[Footnote AN: Zool. Anz. x.x.xvi, pp. 271-279, figs., Oct.

1910.]

BEHAVIOUR.

_Hydra viridis_ is a more sluggish animal than the other species of its genus and does not possess the same power of elongating its column and tentacles. It is, nevertheless, obliged to feed more frequently. Wagner (Quart. J. Micr. Sci. xlviii, p. 586, 1905) found it impossible to use this species in his physiological experiments because it died of starvation more rapidly than other forms. This fact is interesting in view of the theory that the green corpuscles in the cells of _H.

viridis_ elaborate nutritive substances for its benefit. _H. vulgaris_, at any rate in Calcutta, does not ordinarily capture prey more often than about once in three days.

All _Hydrae_ (except possibly the problematical _H. rubra_ of Roux, p.

160) spend the greater part of their time attached by the basal disk to some solid object, but, especially in early life, _H. vulgaris_ is often found floating free in the water, and all the species possess powers of progression. They do not, however, all move in the same way. _H.

viridis_ progresses by "looping" like a geometrid caterpillar. During each forward movement the column is arched downwards so that the peristome is in contact with the surface along which the animal is moving. The basal disk is then detached and the column is twisted round until the basal disk again comes in contact with the surface at a point some distance in advance of its previous point of attachment. The manoeuvre is then repeated. _H. vulgaris_, when about to move, bends down its column so that it lies almost p.r.o.ne, stretches out its tentacles, which adhere near the tips to the surface (p. 153), detaches its basal disk, and then contracts the tentacles. The column is dragged forward, still lying almost p.r.o.ne, the basal disk is bent downwards and again attached, and the whole movement is repeated. Probably _H.

oligactis_ moves in the same way.

When _H. viridis_ is at rest the tentacles and column, according to Wagner, exhibit rhythmical contractions in which those of the buds act in sympathy with those of the parent. In _H. vulgaris_ no such movements have been observed. This species, however, when it is waiting for prey (p. 154) changes the direction of its tentacles about once in half an hour.

All species of _Hydra_ react to chemical and physical stimuli by contraction and by movements of the column and tentacles, but if the stimuli are constantly repeated, they lose the power to some extent. All species are attracted by light and move towards the point whence it reaches them. _H. vulgaris_, however, at any rate in India, is more strongly repelled by heat. Consequently, if it is placed in a gla.s.s vessel of water, on one side of which the sun is shining directly, it moves away from the source of the light[AO]. But if the vessel be protected from the direct rays of the sun and only a subdued light falls on one side of it, the polyp moves towards that side. No species of the genus is able to move in a straight line. Wilson (Amer. Natural. xxv, p.

426, 1891) and Wagner (_op. cit. supra_) have published charts showing the elaborately erratic course pursued by a polyp in moving from one point to another and the effect of light as regards its movements.

[Footnote AO: Mr. F. H. Gravely tells me that this is also the case as regards _H. viridis_ in England, at any rate if freshly captured specimens are placed overnight in a bottle in a window in such a position that the early morning sunlight falls upon one side of the bottle.]

If an individual of _H. vulgaris_ that contains half digested food in its gastral cavity is violently removed from its natural surroundings and placed in a gla.s.s of water, the column and tentacles contract strongly for a few minutes. The body then becomes greatly elongated and the tentacles moderately so; the tentacles writhe in all directions (their tips being sometimes thrust into the mouth), and the food is ejected.

REPRODUCTION.

Reproduction takes place in _Hydra_ (i) by means of buds, (ii) by means of eggs, and (iii) occasionally by fission.

(a) _s.e.xual Reproduction._

The s.e.xual organs consist of ovaries (female) and spermaries (male).

Sometimes the two kinds of organs are borne by the same individual either simultaneously or in succession, but some individuals or races appear to be exclusively of one s.e.x. There is much evidence that in unfavourable conditions the larger proportion of individuals develop only male organs.

In temperate climates most forms of _Hydra_ breed at the approach of winter, but starvation undoubtedly induces a precocious s.e.xual activity, and the same is probably the case as regards other unfavourable conditions such as lack of oxygen in the water and either too high or too low a temperature.

