The tubular shelters occasionally found are very much stouter structures than the tunnels, but are apparently made fundamentally of the same materials; and structures intermediate between them and the tunnels are sometimes produced. The larva as a rule fastens to them branches detached from living colonies of Vorticellid protozoa such as _Epistylis_[AT].

[Footnote AT: Further particulars regarding the life-history of this larva will be found on pp. 114 and 115, J. Asiat.

Soc. Bengal, ii (n. s.) 1906.]

Of animals living in more or less intimate relations with the polyp, I have found two very distinct species of protozoa, neither of which is identical with either of the two commonly found in a.s.sociation with _Hydra_ in Europe, _Trichodina pediculus_ and _Kerona polyporum_. On two occasions, one in January and the other at the beginning of February, I have seen a minute colourless flagellate on the tentacles of the Calcutta polyp. On the first occasion the tentacles were completely covered with this protozoon, so that they appeared at first sight as though encased in flagellated epithelium. The minute organism was colourless, transparent, considerably larger than the spermatozoa of _Hydra_, slightly constricted in the middle and rounded at each end. It bore a long flagellum at the end furthest from its point of attachment, the method of which I could not ascertain. When separated from the polyp little groups clung together in rosettes and gyrated in the water. On the other occasion only a few individuals were observed. Possibly this flagellate was a parasite rather than a commensal, as the individual on which it swarmed was unusually emaciated and colourless, and bore neither gonads nor buds. The larger stinging cells were completely covered by groups of the organism, and possibly this may have interfered with the discharge of stinging threads.

The other protozoon was _Vorticella monilata_, Tatem, which has been found, not in a.s.sociation with _Hydra_, in Europe and S. America. In Calcutta I have only seen it attached to the column of the polyp, but probably it would also be found, if carefully looked for, attached to water-weeds.

Especially in the four-rayed stage, the polyp not infrequently attaches itself to sh.e.l.ls of _Vivipara_, and, more rarely, to those of other molluscs. It is doubtful whether this temporary a.s.sociation between _Hydra_ and the mollusc is of any importance to the latter. Even when the polyp settles on its body and not on its sh.e.l.l (as is sometimes the case) the _Vivipara_ appears to suffer no inconvenience, and makes no attempt to get rid of its burden. It is possible, on the other hand, that the _Hydra_ may protect it by devouring would-be parasites; but of this there is no evidence[AU].

[Footnote AU: In the Calcutta tanks operculate molluscs such as _Vivipara_ are certainly more free from visible attack than non-operculate species. This is the case for instance, as regards the common aquatic glowworm (_Luciola_ sp.), which destroys large numbers of individuals of _Limnophysa_, _Limnaeus_, &c. If it has been starved for several days in an aquarium it will attack an operculate form, but rarely with success. Similarly _Chaetogaster bengalensis_ attaches itself exclusively to non-operculate forms. In the one case the polyp could do very little against an adversary with so stout an integument as the insect, while, in the other, it is doubtful whether the worm does any harm to its host. The polyp would afford very little protection against the snail's vertebrate enemies or against what appears to be its chief foe, namely, drought. As the water sinks in the tank non-operculate species migrate to the deeper parts, but _Vivipara_ and _Ampullaria_ close their sh.e.l.ls, remain where they are, and so often perish, being left high and dry, exposed to the heat of the sun.]

The a.s.sociation, however, is undoubtedly useful to _Hydra_. The mud on the sh.e.l.ls of _Vivipara_ taken on floating objects shows that in cool weather the snail comes up from the bottom to the surface, and it probably goes in the opposite direction in hot weather. Moreover, the common Calcutta species (_V. bengalensis_) feeds very largely, if not exclusively, on minute green algae. It therefore naturally moves towards spots where smaller forms of animal and vegetable life abound and conditions are favourable for the polyp. The polyp's means of progression are limited, and the use of a beast of burden is most advantageous to it, for it can detach itself when it arrives at a favourable habitat. If specimens are kept in water which is allowed to become foul, a very large proportion of them will attach themselves to any snails confined with them. Under natural conditions they would thus in all probability be rapidly conveyed to a more suitable environment.

