In Actinia, as in all Anthozoan zooids, the coelenteron is not a simple cavity, as in a Hydroid, but is divided by a number of radial folds or curtains of soft tissue into a corresponding number of radial chambers.
CORAL, the hard skeletons of various marine organisms. It is chiefly carbonate of lime, and is secreted from sea-water and deposited in the tissues of Anthozoan polyps, the principal source of the coral-reefs of the world (see Anthozoa), of Hydroids (see Hydromedusae), less important in modern reef-building, but extremely abundant in Palaeozoic times, and of certain Algae.
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.
In the common sea-anemone, Actinia equina (which has already been quoted as a type of Anthozoan structure), the mesenteries are numerous and are arranged in cycles.
Stylaster has cyclosystems. In Allopora the cyclostems resemble the calyces of Anthozoan corals.