zooids arise at intervals. In Clavularia and its allies each outgrowth contains several solenia, and the outgrowths may take the form of flat expansions, composed of a number of solenial tubes felted together to form a lamellar surface of attachment. Such outgrowths are called stolons, and a stolon may be simple, i.e. contain only one solenium, as in Cornularia, or may be complex and built up of many solenia, as in Clavularia. Further complications arise when the lower walls of the mother zooid become thickened and interpenetrated with solenia, from which buds are developed, so that lobose, tufted, or branched colonies are formed. The chief orders of the Synalcyonacea are founded upon the different architectural features of colonies produced by different modes of budding. We recognize six orders—the Stolonifera, Alcyonacea, Pseudaxonia, Axifera, Stelechotokea, and Coenothecalia.
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| Fig. 5.A. Skeleton of a young colony of Tubipora purpurea. st, Stolon; p, platform. B. Diagrammatic longitudinal section of a corallite, showing two platforms, p, and simple and cup-shaped tabulae, t. (After S. J. Hickson.) |
Fig. 6.—Portion of a colony of Corallium rubrum, showing expanded and contracted zooids. In the lower part of the figure the cortex has been cut away to show the axis, ax, and the longitudinal canals, lc, surrounding it. |
In the order Stolonifera the zooids spring at intervals from branching or lamellar stolons, and are usually free from one another, except at their bases, but in some cases horizontal solenia arising at various heights from the body-wall may place the more distal portions of the zooids in communication with one another. In the genus Tubipora these horizontal solenia unite to form a series of horizontal platforms (fig. 5). The order comprises the families Cornulariidae, Syringoporidae, Tubiporidae, and Favositidae. In the first-named, the zooids are united only by their bases and the skeleton consists of loose spicules. In the Tubiporidae the spicules of the proximal part of the body-wall are fused together to form a firm tube, the corallite, into which the distal part of the zooid can be retracted. The corallites are connected at intervals by horizontal platforms containing solenia, and at the level of each platform the cavity of the corallite is divided by a transverse calcareous partition, either flat or cup-shaped, called a tabula. Formerly all corals in which tabulae are present were classed together as Tabulata, but Tubipora is an undoubted Alcyonarian with a lamellar stolon, and the structure of the fossil genus Syringopora, which has vertical corallites united by horizontal solenia, clearly shows its affinity to Tubipora. The Favositidae, a fossil family from the Silurian and Devonian, have a massive corallum composed of numerous polygonal corallites closely packed together. The cavities of adjacent corallites communicate by means of numerous perforations, which appear to represent solenia, and numerous transverse tabulae are also present. In Favosites hemisphaerica a number of radial spines, projecting into the cavity of the corallite, give it the appearance of a madreporarian coral.
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| Fig. 7.—The sea-fan (Gorgonia cavolinii). |
In the order Alcyonacea the colony consists of bunches of elongate cylindrical zooids, whose proximal portions are united by solenia and compacted, by fusion of their own walls and those of the solenia, into a fleshy mass called the coenenchyma. Thus the coenenchyma forms a stem, sometimes branched, from the surface of which the free portions of the zooids project. The skeleton of the Alcyonacea consists of separate calcareous spicules, which are often, especially in the Nephthyidae, so abundant and so closely interlocked as to form a tolerably firm and hard armour. The order comprises the families Xeniidae, Alcyonidae and Nephthyidae. Alcyonium digitatum, a pink digitate form popularly known as “dead men’s fingers,” is common in 10-20 fathoms of water off the English coasts.
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Fig. 8. B, Section of the rachis bearing a single pinna, a, Axis; b, metarachidial; c, prorachidial; d, pararachidial stem canals. |
In the order Pseudaxonia the colonies are upright and branched, consisting of a number of short zooids whose proximal ends are imbedded in a coenenchyma containing numerous ramifying solenia and spicules. The coenenchyma is further differentiated into a medullary portion and a cortex. The latter contains the proximal moieties of the zooids and numerous but separate spicules. The medullary portion is densely crowded with spicules of different shape from those in the cortex, and in some forms the spicules are cemented together to form a hard supporting axis. There are four families of Pseudaxonia—the Briareidae, Sclerogorgidae, Melitodidae, and Corallidae. In the first-named the medulla is penetrated by solenia and forms an indistinct axis; in the remainder the medulla is devoid of solenia, and in the Melitodidae and Corallidae it forms a dense axis, which in the Melitodidae consists of alternate calcareous and horny joints. The precious red coral of commerce, Corallium rubrum (fig. 6), a member of the family Corallidae, is found at depths varying from 15 to 120 fathoms in the Mediterranean Sea, chiefly on the African coast. It owes its commercial value to the beauty of its hard red calcareous axis which in life is covered by a cortex in which the proximal moieties of the zooids are imbedded. Corallium rubrum has been the subject of a beautifully-illustrated memoir by de Lacaze-Duthiers, which should be consulted for details of anatomy.
The Axifera comprise those corals that have a horny or calcified axis, which in position corresponds to the axis of the Pseudaxonia, but, unlike it, is never formed of fused spicules; the most familiar example is the pink sea-fan, Gorgonia cavolinii, which is found in abundance in 10-25 fathoms of water off the English coasts (fig. 7). In this order the axis is formed as an ingrowth of the ectoderm of the base of the mother zooid of the colony, the cavity of the ingrowth being filled by a horny substance secreted by the ectoderm. In Gorgonia the axis remains horny throughout life, but in many forms it is further strengthened by a deposit of calcareous matter In the family Isidinae the axis consists of alternate segments of horny and calcareous substance, the latter being amorphous. The order contains six families—the Dasygorgidae, Isidae, Primnoidae, Muriceidae, Plexauridae, and Gorgonidae.
In the order Stelechotokea the colony consists of a stem formed by a greatly-elongated mother zooid, and the daughter zooids are borne as lateral buds on the stem. In the section Asiphonacea the colonies are upright and branched, springing from membranous or ramifying stolons. They resemble and are closely allied to certain families of the Cornulariidae, differing from them only in mode of budding and in the disposition of the daughter zooids round a central, much-elongated mother zooid. The section contains two families, the Telestidae and the Coelogorgidae. The second section comprises the Pennatulacea or sea-pens, which are remarkable from the fact that the colony is not fixed by the base to a rock or other
