The skeleton or corallum of a typical solitary coral - the common Devonshire cupcoral Caryophyllia smithii (fig.
The corallum of Heliopora is of a blue colour, and has the form of broad, upright, lobed, or digitate masses flattened from side to side.
Though the corallum appears to live within the zooid, it is morphologically external to it, as is best shown by its developmental history.
As the microscopic character of the corallum of these extinct forms agrees with that of recent corals, it may be assumed that the anatomy of the soft parts also was similar, and the tetrameral arrange ment, when present, may obviously be referred to a stage when only the first two pairs of Edwardsian mesenteries were present and septa were formed in the intervals between them.
The interspaces between the tubes are filled up by a solid mass of lime, consisting chiefly of calcium carbonate, which replaces the chitinous perisare of ordinary hydroids and forms a stony corallum or coenosteum (fig.
The corallum shaded with dots, the mesogloea represented by a thick line.
The larvae of corals are free swimming ciliated forms known as planulae, and they do not acquire a corallum until they fix themselves.