25. _H. oligactis_, Pallas (1766).

Order ELEUTHEROBLASTEA.

Naked hydrozoa which reproduce their kind by means of buds or eggs, or by fission, without exhibiting the phenomena of alternation of generations.

Family HYDRIDae.

HYDRAIDae, Johnston, Hist. Brit. Zooph. (ed. 2) i, p. 120 (1847).

HYDRIDae, Hincks, Hist. Brit. Hydroid. Zooph. p. 309 (1868).

Small Eleutheroblastea in which the mouth is surrounded by hollow tentacles. Permanent colonies are not formed, but reproduction by budding commonly takes place.

Genus HYDRA, _Linne_.

TYPE, _Hydra viridis_, Linne.

Freshwater polyps which produce eggs with hard chitinous sh.e.l.ls.

Although habitually anch.o.r.ed 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_, Linne (=_H. viridissima_, Pallas), _H.

vulgaris_, Pallas (=_H. grisea_, Linne), and _H. oligactis_, Pallas (=_H. fusca_, Linne). 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 j.a.pan. 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, Mem. 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-sh.e.l.l 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-sh.e.l.l 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-sh.e.l.l smooth or bearing short, simple spines _oligactis_, p. 158.

24. Hydra vulgaris, _Pallas_.

Polypes de la seconde espece, Trembley, Mem. pour servir a 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).

Rosel von Rosenhof, Insecten-Bel.u.s.tigung, iii, Hist.

Polyporum, pls. lxxvi, lxxvii, lxxix-lx.x.xiii (1755).

? _Hydra polypus_, Linne, Fauna Suecica, p. 542 (1761).

_Hydra vulgaris_, Pallas, Elenchus Zoophytorum, p. 30 (1766).

? _Hydra attenuata_, _id_., _ibid_. p. 32.

_Hydra grisea_, Linne (Gmelin), Systema Naturae (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'eponge 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 Bronn's Thier-Reichs, ii (2), pl.

ii, figs. 2_b_, 2_c_, 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.