To the west of the Indian Peninsula, the Maldive and Laccadive Islands form the extremity of another group of atolls, and important madreporic reefs, which extend towards the south, by the Maldives and the Chagos Islands; they consist of low coral formations, densely clothed with cocoa-nut trees. The Maldives, the most southerly cluster, include upwards of a thousand islands and reefs; the Laccadives, seventeen in number, are of similar origin. The Saya de Malha bank, towards the south-east, constitutes a further group of madreporic islets. Finally, the coast of the Mauritius, of Madagascar, of the Seychelles, and even the African continent, from the northern extremity of the Mozambique Channel to the bottom of the Red Sea, are studded with numerous reefs of the same nature. They fail, however, almost completely, along the coast of the Asiatic continent, where, among others, the waters of the Euphrates, the Indus, and the Ganges, enter the sea, and diversify its inhabitants. The western coast of Africa, and the east coast of the American continent, are almost entirely destitute of great madreporic reefs, but they abound in the Caribbean Seas. In the Gulf of Mexico, where the vast fresh-water current of the Mississippi debouches into the sea, they are unknown. It is principally on the north coast and upon the eastern flanks of the chain of West Indian Islands that the madreporic reefs show themselves in these regions.
The polyps which have produced these vast ranges of islands would be set down, at first sight, as the most incapable objects in creation for accomplishing it. In the case of the Pennatulidæ, the case is coriaceous, strengthened with calcareous particles; the interior is a fibrous network containing a transparent jelly in the squares, and permeated by a certain number of longitudinal cartilaginous tubes; the soft part is uniformly gelatinous, but the skin is also coriaceous, with a great number of calcareous spicula placed parallel to one another, adding greatly to its strength and consistency.
The polyps are placed in this external fleshy crust; their position being marked by an orifice on the surface, distinguished by eight star-like rays, which open when the upper portion of the body is forced outwards, in which state it resembles a cylindrical bladder or nipple crowned with a fringe of tentacula, which surround the mouth. Under this orifice is the stomach, occupying the centre of the cylinder. The space between this stomach and the outer envelope is divided into eight equal compartments or cells by as many thin septa, originating in a labial rim or lip between the basis of the tentacula, which descend through the cylinder attached on the one side to the inner tunic of the body, and on the other to the stomach, which is thus retained in its position.
The protruding portion of the polyp is very delicate, the internal viscera being, as it were, enclosed in a bladder formed of two very thin membranes in intimate union, so transparent as to permit a view of their arrangement. At the base of the body, where thickest, it coalesces with the base of the adjacent polyp; thus constituting the common cortical portion into which each animal retreats at will, by a process in many respects resembling that by which a snail draws in its horns. In the greater number of Asteroidæ this common portion secretes carbonate of lime, which is deposited in the meshes of its tissues either in granules or in crystalline spiculæ, which imparts a solid consistency to the whole. The inner tissue meanwhile continues unaltered, being prolonged throughout the polypiferous lining of the cell, the abdominal cavity, and the longitudinal canals which permeate the whole polyp, as well as the tubular network with which the space between the canals is occupied. It is among these inner tissues that the buds or gemmæ are generated, by whose increase and evolution the polyp mass is enlarged, the shape and size depending on the manner in which the buds are evolved; for in some, as in the Pennatulidæ, determinate spots only have the appropriated organization, while in others, as in Alcyonium, the generative faculty appears to be undefined and more diffused.
The Actiniaria.
Here we leave the group of polyps which form united families. The Sea Anemones, of which the Actinia are the type, consist of Zoanthaires, which produce no corals, that is to say, of polyps whose covering remains always soft, and in whose interior nothing solid is produced. This order is usually divided into two families—the Actiniadæ, having the tentacles in uninterrupted circles, with no corallum, and the Minyadinæ, having globose bodies, and very short tentacula.
The modern aquarium exposes the spectator to many wonderful surprises. Coiled up against the transparent crystal walls of the basin, he observes living creatures of the most brilliant shades of colour, and more resembling flowers than animals. Supported by a solid base and cylindrical stem, he sees them terminate like the corolla of a flower, as in the petals of the anemone: these are the animals we call Sea Anemones—curious zoophytes, which, as all persons familiar with the sea shore may have observed, are now seen suspended from the rocks, and presently buried at the bottom of the sea, or floating on its surface. These charming and timid creatures are also called Actinia, as indicating their disposition to form rays or stars, from the Greek ἀκτὶν, a ray.
The body of these animals is cylindrical in form, terminating beneath in a muscular disk, which is generally large and distinct, enabling them to cling vigorously to foreign bodies. It terminates above in an upper disk, bearing many rows of tentacles, which differ from each other only in their size. These tentacles are sometimes decorated with brilliant colours, forming a species of collarette, consisting of contractile and often retractile tubes, pierced at their points with an orifice, whence issue jets of water, which is ejected at the will of the animal. Arranged in multiples of circles, they distribute themselves with perfect regularity round the mouth. These are the arms of this species of zoophyte.