_Hydra socialis_, Linné, Fauna Sueica, p. 542 (1761). _Hydra oligactis_, Pallas, Elench. Zooph. p. 29 (1766). ? _Hydra attenuata_, _id._, _ibid._ p. 32. _Hydra fusca_, Linné, Syst. Nat. (ed. 13), p. 3870 (1782). _Hydra oligactis_, Johnston, Brit. Zooph. i, p. 124, fig. 27 (p. 120) (1847). _Hydra oligactis_, Hincks, Hist. Brit. Hydr. Zooph. i, p. 315, fig. 42 (1868). _Hydra roeselii_, Haacke, Jena Zeitschr. Naturwiss. xiv, p. 135 (1880). ? _Hydra rhætica_, Asper, Zool. Anz. 1880, p. 204, figs. 1-3. _Hydra vulgaris_, Jickeli (_nec_ Pallas), Morph. Jahrb. viii, p. 391, pl. xviii, fig. 3 (1882). _Hydra fusca_, Nussbaum, Arch. mikr. Anat. Bonn, xxix, p. 273, pl. xiv, figs. 34-36, pl. xv, figs. 48-51, &c. (1887). _Hydra fusca_, Brauer, Zeit. wiss. Zool. Leipzig, lii, p. 177, pl. xi, figs. 2, 5, 6; pl. xii, fig. 6 (1891). _Hydra_ sp. ? _id._, _ibid._ pl. xi, figs. 3, 3a, 4, 7, 8; pl. xii, figs. 1, 2, 5-13. _Hydra fusca_, Chun in Brönn's Thier-Reichs, ii (2), pl. ii, figs. 2(_a_), 4, 6 (1892). _Hydra monoecia_, Downing, Science* (5) xii, p. 228. _Hydra fusca_, _id._, Zool. Jahrb. (Anat.) xxi, p. 382 (1905). _Hydra dioecia_, _id._, _ibid._ pl. xxiii, figs. 6, 7, &c. _Hydra fusca_, Hertwig, Biol. Centralbl. xxvi, p. 489 (1906). _Hydra oligactis_, Brauer, Zool. Anz. xxxiii, p. 792, fig. 2 (1908). _Hydra polypus_, _id._, _ibid._ _Hydra fusca_, Frischholz, Ann. Zool. (Würzburg), iii, p. 114, figs. 2-9 (1909). _Hydra oligactis_, Brauer, Süsswasserfauna Deutschl. xix, p. 193, figs. 339-341 (1909). _Hydra polypus_, _id._, _ibid._ figs. 342-344. This species differs from _H. vulgaris_ in the following characters:-- (1) Even when the gastral cavity is empty, the basal part of the column is distinctly more slender than the upper part; (2) even when the animal is at rest, the tentacles are much longer than the column; (3) the nettle-cells of both types are usually smaller and more uniform in size than in the other species; those with barbed threads (fig. 27, p. 131) are always flask-shaped and somewhat narrower in proportion to their length, while those with simple threads are pointed or almost pointed at their distal end; (4) the stinging threads of the more complex form are comparatively stout and short; (5) there are comparatively few nettle-cells in the column; (6) the egg-shell is nearly smooth or covered more or less completely with short, simple spines (fig. 28, p. 137).