Downing states that in N. America (Chicago) _H. vulgaris_ breeds in spring and sometimes as late as December; in Calcutta it has only been found breeding in February and March. Except during the breeding-season s.e.xual organs are absent; they do not appear in the same position on the column in all species.

The spermaries take the form of small mound-shaped projections on the surface of the column. Each consists of a ma.s.s of sperm-mother cells, in which the spermatozoa originate in large numbers. The spermatozoa resemble those of other animals, each possessing a head, which is shaped like an acorn, and a long vibratile tail by means of which it moves through the water. In the cells of the spermary the spermatozoa are closely packed together, with their heads pointing outwards towards the summit of the mound through which they finally make their way into the water. The aperture is formed by their own movements. Downing (Zool.

Jahrb. (Anat.) xxi, p. 379, 1905) and other authors have studied the origin of the spermatozoa in great detail.

[Ill.u.s.tration: Fig. 28.--Eggs of _Hydra_ (magnified).

A=egg of _H. vulgaris_ (after Chun). B=vertical section through egg of _H. oligactis_, form A (after Brauer). C=vertical section through egg of _H. oligactis_, form B (after Brauer).]

The ovaries consist of rounded ma.s.ses of cells lying at the base of the ectoderm. One of these cells, the future egg, grows more rapidly than the others, some or all of which it finally absorbs by means of lobose pseudopodia extruded from its margin. It then makes its way by amoeboid movements between the cells of the ectoderm until it reaches the surface. In _H. vulgaris_ (Mem. Asiat. Soc. Beng. i, p. 350, 1906) the egg is first visible with the aid of a lens as a minute star-shaped body of an intense white colour lying at the base of the ectoderm cells. It increases in size rapidly, gradually draws in its pseudopodia (the rays of the star) and makes its way through the ectoderm to the exterior. The process occupies not more than two hours. The issuing ovum does not destroy the ectoderm cells as it pa.s.ses out, but squeezes them together round the aperture it makes. Owing to the pressure it exerts upon them, they become much elongated and form a cup, in which the embryo rests on the surface of the parent. By the time that the egg has become globular, organic connection has ceased to exist. The embryo is held in position partly by means of the cup of elongated ectoderm cells and partly by a delicate film of mucus secreted by the parent. The most recent account of the oogenesis ("ovogenesis") is by Downing (Zool. Jahrb. (Anat.) xxvii, p. 295, 1909).

(b) _Budding._

The buds of _Hydra_ arise as hollow outgrowths from the wall of the column, probably in a definite order and position in each species. The tentacles are formed on the buds much as the buds themselves arise on the column. There is much dispute as to the order in which these structures appear on the bud, and Haacke (Jenaische Zeitschr. Naturwiss.

xiv, p. 133, 1880) has proposed to distinguish two species, _H.

trembleyi_ and _H. roeselii_, in accordance with the manner in which the phenomenon is manifested. It seems probable, however, that the number of tentacles that are developed in the first instance is due, at any rate to some extent, to circ.u.mstances, for in the summer brood of _H.

vulgaris_ in Calcutta five usually appear simultaneously, while in the winter brood of the same form four as a rule do so. Sometimes buds remain attached to their parents sufficiently long to develop buds themselves, so that temporary colonies of some complexity arise, but I have not known this to occur in the case of Indian individuals.

(c) _Fission._

Reproduction by fission occurs naturally but not habitually in all species of _Hydra_. It may take place either by a horizontal or by a vertical division of the column. In the latter case it may be either equal or unequal. If equal, it usually commences by an elongation in one direction of the circ.u.moral disk, which a.s.sumes a narrowly oval form; the tentacles increase in number, and a notch appears at either side of the disk and finally separates the column into two equal halves, each of which is a complete polyp. The division sometimes commences at the base of the column, but this is very rare. Transverse fission can be induced artificially and is said to occur sometimes in natural conditions. It commences by a constriction of the column which finally separates the animal into two parts, the lower of which develops tentacles and a mouth, while the upper part develops a basal disk. Unequal vertical division occurs when the column is divided vertically in such a way that the two resulting polyps are unequal in size. It is apparently not accompanied by any great increase in the number of the tentacles, but probably starts by one of the tentacles becoming forked and finally splitting down the middle.

