CHAPTER XI.

ZOOPHYTES.

299. THE term Zoophyte, although sometimes used in a wider signification, is properly restricted to the class of Polypifera, or polype-bearing animals, whose composite skeletons or "polyparies" have more or less of a plant-like form; even the Polyzoa (or Bryozoa) being now excluded, on account of their truly Molluscan structure (Chap. XIII), notwithstanding the zoophytic character of their forms and of their habits of life. The true Zoophytes may be divided into two primary groups, the Hydrozoa and the Anthozoa; the Hydra (or fresh-water polype) standing as the type of the one, and the Sea-Anemone as the representative of the other. As the Hydrozoa are essentially microscopic animals, they need to be described with some minuteness; whilst in regard to the Anthozoa, only those points can be dwelt on, which are of special interest to the Microscopist.

300. The Hydra is to be searched for in pools and ditches, where it is most commonly to be found attached to the leaves or stems of aquatic plants, floating pieces of stick, &c. Two species are common in this country, the H. viridis or green polype, and the H. vulgaris, which is usually orange-brown, but sometimes yellowish or red (its color being liable to some variation according to the nature of the food on which it has been subsisting); a third less common species, the H. fusca, is distinguished from both the preceding by the length of its tentacula, which in the former are scarcely as long as the body, whilst in the latter they are, when fully extended, many times longer (Fig. 220). The body of the Hydra consists of a simple bag or sac, which may be regarded as a stomach, and which is capable of varying its shape and dimensions in a very remarkable degree; sometimes extending itself in a straight line, so as to form a long narrow cylinder, at other times being seen (when empty) as a minute contracted globe, whilst, if distended with food, it may present the form of an inverted flask or bottle, or even of a button. At the upper end of this sac is a central opening, the "mouth,” and this is surrounded by a circle of tentacula or "arms," usually from six to ten in number, which are arranged with great regularity around the orifice. The body is prolonged at its lower

end into a narrow base, which is furnished with a suctorial disk; and the Hydra usually attaches itself by this, whilst it allows its

FIG. 220.

tendril-like tentacula to float freely in the water, like so many fishing-lines. The wall of the body is composed of cells, imbedded in a kind of sarcode; and it consists of two principal layers, an outer and more compact, of which the cells form a tolerably even surface, and an inner that lines the stomach, into the cavity of which some of the cells project. Between these layers, there is a space chiefly occupied by "sarcode," having many vacuoles or lacunæ (which often seem to communicate with one another) excavated in its substance. The arms are made up of the same materials as the body; but their surface is beset with little wart-like prominences, which, when carefully examined, are found to be composed of clusters of small "thread-cells," having a single large cell with a long spiculum in the centre of each. The structure of these thread-cells or "urticating or gans" will be described hereafter (§ 310); at present it will be enough to point out, that this apparatus, repeated many times on each tentacle, is doubtless intended to give to the organ a great prehensile power; the minute filaments forming a rough surface, adapted to prevent the object from readily slipping out of the grasp of the arm, whilst the central spiculum is projected into its substance, and probably conveys into it a poisonous fluid secreted by a vesicle at the base of the dart. The latter inference is founded upon the oft-repeated observation, that if the living prey seized by the tentacles have a body destitute of hard integument, as is the case with the minute aquatic Worms which constitute a large part of its aliment, this speedily dies, even if, instead of being swallowed, it escapes from their grasp; on the other hand, minute Entomostracous Crustacea, Insects, and other animals with hard envelopes, may escape without injury, even after having been detained for some time in the polype's embrace. The contractility of the tentacula (the interior of which is traversed by a canal

Hydra fusca, with a young bud at b, and a more advanced bud at c.

which communicates with the cavity of the stomach) is very remarkable, especially in the Hydra fusca; whose arms, when extended in search of prey, are not less than seven or eight inches in length; whilst they are sometimes so contracted, when the stomach is filled with food, as to appear only like little tubercles around its entrance. By means of these instruments, the Hydra is enabled to derive its subsistence from animals, whose activity, as compared with its own slight powers of locomotion, might have been supposed to remove them altogether from its reach; for when, in its movements through the water, a minute worm or a water-flea happens to touch one of the tentacula of the polype, spread out as these are in readiness for prey, it is immediately seized by this, other arms are soon coiled around it, and the unfortunate victim is speedily conveyed to the stomach, within which it may frequently be seen to continue moving for some little time. Soon, however, its struggles cease, and its outline is obscured by a turbid film, which gradually thickens, so that at last its form is wholly lost. The soft parts are soon completely dissolved, and the harder indigestible portions are rejected through the mouth. A second orifice has been observed at the lower extremity of the stomach; but this would not seem to be properly regarded as anal, since it is not used for the discharge of such exuviæ; it is probably rather to be considered as representing, in the Hydra, the entrance to that ramifying cavity, which, in the compound Hydroida, brings into connection the lower extremities of the stomach of all the individual polypes (Fig. 223). A striking proof of the simplicity of the structure of the Hydra, is the fact that it may be turned inside out like a glove; that which was before its external tegument becoming the lining of its stomach, and vice versa.

301. The ordinary mode of multiplication in this animal, is by a gemmation resembling that of Plants. Little bud-like processes (Fig. 220, b, c) are developed from its external surface, which are soon observed to resemble the parent in character, possessing a digestive sac, mouth, and tentacula; for a long time, however, their cavity is connected with that of the parent, but at last the communication is cut off by the closure of the canal of the footstalk, and the young Polype quits its attachment and goes in quest of its own maintenance. A second generation of buds is sometimes observed on the young Polype, before quitting its parent; and as many as nineteen young Hydræ, in different stages of development, have been seen thus connected with one original stock (Fig. 221). Another very curious endowment seems to depend on the same condition, the extraordinary power which one portion possesses of reproducing the rest. Into whatever number of parts a Hydra may be divided, each may retain its vitality, and give origin to a new and entire fabric; so that thirty or forty individuals may be formed by the section of one. The Hydra also propagates itself, however, by a truly

Sexual process; the fecundating apparatus, or vesicle-producing "sperm-cells," and the ovum (containing the "germ-cell," imbedded in a store of nutriment adapted for its early development), being evolved in the substance of the walls of the stomach, the former just beneath the arms, the latter nearer to the lower end of the body. It would appear that sometimes one individual Hydra developes only the male cysts or sperm-cells, while another developes only the female cysts or ovisacs; but the general rule seems to be, that the same individual forms both organs. The fertilization of the ova, however, cannot take place until after the rupture of the spermatic cyst and that of the ovisac also; so that the parent has no further participation in it, than has the Fucus in the analogous fertilization of its germ-cells after their discharge (§ 205). Although the production, from such an egg, of a new Hydra, similar in all respects to its parent, has not yet been witnessed, there seems no reason to doubt the fact. It would seem that this alternation in the method of reproduction, between the gemmiparous and the sexual, is greatly influenced

Fig. 221. Hydra fusca in gemmation; a, mouth; b, base; c, origin of one of the buds. Fig. 222. Development of Medusa-buds in Syncoryna Sarsii :-A, an ordinary polype, with its clubshaped body covered with tentacula:-B, a polype putting forth medusan gemmæ; a, a very young bud; b, a bud more advanced, the quadrangular form of which, with the four nuclei whence the cirrhi afterwards spring, is shown at d; c, a bud still more advanced.

by external temperature; the eggs being produced at the ap proach of winter, and serving to regenerate the species in the

spring, the parents not being able to survive the cold season; whilst the budding process naturally takes place only during the warmer part of the year, but may be made to continue through the whole winter, by keeping the water inhabited by the polypes at a sufficiently high temperature. The Hydra possesses the power of free locomotion, being able to remove from the spot to which it has attached itself, to any other that may be more suitable to its wants; its changes of place, however, seem rather to be performed under the influence of light, towards which the Hydra seeks to move itself, than with reference to the search after food.

302. Some of the simpler forms of the composite Hydrozoa may be likened to a Hydra, whose gemmæ, instead of becoming detached, remain permanently connected with the parent; and as these in their turn may develope gemmæ from their own bodies, a structure of more or less arborescent character may be produced. The form which this will present, and the relation of the component polypes to each other, will depend upon the mode in which the gemmation takes place; in all instances, however, the entire cluster is produced by continuous growth from a single individual; and the stomachs of the several polypes are united by tubes, which proceed from the base of each other, along the stalk and branches, to communicate with the cavity of the central stem. This is the case with the family Corynidae, which are composite fabrics, sometimes quite arborescent in form, but unpossessed of any firm investment, the external wall being only strengthened by a thin horny cuticle. A very beautiful marine species of this family (the Coryne pusilla), is common on seaweeds and stones between the tide-marks; sometimes clustering parasitically round the stalks of Tubularia so as to form a thick beard-like mossiness; each aggregate structure, however, not being more than an inch in length. The tentacula (as in Fig. 222, A) are short, and arise from the whole surface of the body of the polype, instead of from the margin of the mouth alone; and at first it seems difficult to understand how they can be of service in bringing food to the mouth, which is situated at the very extremity of the branch. Observation of the living animal, however, soon removes this difficulty; for the head is so very flexible, that the mouth can bend itself down towards any of the tentacula which may have entrapped prey; all its movements are performed, however, in a very leisurely manner. The freshwater genus Cordylophora has yet been only found in a few localities; and the chief interest attaching to it is derived from the fact of its having been made the subject of an admirable Memoir by Prof. Allman, to which every one should refer, who desires to acquaint himself with the minute organization of this group of Zoophytes. The phenomena of the Reproductive process exhibited by these Hydrozoa, are extremely curious. In Coryne

"Philos. Transact." 1853.