The polypes in this family form colonies united by a fleshy or coriaceous coenosarc, in the shape of a crust or of creeping roots, and they have no power of locomotion. The coenosarc may be strengthened by embedded spicules, adventitious grains of sand, or other foreign bodies. Examples of the family are Zoanthics and Palythoa.
The "Black Corals" or Antipathidae, which compose this group, are always composite, consisting of a number of polypes united by a thin fleshy coenosarc, which is spread over and supported by a simple or more commonly branched horny axis or "sclerobase." The tissues are not furnished with calcareous secretions, and the polypes have in general six simple tentacles.
The corallum or skeleton of the Antipathidae is of a horny consistence, its form simple or branched in a more or less complicated and plant-like manner, and its surface smooth or covered with minute spines. All the Antipathidae form colonies, which are rooted by the base to some foreign object, and which consist of numerous minute polypes united by a fleshy coenosarc (fig. 72). The corallum is secreted by the coznosarc, and thus forms an axis or stem which is completely covered during life by the soft parts of the colony, just as the trunk of a tree is covered by the bark. Owing, further, to the fact that the skeleton is produced wholly by the coenosarc, the corallum is wholly outside the polypes, which are themselves entirely destitute of hard structures. Various other Actinozoa (such as the Gorgonidae) possess, as we shall see, a similar axial skeleton, secreted by the coenosarc; and all such coralla are said to be "sclerobasic." As coralla of this nature are not formed by hard structures deposited within the tissues of the polypes, the general name of "foot-secretion" has been applied to them by Prof. Dana.
Fig. 72. - Part of the living stem of Antipathes anguina, of the natural size.
Sub-order III. Zoantharia sclerodermata or Madre-poraria. - The members of this sub-order include the great bulk of coral-producing or "coralligenous" zoophytes (Madre-poraria) of recent seas. They are defined by the possession of a corallum which is partially or wholly developed within the tissues of the polypes themselves ("sclerodermic"), which does not consist simply of scattered spicules, and in which the parts are very generally disposed in multiples of six. The actinosoma may be simple, consisting of a single polype only, or composite, consisting of many polypes united by a coenosarc.
As regards the anatomy of their soft parts, the simple Zoantharia sclerodermata may be regarded as essentially Sea-anemones, whilst the compound forms are simply colonies of Actin-oid polypes united by a common flesh or coenosarc. It is, therefore, only necessary to consider the nature of the skeleton or corallum of these forms, since the leading peculiarities of the sub-order are to be found in this.
If we examine first a simple coral of this group, we find that we have to deal with an animal in all important respects identical with an ordinary Sea-anemone, but having a more or less complicated skeleton developed in its interior. The animal possesses a base, a column, and a disc - the latter surrounded by tentacles, and perforated centrally by the mouth. The mouth opens into a stomach-sac, connected with the body-walls by mesenteries; and the tentaculate disc and dependent gastric sac remain permanently soft and capable of contraction and expansion. Below the stomach, the soft tissues of the polype are strengthened and supported by a more or less perfect calcareous skeleton or corallum (fig. 74). This is composed of calcareous matter ("sclerenchyma") deposited by and in the tissues themselves, between the endoderm and ectoderm, and the corallum is thus within the polype, and is technically said to be "sclerodermic." The "sclerodermic" corallum is therefore a true " tissue-secretion," and thus differs conspicuously from the "sclerobasis" of the Antipathidae and Gorgonidae, which is secreted by the coenosarc, and is not formed by a calcification of the soft parts of the polypes themselves. The general distinction, arising from their mode of formation, between "sclerobasic" and "sclerodermic" corals, is not, perhaps, of essential importance, and the boundary-line between the two is not very clearly marked ; but it is of considerable practical value. It is, moreover, a distinction which is readily recognised, as a rule, by a simple inspection of the corallum itself. A sclerobasic corallum, namely, being secreted solely by the coenosarc, never exhibits any parts which correspond with the separate polypes of the colony. On the other hand, the sclerodermic corallum (when not composed simply of scattered spicules) either consists of a single cup-like structure corresponding with a single polype (fig. 74), or of several such (fig. 73, A) united by a common skeleton.
Fig. 73. - Sclerodermic and sclerobasic Corals. A. Branch of Dendrophyllia nigres-cens, a sclerodermic coral, showing the cups or thecae (a a) secreted by the separate polypes, and united by the coenenchyma (c c). B, Portion of a sclerobasic coral (Gorgonia) represented diagrammatically: s The solid and branched sclerobase; b A portion of the soft coenosarc with its embedded polypes, investing the sclerobasic axis.