Chapter X: Introduction: To Part II (1)

I.

THE PHYLUM COELENTERATA AND THE CLASS HYDROZOA.

The second of the great groups or phyla into which the metazoa are divided is the Coelenterata, in which are included most of the animals commonly known as zoophytes, and also the corals, sea-anemones and jelly-fish. These animals are distinguished from the sponges on the one hand and from the worms, molluscs, arthropods, vertebrates, etc., on the other by possessing a central cavity (the coelenteron or "hollow inside") the walls of which are the walls of the body and consist of _two_ layers of cells separated by a structureless, or apparently structureless, jelly. This cavity has as a main function that of a digestive cavity.

An ideally simple coelenterate would not differ much in general appearance from an olynthus (p. 27), but it would have no pores in the body-wall and its upper orifice would probably be surrounded by prolongations of the body-wall in the form of tentacles. There would be no collar-cells, and the cells of the body generally would have a much more fixed and definite position and more regular functions than those of any sponge. The most characteristic of them would be the so-called cnidoblasts. Each of these cells contains a capsule[AK] from which a long thread-like body can be suddenly uncoiled and shot out.

[Footnote AK: Similar capsules are found in the tissues of
certain worms and molluscs, but there is the strongest
evidence that these animals, which habitually devour
coelenterates, are able to swallow the capsules uninjured
and to use them as weapons of defence (see Martin, Q. J.
Micro. Sci. London, lii, p. 261, 1908, and Grosvenor, Proc.
Roy. Soc. London, lxxii, p. 462, 1903). The "trichocysts" of
certain protozoa bear a certain resemblance to the
nettle-cells of coelenterates and probably have similar
functions.]

The simplest in structure of the coelenterates are those that constitute the class Hydrozoa. In this class the primitive central cavity is not divided up by muscular partitions and there is no folding in of the anterior part of the body to form an oesophagus or stomatodæum such as is found in the sea-anemones and coral polyps. In many species and genera the life-history is complex, illustrating what is called the alternation of generations. That is to say, only alternate generations attain sexual maturity, those that do so being produced as buds from a sexless generation, which itself arises from the fertilized eggs of a previous sexual generation. The sexual forms as a rule differ considerably in structure from the sexless ones; many medusæ are the sexual individuals in a life-cycle in which those of the sexless generation are sedentary.

An excellent general account of the coelenterates will be found in the Cambridge Natural History, vol. i (by Prof. Hickson).

STRUCTURE OF HYDRA.

_Hydra_, the freshwater polyp, is one of the simplest of the Hydrozoa both as regards structure and as regards life-history. Indeed, it differs little as regards structure from the ideally simple coelenterate sketched in a former paragraph, while its descent is direct from one polyp to another, every generation laying its own eggs[AL]. The animal may be described as consisting of the following parts:--(1) an upright (or potentially upright) column or body, (2) a circle of contractile tentacles at the upper extremity of the column, (3) an oral disk or peristome surrounding the mouth and surrounded by the tentacles, and (4) a basal or aboral disk at the opposite extremity. The whole animal is soft and naked. The column, when the animal is at rest, is almost cylindrical in some forms but in others has the basal part distinctly narrower than the upper part. It is highly contractile and when contracted sometimes assumes an annulate appearance; but as a rule the external surface is smooth.

[Footnote AL: The statement is not strictly accurate as
regards the Calcutta phase of _H. vulgaris_, for the summer
brood apparently does not lay eggs but reproduces its
species by means of buds only. This state of affairs,
however, is probably an abnormality directly due to
environment.]

The tentacles vary in number, but are never very numerous. They are disposed in a single circle round the oral disk and are hollow, each containing a prolongation of the central cavity of the column. Like the column but to an even greater degree they are contractile, and in some forms they are capable of great elongation. They cannot seize any object between them, but are able to move in all directions.

The disk that surrounds the mouth, which is a circular aperture, is narrow and can to some extent assume the form of a conical proboscis, although this feature is never so marked as it is in some hydroids. The basal disk is even narrower and is not splayed out round the edges.

A=capsules from nettle-cells of a single specimen of the summer phase of _H. vulgaris_ from Calcutta, × 480: figures marked with a dash represent capsules with barbed threads. B=a capsule with the thread discharged, from the same specimen, × 480. C=capsule with barbed thread, from a specimen of _H. oligactis_ from Lahore. D=undischarged nettle-cell of _H. vulgaris_ from Europe (after Nussbaum, highly magnified). E=discharged capsule of the same (after the same author). _a_=cnidoblast; _b_=capsule; _c_=thread; _d_=cnidocil. Only the base of the thread is shown in E.]

