VI.

THE FRESH-WATER POLYPES (Hydra viridis and H. fusca).

IF a waterweed, such as duckweed, from a pond, is placed in a glass and allowed to remain undisturbed for a short time, minute gelatinous-looking bodies of a brownish or green colour may frequently be found attached to it, or to the sides of the glass. They have a length of from to of an inch, rarely more, and are cylindrical or slightly conical in form. From the free end delicate filaments, which are often much longer than the body, proceed and spread out with a more or less downward curve, in the water. These threads, which are the tentacles, may vary in number'; if touched they rapidly shorten and together with the body shrink into a rounded mass. After a while, the contracted body and the tentacles elongate and resume their previous form. These are Polypes, the brown ones belonging to the species usually termed Hydra fusca, the green to that called H. viridis. The polypes generally remain attached to one spot for a long time, but they are capable of crawling about by a motion similar to that of the looping caterpillar; and, sometimes, they detach themselves and float passively in the water.

1 In H. Hexactinella, an Australian species, their number is reported to be invariably six,

When any small animal, such as a water-flea, swimming through the water comes in contact with the tentacles, it is grasped, and conveyed by their contraction to the aperture of the wide mouth, which is situated on the summit of a cone (hypostome) in the middle of the circle formed by the bases of the tentacles. It is then taken into a cavity which occupies the whole interior of the body; the nutritive matters which it contains are dissolved out and absorbed by the substance of the Hydra; and the innutritious residuum is eventually cast out by the way it entered. Small pieces of meat, brought within reach of the tentacles, are seized, swallowed and digested in the same manner.

If a Hydra is well fed, bud-like projections make their appearance upon the outer surface of the body. These gradually elongate and become pear-shaped. At the free end a mouth appears; and around it minute processes are developed and grow into tentacles; and thus a young Hydra is formed by gemmation from the parent. This young Hydra becomes detached sooner or later, and leads an independent existence; but, not unfrequently, new buds are developed from other parts of the parent before the first is detached, and the progeny may themselves begin to bud before they attain independence. In this manner, temporarily compound organisms may be formed. Experiments have shewn that these animals may be cut into halves or quarters and that each portion will repair its losses, and grow up into a perfect Hydra; and there is reason to believe that this process of fission sometimes occurs naturally.

The Hydra multiplies by budding through the greater part of the year; but in the summer sexual organs appear in the form of projections of the surface of the body. These, when ripe, may be resolved into a larger and a smaller set. The latter may appear on any part of the

body, but they are not unfrequently restricted to the free end, at or near the bases of the tentacles. Within them (testes) great numbers of minute spermatozoa, each moved by a vibratile cilium, are developed and eventually set free.

The former are large globular bodies, from one to eight in number, usually formed near the attached end of the polype. Each becomes much larger than the testis, and is the ovary. Within it is developed a single large egg-cell, or ovum. This ovum, which is a huge nucleated cell, is impregnated by the spermatozoa and undergoes division into two parts. Each of these again divides into two; and so on, until the ovum is broken up into a number of small embryo-cells. The mass of embryo-cells thus formed becomes surrounded with a thick, usually tuberculated or spinous, case; and, detaching itself from the body, forms the 'egg,' from which a new Hydra is developed.

Microscopic examination shews that the body of the Hydra is a sac, the wall of which is composed of two membranes, a transparent outer (ectoderm), and a coloured inner (endoderm). The tentacles are tubular processes of the sac, and therefore are formed externally by the ectoderm and lined internally by the endoderm. Both the endoderm and the ectoderm are made up of nucleated cells; the inner ends of certain of these are prolonged into delicate fibres, those of the ectoderm, which are most marked, running parallel with the long axis of the body. The green colour of the Hydra viridis results from the presence of chlorophyll grains imbedded in the protoplasm of the endoderm cells.

The cells of the ectoderm, and especially that of the tentacles, contain very singular bodies, the so-called urticating capsules, thread cells, or nematocysts. These are oval bags, with thick and elastic walls, containing a spirally

coiled or looped filament which can be unrolled, presenting the appearance of a long filament attached to the capsule, and often provided with recurved spines near its base. When the nematocyst is ejected these spines or barbs lie in close apposition, with their pointed ends directed forwards and their bases under tension; on reaching the attacked prey they undergo a forcible displacement, fixing the body of the nematocyst and clearing the way for the introduction of the more delicate thread. These thread-cells appear to exert a noxious influence upon the animal's prey. Very rarely, nematocysts are to be found in individual cells of the endoderm; there is reason to believe that they are introduced with the captured prey, but argument from analogy to allied hydroids renders it probable that they may be developed in situ.

The chlorophyll granules contained in the endoderm of the green Hydra are doubtless functional in the manner of those of the plant-cell, but none but faint traces of an 'assimilation product' have yet been observed. The brown or orange-coloured particles predominant in the endoderm of the other species, and rarely present in that of H. viridis, are probably identical with the chlorophyll bodies (see Laboratory work)'.

The larger endoderm cells of Hydra are throughout life amoeboid, and the like is partly true of the ectoderm in at least the young state of one variety (H. viridis var. Bakeri). The Hydra, then, may be compared to an aggregate of Amaba, which are arranged in the form of a double walled sac and have undergone an amount of metamorphosis.

The cavity of the body alone represents a stomach and

1 It has been assumed, upon this, that the green and brown species are mere varieties of one and the same. On the other hand, structural differences in the nematocysts and their parent-cells have been claimed, as sufficient to justify a subdivision into three species.

intestine; there are no organs of circulation, respiration or urinary secretion; the products of digestion are doubtless transmitted, by imbibition, from cell to cell, and those of the waste of the cells exuded directly into the surrounding water. While the Hydra has none of the special apparatuses which are termed glands, definite secretory cells are nevertheless present. Among the more important of these are those developed in the ectoderm of the foot which are utilized for purposes of adhesion, and the secretory cells of the endoderm, most numerous in the hypostome, which is eversible.

Nematocysts are generally, but not invariably, ejected, if any portion of the body which bears them be touched. Certain of the ectoderm cells, usually, if not always, lodging nematocysts, bear each a stiff filament or cnidocil such as can readily be seen in life projecting beyond the free surface of the tentacles. Continuity has been traced between these cells or cnidoblasts and certain small nerve-cells sparsely diffused in the deeper layer of the ectoderm; the whole constituting an elementary neuro-sensiferous apparatus, through the agency of which control of at least the cnidoblasts and their contained urticating capsules is exercised.

The fully formed Hydra may further be compared to those animals previously dealt with, at that stage in their development when the body consists of a double-walled sac (cf. especially the gastrula stage of the Snail and its representative in the Crayfish). The inner layer gives origin, in the latter, to the digestive epithelium and its appended glands; in Hydra it forms the digestive layer. The ectoderm is, in all, protective, and from it such neuro-sensiferous organs as are formed, exclusively arise. Between the applied surfaces of the ectoderm and endoderm there is interposed a gelatinous middle layer, which, under the