ALCYONARIA The Alcyonaria contrast sharply with the two other main groups (Actiniaria and Madreporaria). Alcyonarians almost in variably construct colonies. Their polyps are usually small and muscle down one side, the fibres In the strip being supported by ridges or lamellae of mesogloea and running in a longitudinal direction. The whole strip of muscle is known as a retractor. By their contraction these muscles pull the upper parts of the polyp downwards and inwards during the act of retraction. Each mesentery also possesses a filament ; but the filaments of all the mesenteries are not alike. Those which belong to the two mesen teries farthest away from the siphonoglyph are very long. Each has a groove along its free edge, lined by cells with long cilia; it is Y-shaped in a transverse section. The function of these fila ments is circulatory. The filaments of the other six mesenteries are shorter and consist of an epithelial cord containing many glandular cells; these filaments are digestive organs. The six last-mentioned mesenteries also contain concentrations of sex cells (gonads).
The eggs of Alcyonaria usually develop into free-swimming planulae (see COELENTERATA) which in due course settle down, either upon a hard support or in sand or mud, become transformed into polyps and by budding begin to construct a colony. The proc ess of budding has this peculiarity, however, that no bud is produced direct from a polyp itself ; the polyps give off hollow root-like structures, lined by endoderm (see COELENTERATA) and known as solenia, and from these the new buds arise.
The Alcyonaria offer almost the only examples to be found among the Anthozoa of that phenomenon known as polymorph ism, which is discussed in the articles COELENTERATA and HY DROZOA. In certain Alcyonarian colonies there are two kinds of polyps; the ordinary ones such as those already described, and another kind known as siphonozooids. These latter are polyps whose parts are more or less reduced, with the exception of the siphonoglyph, which is large and strongly ciliated. These indi viduals act as creators of water-currents which circulate through the system of solenial canals penetrating the colony. In cases where a single original polyp becomes transformed into the axis of a colony this constitutes a further differentiation of individuals.
The Alcyonaria are divided into three orders, and since the structure and mode of formation of the colonies, in which rests the main interest of the group, is different in the three cases, it will be convenient from this point onwards to consider them sep arately.
Order 1. Alcyonacea.—The numerous Alcyonaria which be long to this class possess this in common; that although the are extremely uniform in general structure throughout the group; in this they are unlike the polyps of either of the other large series of forms. Diversity among the Alcyonaria therefore af fects not so much the individual as the colony; and the colonies constructed vary most extensively in form, structure, and mode of development. A skeleton sufficiently resistant to retain its shape after the death of the colony is often developed; in other cases either part or the whole of the colony relies for support upon large numbers of minute calcareous bodies, which abound in its tissues, and which are known as spicules. Thus the Alcyona ria include a number of creatures which produce "coral." They are a group of corals which in the main inhabit the shore and the coastal waters down to 55o fathoms. Speaking generally the great depths of the sea as well as considerations of temperature and salinity limit their occurrence. Their centre of distribution is the Indo-Pacific littoral and they form an important element in coral-reefs.
A typical Alcyonarian polyp (fig. 2) is simpler in structure, as well as being less variable from one genus to another than that of most other Anthozoa. It possesses eight, and only eight, tentacles, and these are feathered by the presence of a paired series of lateral branches on each; i.e., they are pinnate. Down one angle of the flattened throat runs a groove lined by strongly ciliated epithelium. This is the siphonoglyph, and it creates a downward current of water into the coelenteron of the polyp. There are eight mesenteries, which alternate in position with the eight tentacles. These mesenteries possess each a special strip of colonies which they form are extremely various, there is never an axial skeleton forming a central support. A conception of the colony-forming activities of the polyps can be gained only by the study of a series of actual colonies. Figs. 3-5 will serve to illustrate the following remarks.
In the simplest colonies the polyps are attached to a foreign surface and connected with one another simply by a few creep ing rootlets (solenia) arising from their bases. Such a condition exists in Cornularia. Somewhat more elaborate are colonies such as those of certain species of Clavularia in which the meshes of the network of solenia have been filled up by solid mesogloea, and the network of rootlets has so been converted into a continu ous mat, with ramifying endodermal tubes inside it, and covered externally both above and below by ectoderm. In neither of these colonies is there any firm skeleton; the support consists simply of an external horny layer covering the rootlets and the lower parts of the polyps (Cornularia) or of spicules (Clavularia).
