When the Hydra has been firmly built into the wall of the shelters and its tentacles fastened down by their bases on the roof, the larva proceeds, sometimes after an interval of some hours, to eat the body, which it does very rapidly, leaving the tentacles attached to its shelter. The meal only lasts for a few minutes; after it the larva enjoys several hours' repose, protected by remains of its victim, which retain a kind of vitality for some time. During this period it remains still, except for certain undulatory movements of the posterior part of the body which probably aid in respiration. Then it leaves the shelter and goes in search of further prey. Its food, even when living in a tunnel, does not consist entirely of Hydra. I have watched a larva building its shelter near a number of rotifers, some of which it devoured and some of which it plastered on to its tunnel.

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]].

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 association 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 association 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 shells of Vivipara, and, more rarely, to those of other molluscs. It is doubtful whether this temporary association between Hydra and the mollusc is of any importance to the latter. Even when the polyp settles on its body and not on its shell (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]].

The association, however, is undoubtedly useful to Hydra. The mud on the shells 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 algæ. 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 prone 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 troisième espèce, Trembley, Mém. 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). Rösel von Rosenhof, Insekt.-Belustigung, iii, Hist. Polyp., pls. lxxxiv-lxxxvi (1755). 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 monœcia, Downing, Science* (5) xii, p. 228. Hydra fusca, id., Zool. Jahrb. (Anat.) xxi, p. 382 (1905). Hydra diœcia, 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:—