The polypes of the stony corals, though extremely small, are essentially the same in structure as the Sea Anemone, but they have no sucker at their base. The Sea Anemone is of soft tissue throughout its whole body. In the polypes of the madrepore corals, on the contrary, the whole of the perpendicular lamellæ which divide the interior of the body into chambers become hard, from being consolidated by particles of carbonate of lime; and their upper edges, which appear as rays round the mouth of the animal, give that starry appearance to the surface of dead madrepores after the soft part of the polypes has been destroyed.
Most of the coral polypes are unarmed; but in some, as, for example, the Caryophyllia Smithii, there are multitudes of dart-cells in the tentacles, besides numerous pellucid filaments or ribbons, full of thread-cells, lying in coils within the chambers which surround the stomach.
We are indebted to Mrs. Thynne’s interesting observations on the Caryophyllia Smithii in her aquarium for the life-history of the animals armed with this formidable artillery. This madrepore, which inhabits many parts of the European seas, at various depths, is a species of the only lamelliform genus of corals which range beyond the tropics. It is a solitary individual polype, with an external calcareous cylindrical coat, wider at the base, when it is fixed to a rock; and the mouth, which has several rows of tentacles, is in the centre of the disk of the cylinder. The tentacles are delicate, transparent, granular tubes, about an inch long, tapering to their extremities, and ending in an opaque white knob full of chambered thread-cells with their darts; but the thread-cells are of a larger size in the ribbons coiled in the chambers round the stomach of the animal. These madrepores are described by Mrs. Thynne as of various tints, from a pure white to a bright apricot colour. At intervals they eject from the mouth a whitish blue fluid, resembling wood smoke, in a stream three or four inches long, sometimes containing a few eggs. But the eggs, though no doubt formed at the base of the lamellæ, become densely packed like fine dust in the hollows of the tentacles, from whence they are expelled by contractions, and escape by the mouth. The eggs lie quiet for a few days in the place where they are deposited: by and by they begin to rotate, slowly at first, then more rapidly, and finally they are developed into most minute madrepores, with the star and colour of the parent. In a few months they become as large as a crown piece, with a very wide mouth and a membranous integument or covering, for they do not get their hard calcareous coat till they are two years old. While in that soft state they propagate by spontaneous division, which always begins at the mouth, and is repeated every few weeks during the second year of their lives. When they split into segments, the broken ends of each segment bend round and unite; and the mouth, which at first is on one side, being a portion of the old one, comes to the centre of the disc, and in addition to the few old tentacles that remain, new ones are added, with their interior chambers, till they amount to five rows, and in this manner a brood of young Caryophylliæ is formed.
Fig. 130. Lobophylla angulosa.
During the second year of their soft state, these madrepores increase by budding. The buds spring from the base of the membranous covering, they expand, get a mouth and tentacles, aid in feeding themselves by greedily taking any small particles of animal food offered to them, and seem also to share in the sustenance provided by the mother, as they dilate when she is fed; ultimately they separate from her. These madrepores have patches of a milk-white fluid substance, which unite and almost cover the space between the mouths and the rows of tentacles: in others of the madrepore tribe these patches are purple, green, yellow, or ultramarine blue. The Caryophylliæ have locomotion while their skin is soft, but no activity; they merely avoid obstacles, and move away from one another; but, as soon as they get their hard calcareous coat, they become permanently fixed, and no longer undergo division or gemmation, but lay eggs.[[29]]
The European Caryophylliæ never have more than one star, but sometimes a great many individuals are united in a spreading bunch, as in the madrepore Lobophylla angulosa ([fig. 130]), or in a branched or tufted mass. Their exterior is invariably striated, and each terminates in a star, with the polypes, mouth and tentacles in its centre. These compound madrepores are inhabitants of warm seas.
The number of tentacles possessed by the Actinian polype varies with the species of the coral. When full grown they have twelve, twenty-four, even forty-eight, or more. When young, they have only four or six, but in general the number increases rapidly as they advance in age. The perpendicular hard lamellæ, which divide the cavity round the stomach of the polype into perpendicular chambers, as in [fig. 129], and form stars round the mouth, consist of thin sheets or plates, either applied or soldered together; and for every new tentacle that is produced at the mouth, a corresponding new chamber is formed immediately below it, between the sheets or leaves of the lamellæ; so that the number of chambers and perpendicular plates is always equal to the number of tentacles, and so the circulation of the fluids is maintained. Since the upper edges of the lamellæ form the rays of the stars round the mouth of the polype, it is clear that the number of rays in a star must always be equal to the number of lamellæ. The new tentacles are always produced exterior to and between the adjacent old ones, so as to form an outer circle, and consequently a new circle of rays will be added to the star round the mouth exterior to the old ones. There may be two, three, or more concentric circles of tentacles round the mouth of the polype, the last being the shortest. However, some polypes never have more than twelve tentacles during the whole course of their lives. The first formed rays of a star are generally, though not always, the longest and most prominent; and sometimes the edges of the lamellæ rise high above the hollow or cup which is the centre of the star, and contains the mouth of the polype. In some families of corals these edges, which form the rays, are toothed or spined.[[30]]
A horny column in the axis of the polype, hardened by sulphate and carbonate of lime, and called the columella, generally shows its top in the centre of the star, and varies in structure in the different genera. Thus the Actinian polypes may be said to possess an internal skeleton, and as they approach maturity they also acquire an external one in the form of a cylindrical coat, or stony wall, which surrounds them, and into which most of them can withdraw the soft upper part of their bodies and tentacles, so as to be partly or altogether concealed. The perpendicular lamellæ are sometimes extended through the stony walls of the polype, so as to form a series of broad, well-developed ribs on its exterior surface.
The stony substance of corals is chiefly carbonate of lime, which the polypes have the power of abstracting from the sea-water, combined with a small quantity of animal matter, and a still smaller quantity of phosphate of lime, with a trace of silver and magnesia. This stony substance takes the crystalline form of needles. By the successive deposition of these needles, a network is formed round the body of the animal, which by a series of these deposits is condensed into a hard impervious coat or wall. During this formative process many characteristic forms may be produced by division and building, depending upon the genus and species of the polype; but they do not lay eggs till they come to maturity.