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.