(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).
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]].
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 diœcious, 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 monœcious. The lower surface of its egg is flattened, adherent, and devoid of spines.
The larger form (A) was originally named Hydra monœcia by Downing, who in 1904 expressed a wish to substitute for the specific name, which had been given through inadvertence, the more appropriate one diœcia. 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 Linné'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 altitude 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 rhætica 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.
Geographical Distribution.—H. oligactis is widely distributed in Europe and N. America, but in India has only been found in and near the city of Lahore in the Punjab.
Biology.—This species was found by Major J. Stephenson, I.M.S., in the basin of a fountain at Lahore and in an ornamental canal in the Shalimar Gardens on the outskirts of the same city. Nothing is known as regards its habits in this country. In N. America, according to Downing, form B breeds in September and October and form A from October to December. The eggs of form B remain attached to the parent until the two cellular layers are formed and then drop off, whereas those of form A are fixed by the parent to some extraneous object, its column contracting until they are in a favourable position for attachment.