In the tanks it is far commoner to find young four-rayed polyps on _Vivipara_ than individuals with five or six rays; but the adults of the species are far less p.r.o.ne to change their position than are the young.

The Calcutta _Hydra_, especially in spring, exhibits a distinct tendency to frequent the neighbourhood of sponges and polyzoa, such as _Spongilla carteri_ and the denser forms of _Plumatella_. Possibly this is owing to the shade these organisms provide.

25. Hydra oligactis, _Pallas_.

Polypes de la troisieme espece, Trembley, Mem. hist.

Polypes,* pl. i, figs. 3, 4, 6; pl. ii, figs. 1-4; pl. iii, fig. 11; pl. v, figs. 1-4; pl. vi, figs. 3-7, 9, 10; pl.

viii, figs. 8, 11; pl. ix (1744).

Rosel von Rosenhof, Insekt.-Bel.u.s.tigung, iii, Hist. Polyp., pls. lx.x.xiv-lx.x.xvi (1755).

_Hydra socialis_, Linne, Fauna Sueica, p. 542 (1761).

_Hydra oligactis_, Pallas, Elench. Zooph. p. 29 (1766).

? _Hydra attenuata_, _id._, _ibid._ p. 32.

_Hydra fusca_, Linne, 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 rhaetica_, 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 Bronn'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. x.x.xiii, p. 792, fig. 2 (1908).

_Hydra polypus_, _id._, _ibid._

_Hydra fusca_, Frischholz, Ann. Zool. (Wurzburg), iii, p.

114, figs. 2-9 (1909).

_Hydra oligactis_, Brauer, Susswa.s.serfauna 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-sh.e.l.l is nearly smooth or covered more or less completely with short, simple spines (fig. 28, p. 137).

_H. oligactis_ is usually a more vigorous form than _H. vulgaris_ and, in spite of its name, has often a considerable number of tentacles. The few Indian specimens examined have, however, been small and have not had more than six tentacles. I have not seen an Indian specimen with more than two buds, but European specimens sometimes produce a great many, and as the daughter buds do not always separate from the parent until they have themselves produced buds, temporary colonies of some complexity arise; Chun figures a specimen with nineteen daughter and granddaughter buds[AV].

[Footnote AV: Pallas writes as regards this "pulcherrime vegetantem varietatem" with his usual critical insight, "Vix tamen peculiaris speciei nomine salutanda videtur." It is probably the _Hydra socialis_ of Linne.]

In Europe and N. America there appear to be two races or phases of the species. To avoid ambiguity they may be called form A and form B and described as follows:--

Form A is of vigorous growth. It is as a rule dioecious, and its reproductive organs may be borne practically at any level on the surface of the column. Its eggs are spherical and as a rule covered almost uniformly with spines.

Form B is smaller and has smaller and more variable nettle-cells. Its reproductive organs are borne only on the distal third or at the base of its column and it is often monoecious. The lower surface of its egg is flattened, adherent, and devoid of spines.

The larger form (A) was originally named _Hydra monoecia_ by Downing, who in 1904 expressed a wish to subst.i.tute for the specific name, which had been given through inadvertence, the more appropriate one _dioecia_.

As, however, it appears to be the commoner of the two in northern Europe, we may regard it as probably being the one named _Hydra oligactis_ by Pallas and therefore may accept it as the _forma typica_ of that species. According to Brauer (1908) the smaller form is Linne's _Hydra polypus_; but the original description of the "species" hardly bears out this view. As reproductive organs have not yet been found in Indian specimens, it is impossible to say to which of the two forms they belong.

A red form of _H. oligactis_ occurs in Tibet in the lake Rham-tso, at an alt.i.tude of about 15,000 feet and has been reported from various small lakes in mountainous parts of Europe. It is probably the form called _Hydra rhaetica_ by Asper, but his figures are lacking in detail and appear to have been drawn from specimens in a state of partial contraction. _H. rubra_, Lewes (Ann. Mag. Nat. Hist. (3) v, p. 71, 1860), may also be identical with this form. Roux, indeed, states that _H. rubra_ is only found living unattached at considerable depths (Ann.

Biol. lacustre ii, p. 266, 1907); but this statement does not accord with the fact that Lewes's specimens were found in ponds on Wimbledon Common.

TYPE not in existence.