In the Gorgonidae, or "Sea-shrubs," there is an arborescent coenosarc permanently rooted and provided with a grooved, or sulcate, branched sclerobasis, associated with true tissue-secretions, termed "dermosclerites.
The sclerobasis of the Gorgonidae varies a good deal in its composition. In some it is corneous, and these have often been confounded with the Antipathidae, amongst the Zoan-tharia. The distinction, however, between them is easy, when it is remembered that the polypes in the Gorgonidae have tentacles in multiples of four, whilst in the Antipathidae they are in sixes. The sclerobasis, too, in the former is always marked by grooves, whereas in the latter it is always either smooth or spinulous. In Isis and Mopsea the sclerobasis consists of alternate calcareous and horny segments, branches being developed in the former from the calcareous, and in the latter from the horny segments.
In Corallium rubrum, the "red coral" of commerce (fig. 83), the sclerobasis is unarticulate, or unjointed, and is entirely calcareous. It is the most familiar member of the family, and is largely imported for ornamental purposes. Red coral consists of a branched, densely calcareous sclerobasis, which is finely grooved upon its surface, is of a bright-red colour, and is in reality composed of fused spicules. The corallum is invested by a coenosarc, also of a red colour, which is studded by the apertures for the polypes, which are white, and possess eight pinnately-fringed tentacles. The entire coenosarc is channelled out by a number of anastomosing canals, which communicate with the somatic cavities of the polypes, and are said to be in direct communication with the external medium by means of numerous perforations in their walls. The entire canal system is filled with a nutrient fluid, containing corpuscles, and known as the "milk."
In the typical Gorgoniae. the sclerobasis is horny, and more or less arborescent, and the same is the case in the "Fan Corals" (Rhipidogorgia), in which the corallum has the form of a regularly reticulate fan-shaped expansion. The soft tissues of the Gorgonidae are abundantly supplied with sclerodermic secretions in the form of calcareous spicules of very various shapes, and often of very brilliant colours, which are in many instances of such characteristic figures that they can be employed as a ground of generic distinction. These spicules ("sclerites") are very generally buried in the soft tissues, but they may project beyond the surface of the coenosarc in such numbers as to render the integument rough and prickly.
Fig. 83. - Red Coral (Corallium rubrum) of the natural size, and a portion enlarged.