A section through the body-wall shows it to consist of the three typical layers of the coelenterates, viz., (i) an outer cellular layer of comparatively small cells, the ectoderm; (ii) an intermediate, structureless or apparently structureless layer, the mesogloea or "central jelly"; and (iii) an internal layer or endoderm consisting of relatively large cells. The cells of the ectoderm are not homogeneous. Some of them possess at their base narrow and highly contractile prolongations that exercise the functions of muscles. Others are gland-cells and secrete mucus; others have round their margins delicate ramifying prolongations and act as nerve-cells. Sense-cells, each of which bears on its external surface a minute projecting bristle, are found in connection with the nerve-cells, and also nettle-cells of more than one type.

The mesogloea is very thin.

The endoderm consists mainly of comparatively large cells with polygonal bases which can be seen from the external surface of the column in colourless individuals. Their inner surface is amoeboid and in certain conditions bears one or more vibratile cilia or protoplasmic lashes. Nettle-cells are occasionally found in the endoderm, but apparently do not originate in this layer.

The walls of the tentacles do not differ in general structure from those of the column, but the cells of the endoderm are smaller and the nematocysts of the ectoderm more numerous, and there are other minor differences.

A more detailed account of the anatomy of _Hydra_ will be found in any biological text-book, for instance in Parker's Elementary Biology; but it is necessary here to say something more as regards the nettle-cells, which are of great biological and systematic importance.

A nettle-cell of the most perfect type and the structures necessary to it consist of the following parts:--

(1) A true cell (the cnidoblast), which contains--
(2) a delicate capsule full of liquid;
(3) a long thread coiled up in the capsule; and
(4) a cnidocil or sensory bristle, which projects from the
external surface of the cnidoblast.

A nerve-cell is associated with each cnidoblast.

In _Hydra_ the nettle-cells are of two distinct types, in one of which the thread is barbed at the base, whereas in the other it is simple. Both types have often two or more varieties and intermediate forms occur, but generally speaking the capsules with simple threads are much smaller than those with barbed ones. The arrangement of the nettle-cells is not the same in all species of _Hydra_, but as a rule they are much more numerous in the tentacles than elsewhere on the body, each large cell being surrounded by several small ones. The latter are always much more numerous than the former.

CAPTURE AND INGESTION OF PREY: DIGESTION.

The usual food of _Hydra_ consists of small insect larvæ, worms, and crustacea, but the eggs of fish are also devoured. The method in which prey is captured and ingested has been much disputed, but the following facts appear to be well established.

If a small animal comes in contact with the tentacles of the polyp, it instantly becomes paralysed. If it adheres to the tentacle, it perishes; but if, as is often the case, it does not do so, it soon recovers the power of movement. Animals which do not adhere are generally those (such as ostracod crustacea) which have a hard integument without weak spots. Nematocysts of both kinds shoot out their threads against prey with considerable violence, the discharge being effected, apparently in response to a chemical stimulus, by the sudden uncoiling of the thread and its eversion from the capsule. Apparently the two kinds of threads have different functions to perform, for whereas there is no doubt that the barbed threads penetrate the more tender parts of the body against which they are hurled, there is evidence that the simple threads do not do so but wrap themselves round the more slender parts. Nussbaum (Arch. mikr. Anat. xxix, pl. xx, fig. 108) figures the tail of a _Cyclops_ attacked by _Hydra vulgaris_ and shows several simple threads wrapped round the hairs and a single barbed thread that has penetrated the integument. Sometimes the cyst adheres to the thread and remains attached to its cnidoblast and to the polyp, but sometimes the thread breaks loose. Owing to the large mass of threads that sometimes congregate at the weaker spots in the external covering of an animal attacked (_e. g._, at the little sensory pits in the integument of the dorsal surface of certain water-mites) it is often difficult to trace out the whole length of any one thread, and as a thread still attached to its capsule is frequently buried in the body of the prey, right up to the barbs, while another thread that has broken loose from its capsule appears immediately behind the fixed one, it seems as though the barbs, which naturally point towards the capsule, had become reversed. This appearance, however, is deceptive. The barbs are probably connected with the discharge of the thread and do not function at all in the same way as those on a spear- or arrow-head, never penetrating the object against which the projectile is hurled. Indeed, their position as regards the thread resembles that of the feathers on the shaft of an arrow rather than that of the barb of the head.

