Polypes de la seconde espèce, Trembley, Mém. pour servir à l'histoire d'un genre de polypes d'eau douce*, pl. i, figs. 2, 5; pl. vi, figs. 2, 8; pl. viii, figs. 1-7; pl. xi, figs. 11-13 (1744).Rösel von Rosenhof, Insecten-Belustigung, iii, Hist. Polyporum, pls. lxxvi, lxxvii, lxxix-lxxxiii (1755).?Hydra polypus, Linné, Fauna Suecica, p. 542 (1761).Hydra vulgaris, Pallas, Elenchus Zoophytorum, p. 30 (1766).?Hydra attenuata,id.,ibid. p. 32.Hydra grisea, Linné (Gmelin), Systema Naturæ (ed. 13), p. 3870 (1782).Hydra pallens,id.,ibid. p. 3871.Hydra vulgaris, Ehrenberg, Abhandl. Akad. Wiss. Berlin, 1836, p. 134, taf. ii.Hydra brunnea, Templeton, London's Mag. Nat. Hist. ix, p. 417 (1836).Hydra vulgaris, Laurent, Rech. sur l'Hydre at l'Éponge d'eau douce (Voy. de la Bonite, Zoophytologie), p. 11, pl. i, pl. ii, figs. 2, 2'' (1844).Hydra vulgaris, Johnston, Hist. British Zoophytes (ed. 2), i, p. 122, pl. xxix, fig. 2 (1847).Hydra vulgaris, Hincks, Hist. British Hydroid Zoophytes, i, p. 314, fig. 41 (1868).Hydra aurantiaca, Kleinenberg, Hydra, p. 70, pl. i, fig. 1, pl. iii, fig. 10 (1872).Hydra trembleyi, Haacke, Zool. Anz. Leipzig, ii, p. 622 (1879).Hydra grisea, Jickeli, Morph. Jahrb. viii, p. 391, pl. xviii, fig. 2 (1883).Hydra grisea, Nussbaum, Arch. mikr. Anat. Bonn, xxix, p. 272, pl. xiii, pl. xiv, figs. 33, 37, 47 (1887).?Hydra hexactinella, v. Lendenfeld, Zool. Jahrb. Jena, ii, p. 96, pl. vi, figs. 13, 14 (1887).?Hydra hexactinella,id., Proc. Linn. Soc. N. S. Wales, x, p. 678, p. xlviii, figs. 1-4 (1887).Hydra grisea, Brauer, Zeit. wiss. Zool. Leipzig, lii, p. 169 (1891).Hydra grisea, Chun, in Brönn's Thier-Reichs, ii (2), pl. ii, figs. 2b, 2c, 5 (1892).Hydra grisea, Downing, Zool. Jahrb. (Anat.) Jena, xxi, p. 381 (1905).Hydra orientalis, Annandale, J. Asiat. Soc. Bengal, (new series) i, 1905, p. 72.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. xxxiii, p. 792, fig. 1 (1908).Hydra orientalis, Annandale, Rec. Ind. Mus. ii, p. 312 (1908).Hydra grisea, Frischholz, Braun's Zool. Annal. (Würzburg), 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 Süsswasserfauna Deutschlands, xix, p. 192, figs. 336-338 (1909).Hydra pentactinella, Powell, Lessons in Practical Biology for Indian Students, p. 24 (Bombay, 1910).
Phaseorientalis*, Annandale.
Colourvariable; in summer usually pale, in winter either deep orange, dull brown, or dark green. The cells do not contain spherical or oval coloured bodies.
Illustration: Fig. 29.—Hydra vulgaris, from Calcutta (phase orientalis)Fig. 29.—Hydra vulgaris, from Calcutta (phaseorientalis).
Fig. 29.—Hydra vulgaris, from Calcutta (phaseorientalis).
A=winter brood; B=summer brood, the same individual in an expanded and a contracted condition. B is more highly magnified than A.
Columnslender and capable of great elongation, normally almost cylindrical, but when containing food often shaped like a wine-glass. The surface is thickly set with nettle-cells the cnidocils of which give it an almost hirsute appearance under themicroscope. 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.
Tentaclesusually 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 aHydrataken 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 shell 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.
