CHAPTER VI.

_CORALS, SEA-ANEMONES, AND JELLY-FISHES._

With the Sea-anemones and Jelly-fishes almost the lowest organised group of living animals is reached. As typified by an ordinary sea-anemone, the body may be described as a simple sac, the orifice of which is inverted for some little distance, and held in position with relation to the outer wall by a series of radiating partitions. One or more rows of tentacles, varying in number and character according to the species, surround the mouth of this partially inverted sac. There is no distinct intestinal track, the whole space enclosed within the outer wall and ramifying among the radiating partitions containing the digestive juices. The radiating membranous partitions develop upon their surfaces the reproductive elements, and in the case of Corals, which are merely skeleton-producing sea-anemones, partly secrete within them the symmetrical radiating calcareous plates so characteristic of the group.

Some thirty odd species of sea-anemones are indigenous to British waters, and one or more of these will be familiar to most readers. The STRAWBERRY-ANEMONE, clinging to the rocks as a hemispherical lump of crimson, green, brown, or red and yellow speckled jelly when the tide is down, and expanding like a beautiful flower when the waters flow back upon it, is the commonest and in many respects the most beautiful of all, the circlet of turquoise beads, regarded as rudimentary eyes, developed around the outer margin of the tentacles, adding a charm possessed by few other species. The DAHLIA-ANEMONE, whose expanded disk and innumerable petal-like tentacles may measure as much as 6 or 8 inches in diameter, is the largest British species. These dimensions are, however, vastly exceeded by its tropical allies. The Australian coast produces giant species which may measure no less than from 18 inches to 2 feet across their expanded disks. These giant anemones are further interesting on account of the circumstance that they are self-constituted "harbours of refuge" to sundry species of fishes and crabs, which nestle among their tentacles like birds in a leafy bower. The anemones are themselves bright in colour, but the associated fishes are even more so. In an example which was photographed by the writer on the Western Australian coast, the anemone was olive-green, with the tips of the tentacles bright mauve. The fishes, of which three examples were present, were brilliant orange-scarlet with white bands. In addition to the fishes a small flat-clawed crab shared the sheltering hospitality of the anemone. Some of the tropical coral-reef-frequenting anemones, which have their tentacles beautifully branched, must be cautiously handled, in consequence of their notable stinging properties. All sea-anemones and corals are, in fact, provided with peculiar stinging-cells, with which they benumb and thus make an easy capture of the living organisms on which they prey. While the majority of the sea-anemones live single or individually separate lives, there are some which form aggregations or colony-stocks of numerous units. These compound growths are brought about by repeated budding, or the sub-division or fission, without complete separation, of an originally single individual. It is by a similar process of recurrent sub-division that the wonderful fabrications of the coral-polyps are built up.

An ordinary coral-animal or polyp, as previously stated, differs in no respect from a sea-anemone, excepting for the possession of a calcareous skeleton secreted within its basal tissues, including portions of the membranous radiating partitions. Some coral-animals, like the majority of the Anemones, are solitary, and form single attached or loosely lying corals. The well-known MUSHROOM-CORAL is one of the latter. One species observed, which was photographed through the water by the writer as it lay expanded in a tide-pool on the Australian Great Barrier Reef, might easily be mistaken for a big sea-anemone allied to the dahlia-anemone. On being disturbed, however, it immediately shrinks back upon its base, ejecting all the water with which its expanded tissues were filled, and revealing the presence of the hard radiating coral beneath. Each of the calcareous radii, which are now clearly defined through the thin semi-transparent skin, corresponds in position with one of the internal membranous partitions, and also with the origin of one of the tentacles. New mushroom-corals are produced as buds thrown off from the parent, which attach themselves and secrete a foot-stalk, to which they remain affixed, like the young of the feather star-fish, for the earlier epoch of their existence. Ultimately, however, they become detached, and, falling from their stalks, lie loosely on the sea-bottom, after the manner of their parents. The huge coral-masses commonly known as MADREPORES, out of which coral-islands and reefs are constructed, all commence as a single coral-animal, with its contained skeleton analogous to the mushroom-coral, though in all instances much smaller. The buds developed by the coral-polyp in these instances remain attached to the parent. If they spread out laterally, they build up by accumulation the large flattened or sub-spherical masses known as BRAIN-CORALS and STAR-CORALS, which are most abundant on coast-line reefs, or form the bases of the outer barrier-reefs. Where, on the other hand, the budding is terminal or oblique, branching tree-like growths such as the STAG'S-HORN CORALS, with their innumerable allies and variations, are produced. The colours of the coral-polyps are as brilliant and diverse as those of ordinary sea-anemones, living reefs, whereon a number of different species are in a condition of healthy growth, yielding a spectacular effect that vies with that of any floral parterre. Sometimes large areas, acres upon acres in extent, may be covered with one almost uniform purple, green, brown, or other coloured growth of the branching stag's-horn species. The aspect presented is not unlike that of a heath-covered common.

