(2) the nematocysts are invariably smaller.
I have seen in Burma an abnormal individual with no tentacles. Its buds, however, possessed these organs.
Type. None of the older types of Hydra are now in existence. That of H. orientalis is, however, in the collection of the Indian Museum.
Geographical Distribution.—H. vulgaris is common in Europe and N. America and is probably found all over tropical Asia. The following are Indian and Ceylon localities:—Bengal, Calcutta and neighbourhood (Annandale, Lloyd); Adra, Manbhum district (Paiva), Rampur Bhulia on the R. Ganges (Annandale); Chakradharpur, Chota Nagpur (Annandale); Pusa, Bihar (Annandale); Puri, Orissa (Annandale): Madras, sea-beach near Madras town (Henderson): Bombay, island of Bombay (Powell): Burma, Mandalay, Upper Burma, and Moulmein, N. Tenasserim (Annandale): Ceylon, Colombo and Peradeniya (Willey, Green). Dr. A. D. Imms tells me that he has obtained specimens that probably belong to this species in the Jumna at Allahabad.
Biology.—In India H. vulgaris is usually found, so far as my experience goes, in stagnant water. In Calcutta it is most abundant in ponds containing plenty of aquatic vegetation, and seems to be especially partial to the plant Limnanthemum, which has floating leaves attached to thin stalks that spring up from the bottom, and to Lemna (duckweed). Dr. Henderson, however, found specimens in a pool of rain-water on the sea-shore near Madras.
There is evidence that each of the two broods which occur in Lower Bengal represents at least one generation; probably it represents more than one, for tentacles are rarely if ever produced after the animal has obtained its full size, and never (or only owing to accident) decrease in number after they have once appeared. The winter form is found chiefly near the surface of the water, especially on the roots of duckweed and on the lower surface of the leaves of Limnanthemum; but the summer form affects deeper water in shady places, and as a rule attaches itself to wholly submerged plants. The latter form is to be met with between March and October, the cold-weather form between October and March, both being sometimes found together at the periods of transition. In the unnatural environment of an aquarium, however, individuals of the winter form lose their colour and become attenuated, in these features resembling the summer form, even in the cooler months. Buds produced in these conditions rarely have more than five tentacles or themselves produce buds freely after liberation.
The buds appear in a fixed order and position, at any rate on individuals examined in winter; in specimens of the summer form the position is fixed, but the order is irregular. Each quadrant of the column has apparently the power of producing, in a definite zone nearer the aboral pole than the mouth, a single bud; but the buds of the different quadrants are not produced simultaneously. If we imagine that the quadrants face north, south, east, and west, and that the first bud is produced in the north quadrant, the second will be produced in the east quadrant, the third in the south, and the fourth in the west. It is doubtful whether more than four buds are produced in the lifetime of an individual, and apparently attached buds never bud in this race. The second bud usually appears before the first is liberated, and this is also the case occasionally as regards the third, but it is exceptional for four buds to be present at one time. About three weeks usually elapse between the date at which the bud first appears as a minute conical projection on the surface of the parent and that at which it liberates itself. This it does by bending down, fixing itself to some solid object by means of the tips of its tentacles, the gland-cells of which secrete a gummy fluid, and then tearing itself free.
Although it is rare for more than two buds to be produced simultaneously, budding is apparently a more usual form of reproduction than sexual reproduction. Individuals that bear eggs have not yet been found in India in natural conditions, although males with functional spermaries are not uncommon at the approach of the hot weather. The few eggs that I have seen were produced in my aquarium towards the end of the cold weather. Starvation, lack of oxygen, and too high a temperature (perhaps also lack of light) appear to stimulate the growth of the male organs in ordinary cases, but perhaps they induce the development of ovaries in the case of individuals that are unusually well nourished.
The spines that cover the egg retain débris of various kinds upon its surface, so that it becomes more or less completely concealed by a covering of fragments of dead leaves and the like even before it is separated from the polyp. Its separation is brought about by its falling off the column of the parent. Nothing is known of its subsequent fate, but probably it lies dormant in the mud through the hot weather. Eggs are sometimes produced that have no shells. This is probably due to the fact that they have not been fertilized.
Reproduction by fission occurs rarely in the Indian Hydra, but both equal and unequal vertical fission have been observed. In the case of equal fission the circumoral area lengthens in a horizontal direction, and as many extra tentacles as those the polyp already possesses make their appearance. The mouth then becomes constricted in the middle and notches corresponding to its constriction appear at either side of the upper part of the column. Finally the whole animal divides into two equal halves in a vertical direction. I have only seen one instance of what appeared to be unequal vertical fission—that of a polyp consisting of two individuals still joined together by the basal disk, but one about half the size of the other. Each had three well-developed tentacles, and in addition a minute fourth tentacle. This was situated on the side opposed to that of the other individual which bore a similar tentacle. Transverse fission has not been observed. The Indian Hydra is a very delicate animal as compared with such a form as H. viridis, and all attempts to produce artificial fission without killing the polyp have as yet failed.