Budsare 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 sexually 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 ofH. 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 thatH. orientalisis 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 sexes are invariably distinct;
(2) the nematocysts are invariably smaller.
I have seen in Burma an abnormal individual with no tentacles.Itsbuds, however, possessed these organs.
Type.None of the older types ofHydraare now in existence. That ofH. orientalisis, however, in the collection of the Indian Museum.
Geographical Distribution.—H. vulgarisis 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 IndiaH. vulgarisis 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 plantLimnanthemum, which has floating leaves attached to thin stalks that spring up from the bottom, and toLemna(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 ofLimnanthemum; 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 thesummer 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 IndianHydra, 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 thewhole 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 IndianHydrais a very delicate animal as compared with such a form asH. 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.
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 sideof 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 ofLimnanthemum, 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 ina 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.
Hydrain 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 oneof 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 ofLimnanthemumleaves had aHydrafixed to them. This occurred in about a third of the occupied shelters examined. TheHydrawas 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 theHydrahas 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 ofHydra. 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 asEpistylis[AT].
Of animals living in more or less intimate relations with thepolyp, I have found two very distinct species of protozoa, neither of which is identical with either of the two commonly found in association withHydrain Europe,Trichodina pediculusandKerona 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 ofHydra, 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 wasVorticella monilata, Tatem, which has been found, not in association withHydra, 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 ofVivipara, and, more rarely, to those of other molluscs. It is doubtful whether this temporary association betweenHydraand 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) theViviparaappears to suffer no inconvenience, and makes no attempt to get rid of its burden. It is possible, on the other hand, that theHydramay protect it by devouring would-be parasites; but of this there is no evidence[AU].
The association, however, is undoubtedly useful toHydra. The mud on the shells ofViviparataken on floating objects showsthat 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 onViviparathan 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 CalcuttaHydra, especially in spring, exhibits a distinct tendency to frequent the neighbourhood of sponges and polyzoa, such asSpongilla carteriand the denser forms ofPlumatella. 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 (necPallas), 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).Hydrasp. ?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 monœcia, Downing, Science* (5) xii, p. 228.Hydra fusca,id., Zool. Jahrb. (Anat.) xxi, p. 382 (1905).Hydra diœcia,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 fromH. vulgarisin 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. oligactisis usually a more vigorous form thanH. vulgarisand, 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 namedHydra monœciaby Downing, who in 1904 expressed a wish to substitute for the specific name, which had been given through inadvertence, the more appropriate onediœcia. As, however, it appears to be the commoner of the two in northern Europe, we may regard it as probably being the one namedHydra oligactisby Pallas and therefore may accept it as theforma typicaof that species. According to Brauer (1908) the smaller form is Linné'sHydra 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 ofH. oligactisoccurs 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 calledHydra rhæticaby 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 thatH. rubrais 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.
Typenot in existence.
Geographical Distribution.—H. oligactisis 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 ofH. oligactisapparently resembles that of the Calcutta winter brood ofH. vulgarisso 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.
[AQ]A small form ofH. viridis(var.bakeri, Marshall) is found in brackish water in England.
[AR]Richard, Mém. Soc. zool. France, vii, p. 237 (1894).
[AS]See Zykoff, Biol. Centralbl. xviii, p. 272 (1898), and Annandale, Rec. Ind. Mus. i, p. 67 (1907).
[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.
[AU]In the Calcutta tanks operculate molluscs such asViviparaare certainly more free from visible attack than non-operculate species. This is the case for instance, as regards the common aquatic glowworm (Luciolasp.), which destroys large numbers of individuals ofLimnophysa,Limnæus, &c. If it has been starved for several days in an aquarium it will attack an operculate form, but rarely with success. SimilarlyChætogaster bengalensisattaches 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, butViviparaandAmpullariaclose their shells, remain where they are, and so often perish, being left high and dry, exposed to the heat of the sun.
[AV]Pallas writes as regards this "pulcherrime vegetantem varietatem" with his usual critical insight, "Vix tamen peculiaris speciei nomine salutanda videtur." It is probably theHydra socialisof Linné.
I.
Status and Structure of the Polyzoa.
