The genera _Sarcophytum_ and _Lobophytum_ occur in shallow water in the tropics of the old world. The former frequently consists of huge toad-stool shaped masses, soft and spongy in {348}consistency, of a green, brown, or yellow colour. On some reefs the colonies of _Sarcophytum_ form a very conspicuous feature, and from their very slimy, slippery surface, add to the minor dangers of wading in these regions. Both genera are dimorphic. Some species of the genus _Sclerophytum_,[374] which occur in the Indian Ocean, are so hard and brittle that they might readily be mistaken for a Zoantharian coral. This character is due to the enormous number of tightly packed spicules borne by the coenenchym. Some of these spicules in _S. querciforme_ are 7 mm. × 1.7 mm.; the largest, though not the longest (_vide_ p. 335) of any spicules occurring in the order. Another very important genus occurring on coral reefs, and of very wide distribution, is _Spongodes_. This genus forms bushy and rather brittle colonies of an endless variety of beautiful shapes and colours. Arising from the neck of each anthocodia there are one or two long, sharp, projecting spicules, which give the surface a very spiny or prickly character. The genera _Siphonogorgia_ and _Chironephthya_ form large brittle, branching colonies which might readily be mistaken for Gorgonians. The strength of the branches, however, is mainly due to the large, densely packed, spindle-shaped spicules at the surface of the coenenchym, the long coelenteric cavities of the zooids penetrating the axis of both stem and branches. _Siphonogorgia_ is usually uniformly red or yellow in colour. _Chironephthya_, on the other hand, exhibits a great variety of colour in specimens from the same reef, and indeed in different branches of the same colony. FAM. 1. XENIIDAE.—Alcyonacea with non-retractile zooids. Spicules very small discs, usually containing a relatively small proportion of lime. _Xenia_, Savigny; Indian Ocean and Torres Straits. _Heteroxenia_, Kölliker; Red Sea, Cape of Good Hope, and Torres Straits. FAM. 2. TELESTIDAE.—Colonies arising from an encrusting membranous or branching stolon. The erect stem and branches are formed by the body-walls of two or three zooids only, from which secondary zooids and branches of the next order arise. _Telesto_, Lamouroux, widely distributed in warm waters of the Atlantic, Pacific, and Indian Oceans. The genus _Fascicularia_, Viguier, from the coast of Algiers, seems to be related to _Telesto_, {349}but the groups of zooids are short, and do not give rise to branches. FAM. 3. COELOGORGIIDAE.—The colony arborescent, attached by stolon-like processes. The stem formed by an axial zooid with thickened body-walls. Branches formed by axial zooids of the second order, and branchlets by axial zooids of the third order, borne either on two sides or in spirals by the main stem. Genus _Coelogorgia_, Zanzibar.