Coral

Coral is the skeleton or shell of various members of the great phylum, the "Coelenterata" (q.v.). A sea anemone is a good example of one of this group which has no hard parts; but in many animals very closely allied to the anemones, the animal is protected by an external coat of hard calcareous matter, which is strengthened by a number of plates radiating from the centre and alternating with the mesenteries. Such is an ordinary simple coral, such as Caryocyathus smithi which lives on the Devonshire coast. The majority of existing corals are not simple but colonial, living in great reefs formed by large numbers of corallites (q.v.), each one of which is based on the plan of a simple coral. Such are the large branching Madrepores or the massive Astrean corals. As popularly used the term includes practically all the calcareous skeletons of the Coelenterata; thus it includes among the Hydrozoa (q.v.), the massive or foliaceous expansions of Millepora (q.v.), and the dendroid branching Stylasters. In these the coral consists of the skeleton formed by a colony of individuals each based on the type of a Hydra (q.v.). Usually each colony consists of a double set of zooids, viz. a central "gastrozooid "surrounded by a ring of comparatively rudimentary "dactylozooids," the whole in Stylaster forming a "cyclosystem." As in Hydra the body cavity is not divided into chambers by mesenteries, there are no "septa" in the corals of this group (Hydrocorallineae). The remaining corals belong to the class Anthozoa (q.v.), and typically all contain septa; this class contains two groups, the Alcyonaria (q.v.) and the Zoantharia (q.v.), each of which contains a series of corals. In the "Dead Men's Fingers" (Alcyonium) of our coasts the skeletal structures consist simply of a number of isolated spicules; but in other cases these spicules have fused into a solid mass, which forms a central axis, as in the "Red Coral" (Corallium rubrum), or a series of tubes united by occasional transverse layers or platforms, as in the "Organ-pipe Coral"' (Tubipora mnsica). In other Alcyonaria the skeleton is of a two-fold origin; in addition to the spicules there is a central axial skeleton developed by an invagination of the base of the external layer (ectodermis); such is Gorgonia or the "Fan Corals," in which the axial sclerobasic skeleton supports the soft tissues by which it is surrounded, and in which are embedded the spicules. The "Blue Coral" (Heliopora) also belongs to the Alcyonaria; its skeleton is composed of tubes of two sizes: the larger (or autopores) are occupied by the sexual zooids, the smaller (siphonopores) by the reduced, sexless zooids; the tubes are crossed by horizontal layers or tabulae, and are strengthened by radial plates similar to the septa of the true corals; as these, however, do not correspond in number to the mesenteries they are known as "pseudo-septa." The extinct family, the Heliolitidae, members of which were abundant in Palaeozoic times, was a close ally of Heliopora. The well-known Halysites catenularis, or the "Dudley Chain Coral," from the Silurian rocks, of Dudley, and the less known Chaetetidae and Auloporidae, both also Palaeozoic, are also probably Alcyonarian corals.

The Zoantharia include most of the true corals, but even here the family of Black Corals (Antipatharia) must be excluded, as they have horny sclerobasic skeletons, and may be compared to the Gorgonias with the zooids of a six-rayed instead of an eight-rayed Alcyonarian type. The Zoantharia includes the Madreporaria or the true corals. Caryophyllia may be taken as a good type of these; it consists of a series of radial vertical plates or septa, which are either six or some multiple of six in number. Externally these septa expand out and unite together so as to form a closed external wall or "theca;" this is often strengthened by an epitheca deposited as a superficial crust around the coral. The axis is usually filled up by a pillar known as the "columella" but this may be absent; the columella often ends above as a knob-shaped expansion supporting the main mass of the soft parts of the coral polype, which lives in the cup-shaped depression (or "calice") at the upper side. Between the columella and the ends of the septa are often one or more circles or "crowns" of plate-like structures known as "pali." The septa may extend out beyond the side wall of the coral and form a series of longitudinal ridges known as costae: both costae and septa usually bear numerous outgrowths, forming either slight teeth or ridges, or long spine-like processes (synapticulae), or flat plates running obliquely through the interseptal chambers (dissepiments), or similar plates placed horizontally and continuous right across the coral (tabulae).

These are the main structures in a simple coral. But two of the three methods of coral reproduction give rise to colonies. The first method is by ovae which may be carried away and grow into independent simple corals like the parent; the second, by budding or gemmation, giving rise typically to a dendroid colony; and, third, by fission, one coral splitting across into two and giving rise to massive forms. The whole colony may be formed simply of young corallites, or these may be embedded in an intermediate tissue - the coenenchyma.

The classification of the true corals is given under "Madreporaria." The corals have a somewhat restricted range both in space and depth: the reef corals are limited to a mean temperature of not below 68° F., and thus occur only in tropical seas and at a depth of rarely below 15 to 30 fathoms. They are usually very sensitive to impurities in the water, and exposure to air and brackish water is fatal; exceptions to these, however, occur. Thus a Madrepora lives in New Zealand in a river that is only very slightly brackish. The simple corals have a much wider range; a few species occur on the British coasts, and some have been dredged from depths of over 1,000 fathoms.