The Polyzoa constitute a class in the third great division of the animal kingdom, the so-called Triploblastea. In this division are included also the worms, molluscs, insects, crustacea, spiders, vertebrates, etc.; for heterogeneous as its elements appear, all these animals may be considered to have essential features in common, in particular a body consisting primarily of three cellular layers. Most of them also possess a body cavity distinct from the alimentary canal. Some authors regard the position of the polyzoa as near that of the higher worms, but the group is an isolated one.
In considering the anatomy of simple forms of animal life such as the sponges it is necessary to pay attention mainly to individual cells, but in discussing more complicated forms our notice is first attracted to tissues and organs, for the cells of which these tissues and organs are composed have each a definite position, a definite structure, and a definite function. The most characteristic feature of the polyzoa, considered from this point of view, is the fact that most of their organs fall into one of two categories and are connected either with what is called the "zoœcium" or with what is known as the "polypide." The zoœcium is a cage in which the polypide is enclosed, but it is a living cage, differing from the shell of a snail or the tubes in which many worms encase themselves in being part of the animal itself. The polypide consists mainly of the organs connected directly and indirectly with nutrition and of part of the muscular system; its name is derived from the fact that it bears a superficial resemblance to a polyp such asHydra.
The shape and structure of the zoœcium differs greatly in different groups of polyzoa. In its simplest form it is merely a cylindrical tube of living matter which secretes an outer horny or gelatinous covering. It is open at the end furthest from its base, at which it is attached either to another zoœcium or to some kind of supporting structure. Certain parts of the polypide can always be extruded from the aperture, which is known technically as the "orifice," or withdrawn through it into the zoœcium.When the polypide is retracted it draws in with it a portion of the zoœcium. The dead outer layer or ectocyst lines part of the portion thus invaginated and forms the walls of a cavity within the orifice. The base of this cavity consists in many forms of a transverse partition pierced in the middle by a circular hole and known as the "diaphragm." The diaphragm, however, does not constitute the limit of the invaginated portion of the zoœcium, for the living inner wall or endocyst is dragged in still further and forms a sheath round the retracted tentacles. When the tentacles are protruded they emerge through the hole in the diaphragm, carrying with them their sheath of endocyst. The invagination above the diaphragm, consisting of both endocyst and ectocyst, is then everted.
The tentacles are a characteristic feature of the polypide. Together with the base to which they are attached they are known as the "lophophore"; they surround the mouth, usually in a circle. They differ widely from the tentacles ofHydrain both structure and function, although they too serve as organs for the capture of prey; they are not highly contractile and are not provided with nettle-cells but are covered with cilia, which are in constant motion. When extruded they form a conspicuous calix-like crown to the zoœcium, but in the retracted condition they are closely pressed together and lie parallel to one another. They are capable individually of motion in all directions but, although they usually move in concert, they cannot as a rule seize objects between them.
The mouth is a hole situated in the midst of the tentacles. It leads directly into a funnel-shaped œsophagus, the upper part of which is lined with cilia and is sometimes distinguished as the "pharynx," while the lower part, the œsophagus proper, is a thin-walled tube that connects the pharynx with the stomach, which it enters on the dorsal side. The stomach is a bulky organ that differs markedly in form and structure in different groups of polyzoa. It is lined internally with glandular cells and the inner wall is sometimes thrown into folds or "rugæ." The part with which the œsophagus communicates is known as the "cardiac" portion, while the part whence the intestine originates is called the "pylorus" or "pyloric" portion. The intestine commences on the ventral side opposite the entrance of the œsophagus and nearly on a level with it, the bulk of the stomach depending between the two tubes. This part of the stomach is often produced into a blind tube, the fundus or cæcum. The alimentary canal may therefore be described as distinctlyY-shaped. The proximal part of the intestine is in some forms lined with cilia, and the tube as a whole is usually divided into two parts—the intestine proper, which is nearest the stomach, and the rectum, which opens by the anus not far from the mouth.
The nervous system consists of a central ganglion or brain, which is situated at the base of the tentacles on the side nearest the anus and gives out radiating nerves in all directions. Closeto the brain and providing a communication between the cavity of the zoœcium and the cavity in which the tentacles are contained (or, in the case of an expanded polyp, the external world) is a ciliated tube known as the "intertentacular organ." Apparently it acts as a passage through which the genital products are expelled; but contradictory statements have been made regarding it, and perhaps it is present only at certain seasons or in certain conditions of the polypide.