Type, Hydra viridis, Linné.
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]].
The three species may be distinguished from one another by the following key:—
| [I. | Colour leaf-green; thecells contain green (chlorophyll) corpuscles of definite form. | |
| A. | Tentaclescomparatively stout, habitually shorter than the column, which iscylindrical. Egg-shell without spines, ornamented with a reticulatepattern | viridis.] |
| II. | Colour neverleaf-green; no chlorophyll corpuscles present in the cells. | |
| A. | Tentacles capable ofgreat elongation but when the animal is at rest never very much longerthan the column, which is cylindrical when the gastral cavity is empty.Largest nettle-cells almost as broad as long. Egg-shell bearing longspines most of which are divided at the tips | vulgaris, p. [148]. |
| B. | Tentacles, even whenthe animal is at rest, much longer than the column, the basal part ofwhich, even when the gastral cavity is empty, is constricted. Largestnettle-cells considerably longer than broad. Egg-shell smooth orbearing 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. Hydra grisea, Linné (Gmelin), Systema Naturæ (ed. 13), p. 3870 (1782). Hydra pallens, id., ibid. p. 3871. Hydra vulgaris, Ehrenberg, Abhandl. Akad. Wiss. Berlin, 1836, p. 134, taf. ii. Hydra brunnea, Templeton, London's Mag. Nat. Hist. ix, p. 417 (1836). Hydra vulgaris, Laurent, Rech. sur l'Hydre at l'Éponge d'eau douce (Voy. de la Bonite, Zoophytologie), p. 11, pl. i, pl. ii, figs. 2, 2'' (1844). Hydra vulgaris, Johnston, Hist. British Zoophytes (ed. 2), i, p. 122, pl. xxix, fig. 2 (1847). Hydra vulgaris, Hincks, Hist. British Hydroid Zoophytes, i, p. 314, fig. 41 (1868). Hydra aurantiaca, Kleinenberg, Hydra, p. 70, pl. i, fig. 1, pl. iii, fig. 10 (1872). Hydra trembleyi, Haacke, Zool. Anz. Leipzig, ii, p. 622 (1879). Hydra grisea, Jickeli, Morph. Jahrb. viii, p. 391, pl. xviii, fig. 2 (1883). Hydra grisea, Nussbaum, Arch. mikr. Anat. Bonn, xxix, p. 272, pl. xiii, pl. xiv, figs. 33, 37, 47 (1887). ? Hydra hexactinella, v. Lendenfeld, Zool. Jahrb. Jena, ii, p. 96, pl. vi, figs. 13, 14 (1887). ? Hydra hexactinella, id., Proc. Linn. Soc. N. S. Wales, x, p. 678, p. xlviii, figs. 1-4 (1887). Hydra grisea, Brauer, Zeit. wiss. Zool. Leipzig, lii, p. 169 (1891). Hydra grisea, Chun, in Brönn's Thier-Reichs, ii (2), pl. ii, figs. 2b, 2c, 5 (1892). Hydra grisea, Downing, Zool. Jahrb. (Anat.) Jena, xxi, p. 381 (1905). Hydra orientalis, Annandale, J. Asiat. Soc. Bengal, (new series) i, 1905, p. 72. Hydra orientalis, id., ibid. (new series) ii, 1906, p. 109. Hydra orientalis, id., Mem. Asiat. Soc. Bengal, i, p. 340 (1906). ? Hydra orientalis, Willey, Spol. Zeylan. Colombo, iv, p. 185 (1907). Hydra grisea, Weltner, Arch. Naturg. Berlin, lxxiii, i, p. 475 (1907). Hydra vulgaris, Brauer, Zool. Anz. xxxiii, p. 792, fig. 1 (1908). Hydra orientalis, Annandale, Rec. Ind. Mus. ii, p. 312 (1908). Hydra grisea, Frischholz, Braun's Zool. Annal. (Würzburg), iii, pp. 107, 134, &c. , figs. 1 and 10-17 (1909). Hydra grisea, id., Biol. Centralbl. Berlin, xxix, p. 184 (1909). Hydra vulgaris, Brauer, Die Süsswasserfauna Deutschlands, xix, p. 192, figs. 336-338 (1909). Hydra pentactinella, Powell, Lessons in Practical Biology for Indian Students, p. 24 (Bombay, 1910).
Phase orientalis*, Annandale.
Colour variable; in summer usually pale, in winter either deep orange, dull brown, or dark green. The cells do not contain spherical or oval coloured bodies.