A different grade of organization appears in certain genera whose polyps tend to grow very tall, and are at the same time not unduly close to one another. In such a case bridges containing solenia may grow across from one polyp to another well above the level of the bases of the polyps (Hicksonia). This process goes a stage farther in Tubipora. In this the polyps become inordi nately long and narrow and diverge from one another as they grow up; and instead of throwing out isolated bridges across the gaps between them they develop a series of plate-like horizontal platforms which run at stated intervals one above the other across the colony. These platforms consist of mesogloea containing solenia, and are covered externally by ectoderm. From each suc cessive platform new polyps arise. The original polyps grow up wards until a considerable number of platforms have been formed ; they then stop, the other polyps continuing the colony. Tubipora produces a skeleton composed of firmly compacted spicules, which lies in the mesogloea of the polyps and platforms during life and is therefore truly internal. The tentacles of the polyps are bright green, the skeleton is crimson; and after the death of the soft parts it remains as a brightly coloured mass of tubes, of ten of considerable size, and is known as "Organ-pipe coral." Colonies of different type are formed by other genera possess ing long polyps. In Alcyonium for instance there is formed a massive colony with a small number of stout, unwieldy lobes. Each lobe consists of a number of polyps which are inordinately elongated vertically; but here, instead of producing platforms, the polyps have filled in the whole space between them with solid mesogloea, so that only the head-end of each polyp projects be yond the general mass. Consequently ectoderm occurs only on the exposed parts of the polyps and on the external surface of the mass connecting these ; the inner parts consist of mesogloea with solenia ramifying through it and connecting the deeper parts of the polyps with each other. New polyps arise from solenia at various levels between the old ones, as the colony grows. In this instance the skeleton consists entirely of scattered spicules ; these occur not only in the mesogloea of the massive portion of the colony but also in the projecting upper ends of the polyps, which are retractile into the solid part. The common British Alcyonium is known as "Dead Men's Fingers" from the suggestive appear ance presented by the colony when removed from the water or stranded by the tide, suspended from the undersides of over hanging rocks. Its appearance when under water with the trans parent glassy polyps fully ex panded is as delicate and beauti ful as its retracted state is repul sive.
The tree-like branching col onies of other Alcyonacea may be produced in more than one way. In Telesto for example, lateral polyps are budded off from solenia lying in the walls of single original polyps, which become extremely elongated.
Since each secondary polyp grows out at an angle from its parent, and itself produces further lateral polyps, it constitutes in time a branch of the main stem formed by the original polyp. A number of such branching systems may be connected with one another by basal stolons.
Lastly in Heliopora, the Blue Coral of tropical Indo-Pacific shores, a massive calcareous skeleton is produced.
Order 2. Gorgonacea.—In these animals the skeleton, ex cept in a few unusual genera which appear to be transitional in status between a mat-like colony and a tree-like one, forms a definite axis running up the centre of a tree-like colony. The soft tissues by which the axis is sur rounded contain spicules.
A typical example of the Gorgonacea is Gorgonia verrucosa, one of the Sea Fans (fig. 6). This forms a tree-like colony at tached by a narrow base and with slender twig-like branches. Each branch has an axis of blackish horny skeleton, clothed on all sides by bright pink flesh con taining solenia, and studded with numerous small translucent pink polyps. In certain relatives of Gorgonia the horny skeleton con tains calcareous inclusions.
In Corallium, the well known Precious Coral of commerce, the "coral" is an axial skeleton sim ilar to that of Gorgonia, but in this case it is stouter and less twig-like, and is formed by the fusion of innumerable spicules into a solid calcareous mass. In Corallium rubrum the flesh of the colony as well as the axis is scarlet, but the polyps are white.
The skeleton of the Gorgona cea, although it varies consider ably in structure from one form to another, is secreted by cells of ectodermal origin which are either embedded in the mesogloea or form a distinct layer round the axis itself ; this is a communal internal skeleton and does not represent an elongated axial polyp.
The colony is started by a polyp which gives rise to solenia in the basal part of its body and from these new polyps arise.
In other Pennatulids the form of the colony varies. In Koplto belemnon, instead of lateral leaf lets the stem bears large iso lated polyps; in Virgularia leaf lets are present but reduced in size; in Umbellula a few large polyps occur in a rosette at the top of a long stem which is other wise bare ; whilst in Renilla the basal peduncle is succeeded by a flat kidney-shaped expansion bearing radially-arranged polyps on its upper side.
General Note on the Skele ton.—Alcyonaria as a whole pro vide an extremely good example of the diversity which may exist in the skeleton within the limits of a single group. Instances have been quoted of both internal and external skeletons ; of horny cov ering skeletons, horny axial skeletons ; tubular skeletons, and solid ones; skeletons formed of diffused spicules, of interlocked spicules, and of fused spicules; cases in which spicules and a solid skeleton co-exist, and in which horny and limy material co exist in the skeleton, not to mention the massive calcareous support of Heliopora. Whatever the nature of the skeleton how ever, it is formed either by the ectoderm (Cornularia), or by cells of ectodermal origin which have penetrated the mesogloea.