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The remaining forms, the Actiniidea, are divisible into the numerous. In some cases—e.g., in some species of MadreMalacactiniae, or soft-bodied sea-anemones, which have already been pora—only two septa are fully developed, the remainder described sufficiently in the course of this article (pp. 454-5), and being very feebly represented. the Scleractiniae (— Madreporaria), or true corals. Though the corallum appears to lie within the zooid, it All recent corals, as has already been said, conform so closely to the anatomy of normal Actinians that they is morphologically external to it, as is best shown by its cannot be classified apart from them, except that they are developmental history. The larvae of corals are free swimdistinguished by the possession of a calcareous skeleton. ming ciliated forms known as planulae, and they do not This skeleton is largely composed of a number of radiating acquire a corallum until they fix themselves. A ring-shaped plates or septa, and it differs both in origin and structure plate of calcite, secreted by the ectoderm, is then formed, from the calcareous skeleton of all Alcyonaria except lying between the embryo and the surface of attachment. Heliopora. It is formed, not from fused spicules, but As the mesenteries are formed, the endoderm of the basal as a secretion 0/ a special layer of cells derived from the disc lying above the basal plate is raised up in the form of basal ectoderm, and known as calicoblasts. The skeleton radiating folds. There may be six of such folds, one in or corallum of a typical solitary coral—the common Devon- each entoccele of the primary cycle of mesenteries; or shire cup-coral Caryophyllia Smithii (Fig. 15) is a good there may be twelve, one in each exoccele and entocoele. The ectoderm beneath each fold becomes detached from the surface of the basal plate, and both it and the mesogloea are folded conformably with the endoderm. The cells forming the limbs of the ectodermic folds secrete nodules of calcite, and these, fusing together, give rise to six (or twelve) vertical radial plates or septa. As growth proceeds new septa are formed simultaneously with the new couples of secondary mesenteries. In some corals, in which all the septa are entocoelic, each new system is embraced by a mesenteric couple; in others, in which the septa are both entocoelic and exocoelic, three septa are formed in every chamber between two primary mesenterial couples, one in the entocoele of the newly-formed mesenterial couple of the secondary cycle, and one in each exocoele between a primary and a secondary couple. These latter are in turn embraced by the couples of the tertiary cycle of mesenteries, and new septa are formed in the exocoeles on either side of them, and so forth.

Fig. 15.—Corallum of Caryophyllia; semi-diagrammatic, ik, theca ; c, costae ; sp, septa ; p, palus ; col, columella. example—exhibits the following parts:—(1) The basal plate, between the zooid and the surface of attachment. (2) The septa, radial plates of calcite reaching from the periphery nearly or quite to the centre of the coral-cup or calicle. (3) The theca or wall, which in many corals is not an independent structure, but is formed by the conjoined thickened peripheral ends of the septa. (4) The columella, a structure which occupies the centre of the calicle, and may arise from the basal plate, when it is called essential, or may be formed by union of trabecular offsets of the septa, when it is called unessential. (5) The costa;, longitudinal ribs or rows of spines on the outer surface of the theca. True costae always correspond to the septa, and are in fact the peripheral edges of the latter. (6) Epitheca, an offset of the basal plate which surrounds the base of the theca in a ring-like manner, and in some corals may take the place of a true theca. (7) Pali, spinous or blade-like upgrowths from the bottom of the calicle, which project between the inner edges of certain septa and the columella. In addition to these parts the following structures may exist in corals :—Dissepiments are oblique calcareous partitions, stretching from septum to septum, and closing the interseptal chambers below. The whole system of dissepiments in any given calicle is often called endotheca. Synapticulce are calcareous bars uniting adjacent septa. Tabulce are stout horizontal partitions traversing the centre of the calicle and dividing it into as many superimposed chambers. The septa in recent corals always bear a definite relation to the mesenteries, being found either in every entocoele or in every entocoele and exocoele. Hence in corals in which there is only a single cycle of mesenteries the septa are correspondingly few in number; where several cycles of mesenteries are present the septa are correspondingly

Fig. 1G.—Tangential section of a larva of Astroides calicularis which has fixed itself on a piece of cork, ec, ectoderm; en, endoderm; mg, mesogloea; m, m, mesenteries; s, septum; 6, basal plate formed of ellipsoids of carbonate of lime secreted by the basal ectoderm ; ep, epitheca. (After von Koch.) It is evident from an inspection of Figs. 16 and 17 that every septum is covered by a fold of endoderm, mesogloea, and ectoderm, and is in fact pushed into the cavity of the zooid from without. The zooid then is, as it were, moulded upon the corallum. When fully extended, the upper part of the zooid projects for some distance out of the calicle, and its wall is reflected for some distance over the lip of the latter, forming a fold of soft tissue extending to a greater or less distance over the theca, and containing in most cases a cavity continuous over the lip of the calicle with the coelenteron. This fold of tissue is known as the edge-zone. In some corals the septa are solid imperforate plates of calcite, and their peripheral ends are either firmly welded together, or are united by interstitial pieces so as to form an imperforate theca. In others the peripheral ends of the septa are united only by bars or trabeculae, so that the theca is perforate, and in many such perforate corals the septa themselves are pierced by numerous perforations. In the former, which have been called aporose corals, the only communication between the cavity of the edge-zone and the general cavity of the zooid is by way of the lip of the calicle; in the latter, or perforate corals, the theca is permeated by numerous branching and anastomosing canals lined by endoderm, which place the cavity of the edge-zone in communication with the general cavity of the zooid.