FAM. 6. GORGONIIDAE.—This family contains some of the commonest and best-known genera of the order. They usually form large flexible branched colonies with delicate horny axes and thin coenenchym. The zooids are usually completely retractile. The principal genera are:—_Gorgonia_, Linn. This genus {357}includes _Gorgonia_ (_Rhipidogorgia_) _flabellum_, the well-known fan Gorgonia with intimately anastomosing branches, from the warm waters of the Atlantic Ocean. The genera _Eugorgia_, Verrill, and _Leptogorgia_, Milne Edwards, differ from _Gorgonia_ in the character of the spicules. In _Xiphigorgia_, Milne Edwards, from the West Indies, the branches are much compressed, forming at the edges wing-like ridges, which bear the zoopores in rows. _Malacogorgia_, Hickson, has no spicules. Cape of Good Hope. FAM. 7. GORGONELLIDAE.—In this family the horny axis is impregnated with lime. The surface of the coenenchym is usually smooth, and the spicules small. The colonies are sometimes unbranched (_Juncella_). In the branching forms the axis of the terminal branches is often very fine and thread-like in dimensions. [Illustration: FIG. 157.—_Verrucella guadaloupensis_, with an epizoic Brittle star (_Oph._) of similar colour.] The principal genera are:—_Gorgonella_, with a ramified flabelliform axis; _Ctenocella_, with a peculiar double-comb manner of branching; and _Juncella_, which forms very long unbranched or slightly branched colonies, with club-shaped spicules. All these genera are found in shallow water in the tropical or semi-tropical regions of the world. _Verrucella_ is a genus with delicate anastomosing branches found principally in the shallow tropical waters of the Atlantic shores. Like many of the Gorgonacea, with branches disposed in one plane (flabelliform) _Verrucella_ frequently carries a considerable number of epizoic Brittle stars, which wind their flexible arms round the branches, and thus obtain a firm attachment to their host. There is no reason to suppose that these Brittle stars are in any sense parasitic, as a specimen that bears many such forms shows no sign of injury or degeneration, and it is possible they may even be of service to {358}the _Verrucella_ by preying upon other organisms that might be injurious. An interesting feature of the association is that the Brittle stars are of the same colour as the host, and the knob-like plates on their aboral surface have a close resemblance to the verrucae (Fig. 157). ORDER V. PENNATULACEA. The Sea-pens form a very distinct order of the Alcyonaria. They are the only Alcyonarians that are not strictly sedentary in habit, that are capable of independent movement as a whole, and exhibit a bilateral symmetry of the colony. No genera have yet been discovered that can be regarded as connecting links between the Pennatulacea and the other orders of the Alcyonaria. Their position, therefore, is an isolated one, and their relationships obscure.