchored by the end of the body furthest from the mouth to extraneous objects, they possess considerable powers of locomotion. They are extremely contractile and change greatly from time to time in both form and size. Only three well-established species of the genus, which is universally distributed and occurs only in fresh or brackish[AQ] water, can be recognized, namely, _H. viridis_, Linné (=_H. viridissima_, Pallas), _H. vulgaris_, Pallas (=_H. grisea_, Linné), and _H. oligactis_, Pallas (=_H. fusca_, Linné). The two latter occur in India, but _H. viridis_ does not appear to have been found as yet anywhere in the Oriental Region, although it is common all over Europe and N. America and also in Japan. The distribution of _H. vulgaris_ is probably cosmopolitan, but there is some evidence that _H. oligactis_ avoids tropical districts, although, under the name _Hydra fusca_, it has been doubtfully recorded as occurring in Tonquin[AR]. [Footnote AQ: A small form of _H. viridis_ (var. _bakeri_, Marshall) is found in brackish water in England.] [Footnote AR: Richard, Mém. Soc. zool. France, vii, p. 237 (1894).] The three species may be distinguished from one another by the following key:-- [I. Colour leaf-green; the cells contain green (chlorophyll) corpuscles of definite form. A. Tentacles comparatively stout, habitually shorter than the column, which is cylindrical. Egg-shell without spines, ornamented with a reticulate pattern _viridis_.] II. Colour never leaf-green; no chlorophyll corpuscles present in the cells. A. Tentacles capable of great elongation but when the animal is at rest never very much longer than the column, which is cylindrical when the gastral cavity is empty. Largest nettle-cells almost as broad as long. Egg-shell bearing long spines most of which are divided at the tips _vulgaris_, p. 148. B. Tentacles, even when the animal is at rest, much longer than the column, the basal part of which, even when the gastral cavity is empty, is constricted. Largest nettle-cells considerably longer than broad. Egg-shell smooth or bearing short, simple spines _oligactis_, p. 158. 24. Hydra vulgaris, _Pallas_. Polypes de la seconde espèce, Trembley, Mém. pour servir à l'histoire d'un genre de polypes d'eau douce*, pl. i, figs. 2, 5; pl. vi, figs. 2, 8; pl. viii, figs. 1-7; pl. xi, figs. 11-13 (1744). Rösel von Rosenhof, Insecten-Belustigung, iii, Hist. Polyporum, pls. lxxvi, lxxvii, lxxix-lxxxiii (1755). ? _Hydra polypus_, Linné, Fauna Suecica, p. 542 (1761). _Hydra vulgaris_, Pallas, Elenchus Zoophytorum, p. 30 (1766). ? _Hydra attenuata_, _id_., _ibid_. p. 32.