Adhesion between the tentacles and the prey is effected partly by the gummy secretion of the glands of the ectoderm, which is perhaps poisonous as well as adhesive, and partly by the threads. Once the prey is fast and has ceased to struggle, it is brought to the mouth, which opens wide to receive it, by the contraction and the contortions of the tentacles, the column, and the peristome. At the same time a mass of transparent mucus from the gastral cavity envelops it and assists in dragging it in. There is some dispute as to the part played by the tentacles in conveying food into the mouth. My own observations lead me to think that, at any rate so far as _H. vulgaris_ is concerned, they do not push it in, but sometimes in their contortions they even enter the cavity accidentally.

When the food has once been engulfed some digestive fluid is apparently poured out upon it. In _H. vulgaris_ it is retained in the upper part of the cavity and the soluble parts are here dissolved out, the insoluble parts such as the chitin of insect larvæ or crustacea being ejected from the mouth. Digestion is, however, to a considerable extent intracellular, for the cells of the endoderm have the power of thrusting out from their surface lobular masses of their cell-substance in which minute nutritive particles are enveloped and dissolved. The movements of the cilia which can also be thrust out from and retracted into these cells, keep the food in the gastral cavity in motion and probably turn it round so as to expose all parts in turn to digestive action. Complete digestion, at any rate in the Calcutta form, takes several days to accomplish, and after the process is finished a flocculent mass of colourless excreta is emitted from the mouth.

COLOUR.

In _Hydra viridis_, a species that has not yet been found in India, the green colour is due to the presence in the cells of green corpuscles which closely resemble those of the cells of certain freshwater sponges. They represent a stage in the life-cycle of _Chlorella vulgaris_, Beyerinck[AM], an alga which has been cultivated independently.

[Footnote AM: Bot. Zeitung, xlviii (1890): see p. 49, _antea_.]

In other species of the genus colour is largely dependent on food, although minute corpuscles of a _dark_ green shade are sometimes found in the cells of _H. oligactis_. In the Calcutta phase of _H. vulgaris_ colour is due entirely to amorphous particles situated mainly in the cells of the endoderm. If the polyp is starved or exposed to a high temperature, these particles disappear and it becomes practically colourless. They probably form, therefore, some kind of food-reserve, and it is noteworthy that a polyp kept in the unnatural conditions that prevail in a small aquarium invariably becomes pale, and that its excreta are not white and flocculent but contain dark granules apparently identical with those found in the cells of coloured individuals (p. 154).

Berninger[AN] has just published observations on the effect of long-continued starvation on _Hydra_ carried out in Germany. He finds that the tentacles, mouth, and central jelly disappear, and that a closed "bladder" consisting of two cellular layers remains; but, to judge from his figures, the colour does not disappear in these circumstances.

[Footnote AN: Zool. Anz. xxxvi, 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 _Hydræ_ (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 prone, 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 prone, 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 glass 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 glass 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) _Sexual Reproduction._

The sexual 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 sex. 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 sexual 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 sexual 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 mass 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.

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 masses 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 passes 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 circumstances, 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 circumoral disk, which assumes 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 circumstances 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 circumstances, 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 mass 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 horny egg-shell 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-shell 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 Fayûm 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 vestita_, 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 medusæ 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 assume 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.

The first two of these species belong to the order Gymnoblastea (Anthomedusæ) and the third to the Calyptoblastea (Leptomedusæ).

(b) _Actinozoa._

(4) _Sagartia schilleriana_, Stoliczka (1869).

_S. schilleriana_, Stoliczka, Journ. As. Soc. Beng. (2)
xxxviii, p. 28, pls. x, xi (1869); _Metridium
schillerianum_, Annandale, Rec. Ind. Mus. i, p. 47, pl. iii
(1907).

This sea-anemone, which has only been found in the delta of the Ganges, offers a most remarkable instance of what appears to be rapid adaptation of a species to its environment. The typical form, which was described in 1869 by Stoliczka from specimens taken in tidal creeks and estuaries in the Gangetic area and in the ponds at Port Canning, is found attached to solid objects by its basal disk. The race (subsp. _exul_), however, that is now found in the same ponds has become elongate in form and has adopted a burrowing habit, apparently owing to the fact that the bottom of the ponds in which it lives is soft and muddy.

In addition to these four species a minute hydroid belonging to the order Gymnoblastea and now being described by Mr. J. Ritchie has been taken in the ponds at Port Canning. It is a very aberrant form.

FRESHWATER COELENTERATES OTHER THAN HYDRA.

_Hydra_ is the only genus of coelenterates as yet found in fresh water in India, but several others have been discovered in other countries. They are:--

(1) _Cordylophora lacustris_, Allman (1843).

Hincks, Hist. Brit. Hydr. Zooph. p. 16, pl. iii, fig. 2
(1868).

This is a branching hydroid that does not produce free medusæ. It forms bushy masses somewhat resembling those formed by a luxuriant growth of _Plumatella fruticosa_ (pl. iii, fig. 1) in general appearance. _C. lacustris_ is abundant in canals, rivers, and estuaries in many parts of Europe and has recently been found in the isolated salt lake Birket-el-Qurun in the Fayûm of Egypt.

(2) _Cordylophora whiteleggei_, v. Lendenfeld (1887).

Zool. Jahrb. ii, p. 97 (1887).

A species or race of much feebler growth; as yet imperfectly known and only recorded from fresh water in Australia.

_Cordylophora_ is a normal genus of the class Hydrozoa and the order Gymnoblastea; the next four genera are certainly Hydrozoa, but their affinities are very doubtful.

(3) _Microhydra ryderi_, Potts (1885).

Potts, Q. J. Micr. Sci. London, l, p. 623, pls. xxxv, xxxvi;
Browne, _ibid._ p. 635, pl. xxxvii (1906).

This animal, which has been found in N. America and in Germany, possesses both an asexual hydroid and a sexual medusoid generation. The former reproduces its species by direct budding as well as by giving rise, also by a form of budding, to medusæ that become sexually mature. The hydroid has no tentacles.

(4) _Limnocodium sowerbii_, Lankester (1880).

Lankester, Q. J. Micr. Sci. London, xx, p. 351, pls. xxx,
xxxi (1880); Fowler, _ibid._ xxx, p. 507, pl. xxxii (1890).

There is some doubt as to the different stages in the life-cycle of this species. The medusa has been found in tanks in hot-houses in England, France and Germany, and a minute hydroid closely resembling that of _Microhydra ryderi_ has been associated with it provisionally.

(5) _Limnocodium kawaii_, Oka (1907).

Oka, Annot. Zool. Japon. vi, p. 219, pl. viii (1907).

Only the medusa, which was taken in the R. Yang-tze-kiang, is as yet known.

(6) _Limnocnida tanganyikæ_, Bohm (1889).

R. T. Günther, Ann. Nat. Hist. (6) xi, p. 269, pls. xiii,
xiv (1893).

Only the medusa, which is found in Lake Tanganyika, Lake Victoria Nyanza and the R. Niger, has been found and it is doubtful whether a hydroid generation exists.

(7) _Polypodium hydriforme_, Ussow (1885).

Morph. Jahrb. xii, p. 137 (1887).

Two stages in this peculiar hydroid, which is found in the R. Volga, are known, (_a_) a spiral ribbon-like form parasitic on the eggs of the sterlet (_Acipenser ruthenus_), and (_b_) a small _Hydra_-like form with both filamentous and club-shaped tentacles. The life-history has not yet been worked out[AP].

[Footnote AP: Since this was written, Lippen has described a
third stage in the life-history of _Polypodium_ (Zool. Anz.
Leipzig, xxxvii, Nr. 5, p. 97 (1911)).]

II.

HISTORY OF THE STUDY OF HYDRA.

