_Hydra orientalis_, _id._, _ibid._ (new series) ii, 1906, p.
109.
_Hydra orientalis_, _id._, Mem. Asiat. Soc. Bengal, i, p.
340 (1906).
? _Hydra orientalis_, Willey, Spol. Zeylan. Colombo, iv, p.
185 (1907).
_Hydra grisea_, Weltner, Arch. Naturg. Berlin, lxxiii, i, p.
475 (1907).
_Hydra vulgaris_, Brauer, Zool. Anz. x.x.xiii, p. 792, fig. 1 (1908).
_Hydra orientalis_, Annandale, Rec. Ind. Mus. ii, p. 312 (1908).
_Hydra grisea_, Frischholz, Braun"s Zool. Annal. (Wurzburg), iii, pp. 107, 134, &c., figs. 1 and 10-17 (1909).
_Hydra grisea_, _id._, Biol. Centralbl. Berlin, xxix, p. 184 (1909).
_Hydra vulgaris_, Brauer, Die Susswa.s.serfauna Deutschlands, xix, p. 192, figs. 336-338 (1909).
_Hydra pentactinella_, Powell, Lessons in Practical Biology for Indian Students, p. 24 (Bombay, 1910).
Phase _orientalis*_, Annandale.
_Colour_ variable; in summer usually pale, in winter either deep orange, dull brown, or dark green. The cells do not contain spherical or oval coloured bodies.
[Ill.u.s.tration: Fig. 29.--_Hydra vulgaris_, from Calcutta (phase _orientalis_).
A=winter brood; B=summer brood, the same individual in an expanded and a contracted condition. B is more highly magnified than A.]
_Column_ slender and capable of great elongation, normally almost cylindrical, but when containing food often shaped like a wine-gla.s.s.
The surface is thickly set with nettle-cells the cnidocils of which give it an almost hirsute appearance under the microscope. When extended to the utmost the column is sometimes nearly 30 mm. (1-1/5 inches) long, but more commonly it is about half that length or even shorter.
_Tentacles_ usually 4-6, occasionally 8. They are always slender except when they are contracted, then becoming swollen at the base and slightly globular at the tip. If the animal is at rest they are not very much longer than the body, but if it is hungry or about to move from one place to another they are capable of very great extension, often becoming like a string of minute beads (the groups of nettle-cells) strung on an invisible wire.
_Nettle-cells._ The capsules with barbed threads (fig. 27, p. 131) are very variable in size, but they are invariably broad in proportion to their length and as a rule nearly spherical. In a _Hydra_ taken in Calcutta during the winter the largest capsules measured (unexploded) 0.0189 mm. in breadth and 0.019 in length, but in summer they are smaller (about 0.012 mm. in breadth). Smaller capsules with barbed threads always occur. The barbed threads are very long and slender. At their base they bear a circle of stout and prominent spines, usually 4 in number; above these there are a number of very small spines, but the small spines are usually obscure. Malformed corpuscles are common. The capsules with unbarbed threads are very nearly as broad at the distal as at the proximal end; they are broadly oval with rounded ends.
_Reproductive organs._ The reproductive organs are confined to the upper part of the body. In India eggs (fig. 28, p. 137) are seldom produced.
They sometimes appear, however, at the beginning of the hot weather. In form they are spherical, and their sh.e.l.l bears relatively long spines, which are expanded, flattened and more or less divided at the tip. The part of the egg that is in contact with the parent-polyp is bare.
Spermaries are produced more readily than ovaries; they are mammillate in form and number from 4 to 24. Ovaries and spermaries have not been found on the same individual.
_Buds_ are confined to a narrow zone nearer the base than the apex of the column. Rarely more than 2 are produced at a time, and I have never seen an attached bud budding. In winter 5 tentacles are as a rule produced simultaneously, and in summer 4. In the former case a fifth often makes its appearance before the bud is liberated.
In Calcutta two broods can be distinguished, a cold-weather brood, which is larger, stouter, and more deeply coloured, produces buds more freely, has larger nematocysts, and as a rule possesses 6 tentacles; and a hot-weather brood, which is smaller, more slender and paler, produces buds very sparingly, has smaller nematocysts, and as a rule possesses only 4 or 5 tentacles. Only the cold-weather form is known to become s.e.xually mature. There is evidence, however, that in those parts of India which enjoy a more uniform tropical climate than Lower Bengal, polyps found at all times of year resemble those found in the hot weather in Calcutta, and sometimes produce spermatozoa or eggs.
I have recently had an opportunity of comparing specimens of the Calcutta hot-weather form with well-preserved examples of _H. vulgaris_, Pallas (=_H. grisea_, Linn.), from England. They differ from these polyps in very much the same way as, but to a greater degree than they do from the winter phase of their own race, and I have therefore no doubt that _H. orientalis_ is merely a tropical phase of Pallas"s species. My description is based on Indian specimens, which seem to differ, so far as anatomy is concerned, from European ones in the following points:--
(1) The s.e.xes are invariably distinct; (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.
Tena.s.serim (_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-sh.o.r.e 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 s.e.xual 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 debris 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 sh.e.l.ls. 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 circ.u.moral 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.
Young individuals are often, and adults occasionally, found floating free in the water, either with the mouth uppermost and the tentacles extended so as to cover as large an area as possible or with the aboral pole at the surface. In the former case they float in mid-water, being of nearly the same specific gravity as the water, and are carried about by any movement set up in it. In the latter case, however, the base of the column is actually attached to some small object such as the cast skin of a water-flea or to a minute drop of mucus originally given out by the polyp"s own mouth; the tentacles either hang downwards or are spread out round the mouth, and the animal is carried about by wind or other agencies acting on the surface.
In addition to this pa.s.sive method of progression the polyp can crawl with considerable rapidity. In doing so it bends its column down to the object along which it is about to move in such a way that it lies almost parallel to the surface, the basal disk, however, being still attached.
The tentacles are then extended and attach themselves near the tips to the surface a considerable distance away. Attachment is effected by the secretion of minute drops of adhesive substance from gland-cells. The basal disk is liberated and the tentacles contract, dragging the column, which still lies p.r.o.ne, along as they do so. The basal disk again affixes itself, the tentacles wrench themselves free, the surface of their cells being often drawn out in the process into pseudopodia-like projections, which of course are not true pseudopodia[AS] but merely projections produced by the mechanical strain. The whole action is then repeated. The polyp can also pull itself across a s.p.a.ce such as that between two stems or leaves by stretching out one of its tentacles, fixing the tip to the object it desires to reach, pulling itself free from its former point of attachment, and dragging itself across by contracting the fixed tentacle. The basal disk is then turned round and fixed to the new support.
[Footnote AS: See Zykoff, Biol. Centralbl. xviii, p. 272 (1898), and Annandale, Rec. Ind. Mus. i, p. 67 (1907).]
The Indian polyp, like all its congeners, is attracted by light, but it is more strongly repelled by heat. Probably it never moves in a straight line, but if direct sunlight falls on one side of a gla.s.s aquarium, the polyps move away from that side in a much less erratic course than is usually the case. If conditions are favourable, they often remain in one spot for weeks at a time, their buds congregating round them as they are set free. In a natural environment it seems that regular migrations take place in accordance with changes in temperature, for whereas in cool weather many individuals are found adhering to the lower surface of the floating leaves of _Limnanthemum_, few are found in this position immediately after a rise in the thermometer. If the rise is only a small one, they merely crawl down the stems to the end of which the leaves are attached, but as soon as the hot weather begins in earnest, the few that survive make their way to the deepest and most shady part of the pond.
In captivity the polyps seek the bottom of any vessel in which they are contained, if sunlight falls on the surface of the water.
The chief function of the tentacles is that of capturing prey. The Indian polyp feeds as a rule in the early morning, before the day has become hot. In an aquarium at any rate, the tentacles are never more than moderately extended during the night. If the polyp is hungry, they are extended to their greatest length in the early morning, and if prey is not captured, they sometimes remain in this condition throughout the day. In these circ.u.mstances they hang down or stand up in the water closely parallel to one another, and often curved in the middle as if a current were directed against them. Prey that comes in contact with one of them has little chance of escape, for nematocysts from all the tentacles can be readily discharged against it. Approximately once in half an hour the direction of the tentacles is changed, but I have been unable to observe any regular rhythmical movements of the tentacles or any correlation between those of a parent polyp and the buds still attached to it.
The prey consists chiefly of the young larvae of midges (Chironomidae) and may-flies, but small copepod and phyllopod crustacea are also captured.
As soon as the prey adheres firmly to the tentacles and has become paralysed it is brought to the mouth by their contracting strongly and is involved in a ma.s.s of colourless mucus extruded from the digestive cavity. Partly by the contraction of muscle-fibres in the body-wall and partly by movements of the mouth itself a.s.sisted by the mucus, which apparently remains attached to the walls of the cavity, the food is brought into the mouth. If it is at all bulky, it remains in the upper part of the cavity, the gland-cells pouring out a digestive fluid upon it and so dissolving out soluble substances. A large share of the substances thus prepared falls down to the bottom of the cavity and are there digested by the endoderm cells. The insoluble parts of the food are, however, ejected from the mouth without ever reaching the base of the cavity.
The colour of the polyp appears to be due mainly to the results of digestion. Brown or orange individuals recently captured in a pond and kept in favourable conditions take three or four days to digest their food, and the excreta ejected from the mouth then take the form of a white flocculent ma.s.s. If, however, the same individuals are kept for long in a gla.s.s aquarium, they lose their colour, even though they feed readily. Digestion is then a much more rapid process, and the excreta contain minute, irregular, coloured granules, which appear to be identical with those contained in the endoderm cells of individuals that have recently digested a meal fully. Starved individuals are always nearly colourless. It seems, therefore, that in this species colour is due directly to the products of digestion, and that digestion does not take place so fully in unfavourable conditions or at a high temperature as it does in more healthy circ.u.mstances. The dark green colour of some polyps is, however, less easily explained. I have noticed that all the individuals which have produced eggs in my aquarium have been of this colour, which they have retained in spite of captivity; whereas individuals that produced spermatozoa often lost their colour completely before doing so, sometimes becoming of a milky white owing to the acc.u.mulation of minute drops of liquid in their endoderm cells. Even in green individuals there is never any trace in the cells of coloured bodies of a definite form.
The Indian polyp, unlike European representatives of its species, is a very delicate little animal. In captivity at any rate, three circ.u.mstances are most inimical to its life: firstly, a sudden rise in the temperature, which may either kill the polyp directly or cause it to hasten its decease by becoming s.e.xually mature; secondly, the lack of a free current of air on the surface of the aquarium; and thirdly, the growth of a bacterium, which forms a sc.u.m on the top of the water and clogs up the interstices between the leaves and stems of the water-plants, soon killing them. If adult polyps are kept even in a shallow opaque vessel which is shut up in a room with closed shutters they generally die in a single night; indeed, they rarely survive for more than a few days unless the vessel is placed in such a position that air is moving almost continuously over its surface. The bacterium to which I allude often almost seals up the aquarium, especially in March and April, in which months its growth is very rapid. Strands of slime produced by it surround the polyp and even enter its mouth. In this event the polyp retracts its tentacles until they become mere prominences on its disk, and shrinks greatly in size. The colouring matter in its body becomes broken up into irregular patches owing to degeneracy of the endoderm cells, and it dies within a few hours.
_Hydra_ in Calcutta is often devoured by the larva of a small midge (_Chironomus fasciatipennis_, Kieffer) common in the tanks from November to February. In the early stages of its larval life this insect wanders free among communities of protozoa (_Vorticella_, _Epistylis_, &c.) and rotifers on which it feeds, but as maturity approaches begins to build for itself a temporary shelter of one of two kinds, either a delicate silken tunnel the base of which is formed by some smooth natural surface, or a regular tube the base of which is fixed by a stalk situated near the middle of its length to some solid object, while the whole surface is covered with little projections. The nature of the covering appears to depend partly on that of the food-supply and partly on whether the larva is about to change its skin.
I had frequently noticed that tunnels brought from the tank on the under surface of _Limnanthemum_ leaves had a _Hydra_ fixed to them. This occurred in about a third of the occupied shelters examined. The _Hydra_ was always in a contracted condition and often more or less mutilated.
By keeping a larva together with a free polyp in a gla.s.s of clean water, I have been able to observe the manner in which the polyp is captured and entangled. The larva settles down near the base of its column and commences to spin a tunnel. When this is partially completed, it pa.s.ses a thread round the polyp"s body to which it gives a sharp bite. This causes the polyp to bend down its tentacles, which the larva entangles with threads of silk, doing so by means of rapid, darting movements; for the nettle-cells would prove fatal should they be shot out against its body, which is soft. Its head is probably too thickly coated with chitin to excite their discharge. Indeed, small larvae of this very species form no inconsiderable part of the food of the polyp, and, so far as my observations go, a larva is always attacked in the body and swallowed in a doubled-up position.
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.