- Metagenetic colony-forming Hydromedusae, in which the polyp-colony forms a massive, calcareous corallum into which the polyps can be retracted; polyp-individuals always of two kinds, gastrozoids and dactylozoids; gonosome either free medusae or sessile gonophores.
- Portion of cyclosystem is covered by a cap or operthe corallum of A stylus culum.
- Portion of the calcareous corallum of Millepora nodosa, showing the cyclical arrangement of the pores occupied by the " persons " or hydranths.
The most familiar Anthozoan is the common sea-anemone, Actinia equina, L., and it will serve, although it does not form a skeleton or corallum, as a good example of the structure of a typical Anthozoan polyp or zooid.
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.
- Corallum of Caryophyllia; semi-diagrammatic. th, Theca; c, costae; sp, septa; p, palus; col, columella.
Though the corallum appears to live within the zooid, it is morphologically external to it, as is best shown by its developmental history.
The larvae of corals are free swimming ciliated forms known as planulae, and they do not acquire a corallum until they fix themselves.
The corallum shaded with dots, the mesogloea represented by a thick line.
In the former case the young daughter zooid, with its corallum, arises wholly outside the cavity of the parent zooid, and the component parts of the young corallum, septa, theca, columella, &c., are formed anew in every individual produced.
On the other hand, the study of the anatomy and development of the zooids has thrown much light upon the manner in which the corallum is formed, and it is now possible to infer the structure of the soft parts from a microscopical examination of the septa, theca, &c., with the result that unexpected relationships have been shown to exist between corals previously supposed to stand far apart.
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.