Chapter XI: Introduction: To Part II (2)

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.

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 passive 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 prone, 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 space 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 glass 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 circumstances 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 larvæ of midges (Chironomidæ) 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 mass 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 assisted 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 mass. If, however, the same individuals are kept for long in a glass 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 circumstances. 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 accumulation 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 circumstances 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 sexually 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 scum 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 glass 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 passes 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 larvæ 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.

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].

[Footnote AT: Further particulars regarding the life-history
of this larva will be found on pp. 114 and 115, J. Asiat.
Soc. Bengal, ii (n. s.) 1906.]

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].

[Footnote AU: In the Calcutta tanks operculate molluscs such
as _Vivipara_ are certainly more free from visible attack
than non-operculate species. This is the case for instance,
as regards the common aquatic glowworm (_Luciola_ sp.),
which destroys large numbers of individuals of _Limnophysa_,
_Limnæus_, &c. If it has been starved for several days in an
aquarium it will attack an operculate form, but rarely with
success. Similarly _Chætogaster bengalensis_ attaches itself
exclusively to non-operculate forms. In the one case the
polyp could do very little against an adversary with so
stout an integument as the insect, while, in the other, it
is doubtful whether the worm does any harm to its host. The
polyp would afford very little protection against the
snail's vertebrate enemies or against what appears to be its
chief foe, namely, drought. As the water sinks in the tank
non-operculate species migrate to the deeper parts, but
_Vivipara_ and _Ampullaria_ close their shells, remain where
they are, and so often perish, being left high and dry,
exposed to the heat of the sun.]

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 monoecia_, Downing, Science* (5) xii, p. 228.

_Hydra fusca_, _id._, Zool. Jahrb. (Anat.) xxi, p. 382
(1905).

_Hydra dioecia_, _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:--

(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].

[Footnote AV: Pallas writes as regards this "pulcherrime
vegetantem varietatem" with his usual critical insight, "Vix
tamen peculiaris speciei nomine salutanda videtur." It is
probably the _Hydra socialis_ of Linné.]

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 dioecious, 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
monoecious. The lower surface of its egg is flattened,
adherent, and devoid of spines.

The larger form (A) was originally named _Hydra monoecia_ by Downing, who in 1904 expressed a wish to substitute for the specific name, which had been given through inadvertence, the more appropriate one _dioecia_. 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.

The colour of Indian examples of _H. oligactis_ apparently resembles that of the Calcutta winter brood of _H. vulgaris_ so far as visual effect is concerned, but I have noticed in specimens from Lahore and the neighbourhood that very minute spherical bodies of a dark green colour are present in the endoderm cells.

PART III.

FRESHWATER POLYZOA

(CTENOSTOMATA & PHYLACTOLÆMATA).