A typical simple sclerodermic corallum (fig. 74) is secreted by a single polype, and its structure presents an obvious correspondence with that of the animal which produces it. It is generally more or less conical in shape, sometimes discoid, consisting of an outer wall and included space. The wall is secreted by the mesoderm of the column and base, and is known as the "theca." It may be very imperfect, or may be strengthened by a secondary calcareous investment ("epitheca.") The theca encloses a space which corresponds with the lower part of the body-cavity of the polype, and is known as the "visceral chamber." Superiorly the theca terminates in a shallower or deeper cup-shaped depression, which contains the stomach-sac of the polype, and is known as the "calice." Below the calice, the visceral chamber is subdivided into a number of vertical compartments ("loculi") by a series of upright partitions or "septa," which spring from the inner surface of the theca, and are directed inwards towards the centre. The septa are calcifications formed within the mesenteries, to which, therefore, they correspond in number and size. Like the mesenteries, the septa are thus "primary," "secondary," and "tertiary," according to their width (fig. 75).
Fig 74. - Caryophyllia borealis. A simple sclerodermic Coral, twice the natural size. (After Sir Wyville Thomson.)
Fig. 75. - Diagram of the arrangement of the septa in the Zoantharia sclerodermata and Rugosa. A, Transverse section of a simple sclerodermic Coral (Turbinolid), showing the theca, with its projecting ridges or "costae" outside, the visceral chamber and radiating septa inside, and the columella in the centre. B, Transverse section of a simple Rugose Coral (Cyathophyllum), showing the wall, costae, and septa.
The septa in the adult sclerodermic corallum are typically some multiple of six in number, arranged in six systems; but this rule is not of universal application, and the typical hexameral arrangement may be departed from altogether. The laws of development of the septa are complicated, and need not be discussed here. On the outside of the theca are vertical ridges ("costae"), corresponding with the septa within; and the centre of the visceral chamber may be vacant, or may be occupied by an axial rod-like structure, which is termed the "columella." The continuity of the "interseptal loculi" is liable to be more or less interfered with by the development of the structures known as "synapticulae" "dissepiments," and "tabulae." The "synapticulae" are transverse calcareous bars which stretch across the interseptal loculi, and form a kind of trellis-work, uniting the opposite faces of adjacent septa. They are characteristic of the Fungidae. The "dissepiments" are commonly present in a great many corals, and have the form of incomplete, approximately horizontal plates, which stretch between adjacent septa, and break up the interseptal loculi into secondary compartments or cells. Lastly, the "tabulae" may be regarded as highly developed dissepiments, and, like them, are approximately horizontal, as a rule, at any rate. They differ from the dissepiments in the fact that they cut across the interseptal loculi at the same level. When fully developed (fig. 76, D), they are transverse plates, which extend completely across the visceral chamber, and divide it into a series of storeys placed one above the other, the only living portion of the coral being above the last - formed tabula. Tabulae are found in various of the Zoantharia sclerodermata, in some of the Alcyonaria, and in a great many of the Rugosa. The above gives the general structure of a typical simple sclerodermic corallum, as secreted by a single polype. A compound sclerodermic corallum is the aggregate skeleton produced by a colony of such polypes, and varies in form and size according to the characters of the colony by which it is produced. In general, such a colony consists (fig. 77) of a number of polypes, which may spring directly from one another, or may be united by a common flesh or coenosarc; and corresponding elements are found in the corallum. In the former instance, the compound corallum consists of an assemblage of separate "corallites," as the skeletons of the individual polypes are called, these being united with one another directly and in various ways. In the latter instance, the corallum consists of a number of "corallites," and of a common calcareous basis or tissue, which unites the various corallites into a whole, is secreted by the coenosarc, and is known as the "coenen-chyma."
Fig. 76. - A, Portion of the corallum of Pocillopora aspera, var. lata, Verrill, of the natural size. B, Part of the surface of same, enlarged. C, Section of the corallites of the same, showing the columella, enlarged. D, Vertical section of the same, enlarged, showing tabulae. (After Dana.)
The compound coralla are, of course, primitively simple, and they become composite either by budding or by cleavage of the original polype. The following are the principal methods in which this increase is effected; and in considering this subject briefly, it will be as well to take into account not only the Zoantharia sclerodermata, but also the Rugosa, the modes of increase in the two groups being very similar: (1.) Lateral or parietal gemmation. - In this mode of increase the original polype throws out buds from some point on its sides between the base and the circle of tentacles, and these buds on becoming perfect corallites may repeat the process. This is one of the commonest modes of growth amongst the recent corals, and it gives rise chiefly to dendroid or tree-like corals.
Fig. 77. - Astraea pallida, a compound sclerodermic Coral, in its living condition.
(2.) Basal gemmation. - In this method the original polype gives forth from its base a rudimentary coenosarc, from which new buds are thrown up, and which may have the form of foot-like prolongations or of a continuous horizontal expansion. The resulting coralla are usually massive or encrusting, and the youngest corallites are, of course, those placed on the periphery of the colony.
(3.) Calicular gemmation. - This consists in the production of buds from the calicine disc of the parent corallite, which may or may not continue to grow thereafter, whilst the new corallites thus produced generally repeat the process. This mode of growth is exceedingly rare amongst the Zoantharia sclerodermata, and is never typically exhibited; but it is a characteristic feature in many of the Rugose corals. In many of these (fig. 78), the original polype throws up from its calicine disc one or more new corallites, which kill the parent. These, in turn, produce others after a similar fashion, till the entire corallum assumes the form of an inverted pyramidal mass resting upon the original budding polype. In other Rugose corals the calicine disc gives off but a single bud, which may repeat the process indefinitely till the corallum presents the appearance of a succession of inverted cones placed one above the other.
Fig. 78. - Calicular gemmation as seen in Lonsdaleia floriformis. Carboniferous.
(4.) Fission. - This process in the coralligenous Actinozoa is usually effected by "oral cleavage," the divisional groove commencing at the oral disc, and deepening to a greater or less extent, the proximal extremity always remaining undivided. According to Dana, in fission a new mouth is formed in the disc near the old mouth, and a new stomach is formed for the new mouth, round which the new tentacles are then developed. This, therefore, is not, strictly speaking, a subdivision into halves; since one half carries off the old mouth and stomach. More rarely, fission "is effected by the separation of small portions from the attached base of the primitive organism, whose form and structure they subsequently, by gradual development, tend to assume."
"The coral-structures which result from a repetition of the fissiparous process are of two principal kinds, according as they tend most to increase in a vertical or in a horizontal direction. In the first of these cases the corallum is caespitose, or tufted, convex on its distal aspect, and resolvable into a succession of short diverging pairs of branches, each resulting from the division of a single corallite." In the second case the coral becomes lamellar. "Here the secondary corallites are united throughout their whole height, and disposed in a linear series, the entire mass presenting one continuous theca." Both these forms of corallum are "liable to become massive by the union of several rows or tufts of corallites throughout the whole or a portion of their height. An illustration of this is afforded by the large gyrate corallum of Meandrina, over the surface of whose spheroidal mass the calicine region of the combined corallites winds in so complex a manner as at once to suggest that resemblance to the convolutions of the brain which its popular name of Brain-stone Coral has been devised to indicate " (Greene).
The Zoantharia sclerodermata are divided into the two following groups, founded upon the characters of the corallum: