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 cœlenterates, viz., (i) an outer cellular layer of comparatively small cells, the ectoderm; (ii) an intermediate, structureless or apparently structureless layer, the mesoglœa 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 mesoglœa 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 amœboid 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.
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.
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 manœuvre 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.
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.
Fig. 28.—Eggs of Hydra (magnified).
A=egg of H. vulgaris (after Chun). B=vertical section through egg of H. oligactis, form A (after Brauer). C=vertical section through egg of H. oligactis, form B (after Brauer).
The ovaries consist of rounded 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 amœboid 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. rœselii, 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.
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).
Cœlenterates of Brackish Water.
Marine cœlenterates 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 Mœrisia 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).
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).
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).
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).
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 Cœlenterates other than Hydra.
Hydra is the only genus of cœlenterates as yet found in fresh water in India, but several others have been discovered in other countries. They are:—
(1) Cordylophora lacustris, Allman (1843).
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).
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).
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).
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).
Only the medusa, which was taken in the R. Yang-tze-kiang, is as yet known.
(6) Limnocnida tanganyikæ, Bohm (1889).
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).
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].
[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 cœlenterates, 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 cœlenterates and probably have similar functions.
[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.
[AM] Bot. Zeitung, xlviii (1890): see p. 49, antea.
[AN] Zool. Anz. xxxvi, pp. 271-279, figs., Oct. 1910.
[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.
[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 Cœlenterates, 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 Cœlenterates, 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.
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).
| Aboral (or basal disk) | The disk by means of which a free polyp 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) corpuscles | Minute green bodies contained in cells of polyps or other animals and representing a stage in the life-history of an alga (Chlorella). |
| Mesoglœa | 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 cœlenterate 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. |
Class HYDROZOA.
Order ELEUTHEROBLASTEA.
Family HYDRIDÆ.
Genus Hydra, Linné (1746).
24. H. vulgaris, Pallas (1766).
25. H. oligactis, Pallas (1766).
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.
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].
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.