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.