The axis in this sub-order consists of numerous spicules tightly packed together, or cemented together by a substance which is probably allied to horn in its chemical composition. This substance may be considerable in amount, in which case it remains after decalcification as a spongy, porous residue; or it may be so small in amount, as in _Corallium_, that the axis appears to be composed of solid carbonate of lime. The statement is usually made that the axis is penetrated by nutritive canals in certain genera, but the evidence upon which this is based is unsatisfactory and in some cases unfounded. There can be no doubt, however, that in some genera the axis is porous and in others it is not, and this forms a useful character for the separation of genera. FAM. 1. BRIAREIDAE.—The medullary substance consists of closely packed but separate spicules embedded in a soft horny matrix, which is uniform in character throughout its course. Nearly all the genera form dendritic colonies of considerable size. The principal genera are:—_Solenocaulon_, Gray; Indian Ocean and North Australia. Many of the specimens of this genus have fistulose stems and branches. The tubular character of the stem and branches is probably caused by the activity of a Crustacean, {351}_Alpheus_, and may be regarded as of the nature of a gall-formation.[376] _Paragorgia_, M. Edwards; Norwegian fjords, in deep water. This genus forms very large tree-like colonies of a ruby-red or white colour. It is perhaps the largest of the dendritic Alcyonarians. It is dimorphic. _Spongioderma_, Kölliker; Cape of Good Hope. The surface of this form is always covered by an encrusting sponge. _Iciligorgia_, Ridley; Torres Straits. The stem and branches are compressed and irregular in section. FAM. 2. SCLEROGORGIIDAE.—The medullary mass forms a distinct axis consisting of closely packed elongate spicules with dense horny sheaths. _Suberogorgia_, Gray, has a wide distribution in the Pacific Ocean, Indian Ocean, and the West Indies. _Keroeides_, W. and S., comes from Japan. FAM. 3. MELITODIDAE.—The axis in this family exhibits a series of nodes and internodes (Fig. 155), the former consisting of pads formed of a horny substance with embedded spicules, the latter of a calcareous substance with only traces of a horny matrix. The internodes are quite rigid, the nodes however give a certain degree of flexibility to the colony as a whole. Neither the nodes nor the internodes are penetrated by nutritive canals, but when dried the nodes are porous. [Illustration: FIG. 155.—_Melitodes dichotoma_, showing the swollen nodes and the internodes.]