Since the publication of Laval's description, many circular isles, or groups of islands, analogous to these atollons, since called _atolls_, have been discovered in the Pacific Ocean and other seas. The naturalist Forster, who accompanied Cook in his voyage round the world, first made known the more remarkable characteristics of these gigantic formations. He perfectly comprehended their origin, which he was the first to attribute to the development of the calcareous zoophytic polypier. After Forster, many other naturalists--Lamouroux, Chamisso, Quoy, Gaimard, Ehrenberg, Ellis, Darwin, Couthony, and Dana--have furnished Science with many precious lessons on the natural history of coral islands and madreporic reefs. We can only glance at a few of the more remarkable genera of these interesting creatures. "Those occupying the same Coral," says Frédol, "live in perfect harmony; they constitute a family of brothers, physically united in the closest bonds of union. They occupy the same dwelling, each having its separate chamber; but the power of abandoning it is denied them. Attached each to its cell, they are driven to trust in Providence for the food which never fails them; moreover, what is eaten by each mouth profits the whole community. Urged on by a wonderful instinct, the polypes labour together at the same work; isolated, they would be weak and helpless; in combination, they are strong." M. Lacaze-Duthiers has even demonstrated that _Antipathes glaberrima_, _Gorgonia tuberculata_ (Lamarck), _Leiopathes glaberrima_ (Gray), and _Leiopathes Lamarckii_ (Haime), were present on the same coral, the _Gerardia_ of Lamarck. It is thus recognized that, under the general denomination of polyps, very distinct genera are found, some being of the _Hydra_ type, others belonging to the _Plumularia_. The first are very common on our coast: they include the _Tubularia_, the _Campanularia_, and the _Sertularia_. The Reed Tubularia (_T. indivisa_) is remarkably curious: its numerous stems are horny, yellow, and marked at intervals with irregular knots, resembling the joints of a straw. Their lower extremity is tortuous, and apt to adhere to foreign bodies; the upper part is nearly upright, and slightly flexuous, the whole resembling some flowering plant, without leaves or lateral branches. The _Campanularias_ are altogether different; the end of the branches whence the polyps issue are broad and bell-shaped, _C. dichotoma_ presenting a stem of brownish colour, thin as a silken thread, but strong and elastic. The polyps are numerous, a branch eight inches in height being inhabited by as many as twelve hundred individuals.