Hydra was discovered by Leeuwenhoek at the beginning of the eighteenth century and had attracted the attention of several skilful and accurate observers before that century was half accomplished. Among them the chief was Trembley, whose "Mémoires pour servir à l'histoire d'un genre de Polype d'eau douce"* was published at Paris 1744, and is remarkable not only for the extent and accuracy of the observations it enshrines but also for the beauty of its plates. Baker in his work entitled "An attempt towards a natural history of the Polyp"* (London, 1743) and Rösel von Rosenhof in the third part of his "Insecten-Belustigung" (Nurenberg, 1755) also made important contributions to the study of the physiology and structure of _Hydra_ about the same period. Linné invented the name _Hydra_, and in his "Fauna Sueica" and in the various editions of his "Systema Naturæ" described several forms in a manner that permits some of them to be recognized; but Linné did not distinguish between the true _Hydra_ and other soft sessile Coelenterates, and it is to Pallas ("Elenchus Zoophytorum," 1766) that the credit properly belongs of reducing the genus to order. It is a tribute to his insight that three of the four species he described are still accepted as "good" by practically all students of the Coelenterates, while the fourth was a form that he had not himself seen.

In the nineteenth century the freshwater polyp became a favourite object of biological observation and was watched and examined by a host of observers, among the more noteworthy of whom were Kleinenberg, Nussbaum, and Brauer, who has since the beginning of the present century made an important contribution to the taxonomy of the genus.

BIBLIOGRAPHY OF HYDRA.

_Hydra_ has been examined by thousands of students in biological laboratories all over the civilized world, and the literature upon it is hardly surpassed in magnitude by that on any other genus but _Homo_. The following is a list of a few of the more important general memoirs and of the papers that refer directly to Asiatic material. A systematic bibliography is given by Bedot in his "Matériaux pour servir a l'Histoire des Hydroïdes," Rev. Suisse Zool. xviii, fasc. 2 (1910).

(a) _General._

1743. BAKER, "An attempt towards a natural history of the Polyp"* (London).

1744. TREMBLEY, "Mémoires pour servir à l'histoire d'un genre de polypes d'eau douce"* (Paris).

1755. RÖSEL VON ROSENHOF, "Insecten-Belustigung: iii, Hist. Polyporum."

1766. PALLAS, "Elenchus Zoophytorum."

1844. LAURENT, "Rech. sur l'Hydre et l'Eponge d'eau douce" ("Voy. de la Bonite, Zoophytologie").

1847. JOHNSTON, "A History of the British Zoophytes" (2nd edition).

1868. HINCKS, "History of British Hydroid Zoophytes."

1872. KLEINENBERG, "Hydra. Eine Anatomisch Entwicklungsgeschichtliche Untersuchung."

1882. JICKELI, "Der Bau der Hydroidpolypen," Morph. Jahrb. viii, p. 373.

1887. NUSSBAUM, "Ueber die Theilbarkeit der lebendigen Materie. II. Mittheilung. Beiträge zur Naturgeschichte des Genus Hydra," Arch. mikr. Anat. Bonn, xxix, p. 265.

1891. BRAUER, "Über die Entwicklung von Hydra," Zeitschr. wiss. Zool. Leipzig, lii, p. 169.

1892. CHUN, "Coelenterata (Hohlthiere)," in Bronn's Thier-Reichs II (2).

1905. DOWNING, "The spermatogenesis of Hydra," Zool. Jahrb. (Anat.) xxi, p. 379.

1908. BRAUER, "Die Benennung und Unterscheidung der Hydra-Arten," Zool. Ann. xxxiii, p. 790.

1909. FRISCHHOLZ, "Biologie und Systematik im Genus Hydra," Braun's Annal. Zool. (Würzburg) iii, p. 105.

1910. BERNINGER, "Über Einwirkung des Hungers auf Hydra," Zool. Anz. xxxvi, p. 271.

(b) _Asiatic References._

1894. RICHARD, "Sur quelques Animaux inférieurs des eaux douces du Tonkin (Protozoaires, Rotifères, Entomostracés)," Mém. Soc. zool. France, vii, p. 237.

1904. VON DADAY, "Mikroskopische Süsswasserthiere aus Turkestan," Zool. Jahrb. (Syst.) xix, p. 469.

1906. ANNANDALE, "Notes on the Freshwater Fauna of India. No. IV. _Hydra orientalis_ and its bionomical relations with other Invertebrates," J. Asiat. Soc. Bengal (new series), ii, p. 109.

1906. ANNANDALE, "The Common _Hydra_ of Bengal: its Systematic Position and Life History," Mem. As. Soc. Bengal, i, p. 339.

1907. ANNANDALE, "Notes on the Freshwater Fauna of India. No. X. _Hydra orientalis_ during the Rains," J. Asiat. Soc. Bengal (new series), iii, p. 27.

