Two mesenteries only-are complete, and they correspond to the long axis of the body: they have mesenterial filaments, and carry the sexual products. Two short mesenteries correspond to each end of the shorter diameter, and two still more reduced lie one on either side of each complete mesentery. There are con-sequently ten mesenteries in all. A canal traverses the colony beneath the bases of the zooids, but whether or not it is in connection with them is uncertain. The surface of the colony secretes in Gerardia and some species of Antipathes a viscid mucus which entangles foreign bodies, such as spicules of Axifera, and Sponges, grains of sand. The nematocysts of the ectoderm in the two genera named are aggregated into groups. The sexes appear to be separate.
1In large specimens the mesenterial filaments undergo a change close to the lower border of the stomodaeum. They are drawn out into a number of simple or branched 'mesenterial threads.' These structures commence as small papillate projections of the mesentery, bordered of course by the mesenterial filament. The papillae grow out, become filiform, and may branch. Hence a cross section of such a thread shows as it were two mesenterial filaments back to back. An acontium differs from a mesenterial thread (I) in being attached to the surface, not the edge, of a mesentery; (2) in being simple and filiform, and single in cross section; and (3) in containing very large numbers of nematocysts.
The Madreporaria are simple or colonial, and differ most markedly from other Zoantharia in the presence of a continuous calcareous skeleton secreted by cells or calycoblasts, which either actually are, or represent the basal ectoderm1. The skeletal structures are therefore, strictly speaking, external to the animals to which they belong. The form of the skeleton is very variable. In a typical simple, i. e. non-colonial Madreporarian, it consists of a theca, which is as a rule attached by a base, large or small, to some foreign object, but it may, as in Flabelhun, etc, become free when adult. The theca is excavated terminally by a depression, the calycle or calice in which the animal is lodged, the soft parts extending over its lip to a variable extent. The theca is sometimes flattened, e.g. Bathyactis, Fungia, and it may then, as in the genera named, be covered completely on its lower surface by soft tissues. The inner aspect of the calycle is not smooth, but bears a number of radial calcareous ridges or septa, differing in size and extent, and accordingly distinguishable into systems, primary, secondary, etc. Certain of these septa may meet in the centre of the cup, where they form a pillar or columella, which may be prolonged more or less upwards.
They may unite together in various degrees, and there may be at or near their central ends one or more circles of plate-like upgrowths, known as pali. The outer surface of the theca may be marked with ridges or costae, which do or do not correspond to the septa, and are separated by intercostal spaces: it may be covered to a greater or less height by an epitheca or calcareous layer distinct from the theca itself, which is sometimes extended into radicles2. In the colonial forms the shape of the colony depends on the mode of growth and multiplication of the individual; it is massive, branched, lamellate, cup-shaped, etc. The soft parts of the zooids may become independent, or remain in continuity over larger or smaller regions. As the coral grows, portions of the skeleton often become exposed to a variable extent, and the exposed surface, like the exposed surface of a simple coral, is attacked by the water, and by organisms, vegetable or animal. The calycles are in the majority of colonial corals connected by a calcareous coenenchyma or common skeleton, which is either perfectly solid, or excavated by internal spaces left during growth, but not connected with the calycles nor open in the macerated state; or it is traversed within and without by tubular channels which lodge in the living condition canals connecting the gastric cavities of the zooids, and therefore in the macerated skeleton open superficially.
Hence the division of Madreporaria into two great sections, the Aporosa and the Perforata. The skeleton itself is composed of calcareous ellipsoids, themselves made up of typical rhombs of Calcite. Little is accurately known as to its development. In Astroides calycularis it appears first as a calcareous ring beneath the base of the solitary larva and the foreign object to which it is fixed. The ring becomes a disc, and on the upper surface of the disc are formed twelve radial ridges, the first septa, which bifurcate at their outer extremities. They correspond to vertical folds of the base of the zooid, within the hollow of which they are laid down. It is a disputed point whether or not the wall or theca arises by a fusion of the outer ends of the septa (von Koch) or is a ring-like thickening of the base independent of the septa (de Lacaze Duthiers). The epitheca appears at the spot where the ectoderm of the base passes into the ectoderm of the wall of the zooid. Of the twelve septa, six grow more rapidly at first than the remaining six, a difference afterwards equalised. A second series of twelve septa, and then a third of twenty-four appear. The young Astroides begins to bud and form colonies.
In the larger calycles the second set of twelve septa become equalised with the first twelve, and a fourth series of forty-eight septa is intercalated. An even and regular development of septa in cycles is probably the rule: but (I) cycles may become equalised, and (2) irregularities affecting a part or the whole of a calycle may occur. Hence there are many difficulties to be encountered in interpreting the skeleton of coralsl. Moreover 'nearly allied
1 Vegetable parasites are often found in the coralla of Madreporaria, Heliopora, and the coenosteum of Millepora. See Moseley, 'Hydrocorallinae,' Challenger Reports, ii. p. 30; Martin Duncan, P. R. S. xxv. 1877, and Quart. Journal Geol. Soc. xxxii. 1876.
2For definitions of descriptive terms relating to Corals, see Martin Duncan, J. L. S. xviii. pp. 200-2.
1Von Koch has recently given an account (M. J. xii. (I) 1886) of the morphology of the Madreporarian skeleton according to his latest views. He regards (1) the epitheca as formed on the outer surface of the wall of the zooid, (2) the theca as concentric with it, and (3) the two structures as separated by spaces crossed by mesenteries and continuous with the intra-calycular portions of the inter- and intra-septal chambers at the free edge or lip of the theca. The theca and epitheca are, however, never separated, so far as the specimens and descriptions accessible to me show, by anything more than a minimal space, and there is no evidence for the existence of soft pails between them. The epitheca appears to be secreted by the free edge of the soft parts or limbus covering the theca. At present it is by no means a settled point whether or no the theca may grow in such a way that it cuts the mesenteries, as von Koch contends it does, into an extra- and an intra-thecal portion, or whether the edge or limbus of the zooid simply overlaps the edge of the theca. It is probable that both modes of growth occur.