In addition to the solid, calcareous-skeletoned Madrepores, or "Stony Corals," as they are often termed, there are a number of species in which a skeleton composed only of loosely aggregated calcareous spicules is produced. The so-called FLEXIBLE CORALS, or SEA-FANS, belong to this category, as also the precious CORAL OF COMMERCE. In the last-named species the solid, brilliantly coloured skeleton so much prized as an article of jewellery is deposited as a supplementary basis outside the tissues by which the star-patterned skeletons of the stony corals are secreted.

A group which demands brief notice is that of the HYDROID POLYPS. These include the majority of the JELLY-FISHES, a few coral-secreting species, and the organisms whose seaweed-like horny skeletons, known as SEA-FIRS, are, in common with those of Sea-mats, included among the flotsam and jetsam on every sea-beach. In the COMMON HYDRA, or FRESHWATER POLYP, an exceptional fresh-water representative of this group is presented. It may be likened to a tiny sea-anemone, having, when extended, a slender foot-stalk and long thread-like tentacles. Like a sea-anemone, it will shrink up when disturbed into a mere button of jelly. Its organisation is more simple than that of the anemone, its body-cavity being a simple sac, without any intucking of the orifice, or strengthening by supplementary membranous partitions. A similar simple structural plan is characteristic of all the organisms belonging to the series. An interesting phenomenon connected with the fresh-water hydra is the circumstance, demonstrated now over a century ago, that, if one of these animals be cut up into little pieces, each separate fragment is capable of repairing itself and growing into a new polyp.

The JELLY-FISHES, or MEDUSAS, and their allies would appear at first sight to possess but little structurally in common with the Coral-polyps and Sea-anemones. In their most familiar form they are represented by a more or less translucent bell-shaped body, which drifts with the current or propels itself through the water by its alternate expansions and contractions. In the centre of the lower surface, occupying the position of the bell's clapper, a polyp-like, tubular mouth is usually discernible, and this is frequently surrounded by a circle of tentacles, sometimes simple and sometimes elaborately ramified. Long, thread-like tentacles are also commonly developed around the margin of the swimming-bell.

The larger number of the jellyfishes are, as a matter of fact, transitional phases only of the fixed hydroid polyps previously referred to. In certain instances the body of the fixed polyp becomes elongated, and splits up horizontally into a series of jelly-fishes, or medusas, resembling a pile of saucers, which consecutively break away and lead a free-roving existence. In other forms a compound tree-like growth gives birth to medusa-like buds, like the flowers on a plant, which ultimately become detached and swim away. What are known as the COMB-BEARING JELLY-FISHES--their locomotive organs consisting of comb-like bands of vibratile hairs--are especially noteworthy. In some of these the body is nearly spherical or ovate, one of the species, in reference to its shape, being popularly known as the SEA-LEMON. A notable feature of these medusas is their remarkable glass-like transparency, their presence in the water in many instances being recognisable only by the prismatic glimmerings of their rows of vibratile hairs when the light falls upon them at a favourable angle. The most remarkable member of this particular group is undoubtedly the form known as VENUS'S GIRDLE. This species takes the form of a long, ribbon-like band of transparent jelly. The edges of the ribbon are clothed with vibratile hairs, and the mouth is situated in the centre of one of the edges. The animal progresses by the action of its hairs alone, or may be assisted by the twistings and undulations of its ribbon-like body.

Many jelly-fishes possess an unenviable reputation with reference to their stinging properties. The so-called PORTUGUESE MAN-OF-WAR is one of the more noteworthy of these. The organism consists of an ovately pointed air-bladder, which floats on the water, and from which depend numerous nutritive polyps and a mass of capturing-filaments, or tentacles.

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