The question of the regeneration of lost parts in _Hydra_ cannot well be separated from that of reproduction by fission. Over a hundred and fifty years ago Trembley found that if a polyp were cut into several pieces, each piece produced those structures necessary to render it a perfect polyp. He also believed that he had induced a polyp that had been turned inside out to adapt itself to circ.u.mstances and to reverse the functions and structure of the two cellular layers of its body. In this, however, he was probably mistaken, for there can be little doubt that his polyp turned right side out while not under his immediate observation. Many investigators have repeated some of his other experiments with success in Europe, but the Calcutta _Hydra_ is too delicate an animal to survive vivisection and invariably dies if lacerated. It appears that, even in favourable circ.u.mstances, for a fresh polyp to be formed by artificial fission it is necessary for the piece to contain cells of both cell-layers.

DEVELOPMENT OF THE EGG.

The egg of _Hydra_ is said to be fertilized as it lies at the base of the ectoderm, through which the fertilizing spermatozoon bores its way.

As soon as the egg has emerged from the cells of its parent it begins to split up in such a manner as to form a hollow ma.s.s of comparatively large equal cells. Smaller cells are separated off from these and soon fill the central cavity. Before segmentation begins a delicate film of mucus is secreted over the egg, and within this film the larger cells secrete first a thick chitinous or h.o.r.n.y egg-sh.e.l.l and within it a delicate membrane. Development in some cases is delayed for a considerable period, but sooner or later, by repeated division of the cells, an oval hollow embryo is formed and escapes into the water by the disintegration of the egg-sh.e.l.l and the subsequent rupture of the inner membrane. Tentacles soon sprout out from one end of the embryo's body and a mouth is formed; the column becomes more slender and attaches itself by the aboral pole to some solid object.

ENEMIES.

_Hydra_ seems to have few natural enemies. Martin (Q. J. Micr. Sci.

London, lii, p. 261, 1908) has, however, described how the minute worm _Microstoma lineare_ attacks _Hydra "rubra"_ in Scottish lochs, while the larva of a midge devours _H. vulgaris_ in considerable numbers in Calcutta tanks (p. 156).

COELENTERATES OF BRACKISH WATER.

Marine coelenterates of different orders not infrequently make their way or are carried by the tide up the estuaries of rivers into brackish water, and several species have been found living in isolated lagoons and pools of which the water was distinctly salt or brackish. Among the most remarkable instances of such isolation is the occurrence in Lake Qurun in the Faym of Egypt of _Cordylophora lacustris_ and of the peculiar little hydroid recently described by Mr. C. L. Boulenger as _Moerisia lyonsi_ (Q. J. Micr. Sci. London, lii, p. 357, pls. xxii, xxiii, 1908). In the delta of the Ganges there are numerous ponds which have at one time been connected with estuaries or creeks of brackish water and have become isolated either naturally or by the hand of man without the marine element in their fauna by any means disappearing (p.

14). The following species have been found in such ponds:--

(_a_) _Hydrozoa._

(1) _Bimeria vest.i.ta_, Wright (1859).

Hincks, Hist. Brit. Hydr. Zooph. p. 103, pl. xv, fig. 2 (1868); Annandale, Rec. Ind. Mus. i, p. 141, fig. 3 (1907).

This is a European species which has also been found off S. America. It occurs not uncommonly in the creeks that penetrate into the Ganges delta and has been found in pools of brackish water at Port Canning. The Indian form is perhaps sufficiently distinct to be regarded as a subspecies. The medusoid generation is suppressed in this genus.

(2) _Syncoryne filamentata_, Annandale (1907).

Annandale, Rec. Ind. Mus. i, p. 139, figs. 1, 2 (1907).

Both hydroid and medusae were found in a small pool of brackish water at Port Canning. The specific name refers to the fact that the ends of the rhizomes from which the polyps arise are frequently free and elongate, for the young polyp at the tip apparently takes some time to a.s.sume its adult form.

(3) _Irene ceylonensis_, Browne (1905).

Browne, in Herdman's Report on the Pearl Fisheries of Ceylon, iv, p. 140, pl. iii, figs. 9-11 (1905); Annandale, Rec. Ind. Mus. i, p. 142, fig. 4 (1907).

The medusa was originally taken off the coast of Ceylon, while the hydroid was discovered in ponds of brackish water at Port Canning. It is almost microscopic in size.