{336}[Illustration: FIG. 149.—Spicules of Alcyonaria. 1, Club of _Juncella_; 2, warted cross of _Plexaurella_; 3, torch of _Eunicella_; 4, needle of _Renilla_; 5, warted spindle of _Gorgonella_; 6, spicule of _Pennatula_; 7, foliate club of _Eunicea_; 8, irregular spicule of _Paramuricea_; 9, scale of _Primnoa_; 10, spicules of _Trichogorgia_. (5 and 10 original, the remainder after Kölliker.)] In _Paragorgia_ and some other closely related genera the spicules of the axis of the colony also become tightly wedged together, but the core thus formed is far more porous and brittle than it is in the Coralliidae. In _Tubipora_ (the organ-pipe coral) and in _Telesto rubra_ the spicules of the body-walls of the zooids fuse to form perforated calcareous tubes. In some species of _Sclerophytum_ the large spicules of the coenenchym become so closely packed that they form dense stony masses, almost as hard as a Perforate Madreporarian coral. The horny substance, allied chemically to keratin, plays an {337}important part in the building up of skeletal structures in many Alcyonaria. In _Clavularia viridis_ and in _Stereosoma_ a change in the chemical character of the mesogloea of the body-walls of the polyps leads to the formation of a horny tube, which in the former case is built up of interlacing fibres, and in the latter is formed as a homogeneous sheath. In many of the Alcyonacea which have a compact axial skeleton the spicules are cemented together by a horny matrix. In the Gorgonellidae and some others the hard axis is formed of a horny substance impregnated with a crystalline form of calcium carbonate; but in the Gorgoniidae, many of the Pennatulacea and some other genera very little or no carbonate of lime is found in the horny axis. The skeleton of the genus _Heliopora_ differs from that of all the other Alcyonaria in its development, structure, and form. In the words of Dr. G. C. Bourne,[365] "the calcareous skeleton of _Heliopora_ is not formed from spicules developed within cells but is a crystalline structure formed by crystallisation of carbonate of lime, probably in the form of aragonite, in an organic matrix produced by the disintegration of cells which I have described as calicoblasts." It is further characterised by its blue colour. A peculiar form of the axial skeleton (Fig. 155), consisting of alternate nodes mainly composed of keratin, and internodes mainly composed of calcium carbonate, is seen in the families Isidae and Melitodidae. In the Melitodidae the nodes contain a considerable number of loose spicules, and the internodes are mainly composed of spicules in close contact but firmly cemented together by a sparse horny matrix. In the Isidae the scanty calcareous substance of the nodes, and the bulk of the substance of the internodes, is formed of amorphous crystalline limestone.