1907. ANNANDALE, "Notes on the Freshwater Fauna of India. No. XI. Preliminary Note on the occurrence of a Medusa (_Irene ceylonensis_, Browne) in a brackish pool in the Ganges Delta and on the Hydroid Stage of the species," J. Asiat. Soc. Bengal (new series), iii, p. 79.

1907. WILLEY, "Freshwater Sponge and Hydra in Ceylon," Spolia Zeylan. Colombo, iv, p. 184.

1908. ANNANDALE, "Observations on specimens of _Hydra_ from Tibet, with notes on the distribution of the genus in Asia," Rec. Ind. Mus. ii, p. 311.

1910. POWELL, "Lessons in Practical Biology for Indian Students" (Bombay).

1910. LLOYD, "An Introduction to Biology for Students in India" (London).

GLOSSARY OF TECHNICAL TERMS USED IN PART II.

_Aboral_ (or _basal_) The disk by means of which a free polyp
_disk_ attaches itself to external objects.

_Cnidoblast_ The living cell of the nematocyst or
nettle-cell (_q. v._).

_Cnidocil_ A minute bristle that projects on the
surface in connection with a nettle-cell
(_q. v._).

_Column_ The upright or potentially upright
part of a polyp (_q. v._).

_Ectoderm_ The external cell-layer of the body-wall.

_Endoderm_ The internal cell-layer of the body-wall.

_Green (chlorophyll) Minute green bodies contained in cells
corpuscles_ of polyps or other animals and
representing a stage in the life-history
of an alga (_Chlorella_).

_Mesogloea_ The intermediate, gelatinous layer of
the body-wall.

_Nettle-cell (nematocyst)_ A cell capsule full of liquid in which
an eversible thread is coiled up.

_Oral disk_ The eminence that surrounds the mouth
and is surrounded by tentacles.

_Peristome_ See "oral disk."

_Polyp_ An individual coelenterate of simple
structure that is fixed temporarily or
permanently by one end of a more
or less cylindrical body and possesses
a mouth at the other end.

_Tentacles_ Filamentous outgrowths (in _Hydra_
hollow) of the body-wall round the
mouth.

LIST OF THE INDIAN HYDRIDA.

Class HYDROZOA.

Order ELEUTHEROBLASTEA.

Family HYDRIDÆ.

Genus HYDRA, _Linné_ (1746).

24. _H. vulgaris_, Pallas (1766).

25. _H. oligactis_, Pallas (1766).

Order ELEUTHEROBLASTEA.

Naked hydrozoa which reproduce their kind by means of buds or eggs, or by fission, without exhibiting the phenomena of alternation of generations.

Family HYDRIDÆ.

HYDRAIDÆ, Johnston, Hist. Brit. Zooph. (ed. 2) i, p. 120
(1847).
HYDRIDÆ, Hincks, Hist. Brit. Hydroid. Zooph. p. 309 (1868).

Small Eleutheroblastea in which the mouth is surrounded by hollow tentacles. Permanent colonies are not formed, but reproduction by budding commonly takes place.

Genus HYDRA, _Linné_.

TYPE, _Hydra viridis_, Linné.

Freshwater polyps which produce eggs with hard chitinous shells. Although habitually anchored by the end of the body furthest from the mouth to extraneous objects, they possess considerable powers of locomotion. They are extremely contractile and change greatly from time to time in both form and size.

Only three well-established species of the genus, which is universally distributed and occurs only in fresh or brackish[AQ] water, can be recognized, namely, _H. viridis_, Linné (=_H. viridissima_, Pallas), _H. vulgaris_, Pallas (=_H. grisea_, Linné), and _H. oligactis_, Pallas (=_H. fusca_, Linné). The two latter occur in India, but _H. viridis_ does not appear to have been found as yet anywhere in the Oriental Region, although it is common all over Europe and N. America and also in Japan. The distribution of _H. vulgaris_ is probably cosmopolitan, but there is some evidence that _H. oligactis_ avoids tropical districts, although, under the name _Hydra fusca_, it has been doubtfully recorded as occurring in Tonquin[AR].

[Footnote AQ: A small form of _H. viridis_ (var. _bakeri_,
Marshall) is found in brackish water in England.]

[Footnote AR: Richard, Mém. Soc. zool. France, vii, p. 237
(1894).]

