As minute social organisms the corals play an important part in the general economy of nature, although they do not, as people began to believe after Capt. Cook’s voyages of discovery, build up islands or enlarge continents from almost unfathomable depths of the ocean. They excite the liveliest interest, whether regarded as physiological objects, and as illustrating the various gradations of animal form, or in connection with the geography of plants, and the geognostic relations of the earth’s crust. According to the comprehensive views of Leopold von Buch, the whole Jura-formation consists of “large elevated coral-banks of the ancient world, surrounding at a certain distance the old mountain chains.” According to Ehrenberg’s classification,[KS] coral-animals, (in English works often incorrectly termed coral-insects,) are separable into the monostomous _Anthozoa_, which are either free and with the power of detaching themselves, as _Animal-corals_; or are attached in the manner of plants, as _Phyto-corals_. To the first order (Zoocorallia) belong the Hydras or Armpolyps of Trembley, the Actiniæ, radiant with the most splendid colours, and the mushroom-corals; and to the second order belong the Madrepores, the Astrææ, and the Ocellinæ. The Polyps of the second order are those which from their cellular, wave-resisting, wall-works are the principal subject of this illustration. The wall-work is composed of the aggregate of the coral-trunks, which, however, do not suddenly lose their combined vitality, like a dead forest tree. Every coral-trunk arises by a process of gemmation in accordance with certain laws, and forms one complete structure, each portion being formed by a great number of organically distinct individual animals. In the group of Phyto-corals these cannot separate themselves spontaneously, but remain united with one another by lamellæ of carbonate of lime. Hence each coral-trunk by no means possesses a central point of common vitality.[KT] The propagation of coral-animals, according to the difference of the orders, is by eggs, spontaneous division or gemmation. This last kind of propagation presents the greatest variety of forms in the development of individuals.