The three species may be distinguished from one another by the following key:--

[I. Colour leaf-green; the cells contain green
(chlorophyll) corpuscles of definite form.
A. Tentacles comparatively stout, habitually
shorter than the column, which is cylindrical.
Egg-shell without spines, ornamented
with a reticulate pattern _viridis_.]
II. Colour never leaf-green; no chlorophyll
corpuscles present in the cells.
A. Tentacles capable of great elongation but
when the animal is at rest never very much
longer than the column, which is cylindrical
when the gastral cavity is empty.
Largest nettle-cells almost as broad as
long. Egg-shell bearing long spines most
of which are divided at the tips _vulgaris_, p. 148.
B. Tentacles, even when the animal is at rest,
much longer than the column, the basal
part of which, even when the gastral
cavity is empty, is constricted. Largest
nettle-cells considerably longer than
broad. Egg-shell smooth or bearing
short, simple spines _oligactis_, p. 158.

24. Hydra vulgaris, _Pallas_.

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. 2_b_, 2_c_, 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).

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.

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-glass. 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 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.

_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 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 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 sexes 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. Tenasserim (_Annandale_): CEYLON, Colombo and Peradeniya (_Willey_, _Green_). Dr. A. D. Imms tells me that he has obtained specimens that probably belong to this species in the Jumna at Allahabad.

BIOLOGY.--In India _H. vulgaris_ is usually found, so far as my experience goes, in stagnant water. In Calcutta it is most abundant in ponds containing plenty of aquatic vegetation, and seems to be especially partial to the plant _Limnanthemum_, which has floating leaves attached to thin stalks that spring up from the bottom, and to _Lemna_ (duckweed). Dr. Henderson, however, found specimens in a pool of rain-water on the sea-shore near Madras.

There is evidence that each of the two broods which occur in Lower Bengal represents at least one generation; probably it represents more than one, for tentacles are rarely if ever produced after the animal has obtained its full size, and never (or only owing to accident) decrease in number after they have once appeared. The winter form is found chiefly near the surface of the water, especially on the roots of duckweed and on the lower surface of the leaves of _Limnanthemum_; but the summer form affects deeper water in shady places, and as a rule attaches itself to wholly submerged plants. The latter form is to be met with between March and October, the cold-weather form between October and March, both being sometimes found together at the periods of transition. In the unnatural environment of an aquarium, however, individuals of the winter form lose their colour and become attenuated, in these features resembling the summer form, even in the cooler months. Buds produced in these conditions rarely have more than five tentacles or themselves produce buds freely after liberation.

The buds appear in a fixed order and position, at any rate on individuals examined in winter; in specimens of the summer form the position is fixed, but the order is irregular. Each quadrant of the column has apparently the power of producing, in a definite zone nearer the aboral pole than the mouth, a single bud; but the buds of the different quadrants are not produced simultaneously. If we imagine that the quadrants face north, south, east, and west, and that the first bud is produced in the north quadrant, the second will be produced in the east quadrant, the third in the south, and the fourth in the west. It is doubtful whether more than four buds are produced in the lifetime of an individual, and apparently attached buds never bud in this race. The second bud usually appears before the first is liberated, and this is also the case occasionally as regards the third, but it is exceptional for four buds to be present at one time. About three weeks usually elapse between the date at which the bud first appears as a minute conical projection on the surface of the parent and that at which it liberates itself. This it does by bending down, fixing itself to some solid object by means of the tips of its tentacles, the gland-cells of which secrete a gummy fluid, and then tearing itself free.

Although it is rare for more than two buds to be produced simultaneously, budding is apparently a more usual form of reproduction than sexual reproduction. Individuals that bear eggs have not yet been found in India in natural conditions, although males with functional spermaries are not uncommon at the approach of the hot weather. The few eggs that I have seen were produced in my aquarium towards the end of the cold weather. Starvation, lack of oxygen, and too high a temperature (perhaps also lack of light) appear to stimulate the growth of the male organs in ordinary cases, but perhaps they induce the development of ovaries in the case of individuals that are unusually well nourished.

The spines that cover the egg retain débris of various kinds upon its surface, so that it becomes more or less completely concealed by a covering of fragments of dead leaves and the like even before it is separated from the polyp. Its separation is brought about by its falling off the column of the parent. Nothing is known of its subsequent fate, but probably it lies dormant in the mud through the hot weather. Eggs are sometimes produced that have no shells. This is probably due to the fact that they have not been fertilized.