This skeleton is largely composed of a number of radiating plates or septa, and it differs both in origin and structure from the calcareous skeleton of all Alcyonaria except Heliopora.
Nearly all the Alcyonaria are colonial.
These are the Alcyonaria and the Zoantharia.
The order Coenothecalia is represented by a single living species, Heliopora coerulea, which differs from all recent Alcyonaria in the fact that its skeleton is not composed of spicules, but is formed as a secretion from a layer of cells called calicoblasts, which originate from the ectoderm.
The arrangement of the muscle-banners on the mesenteries is characteristic. On six of the mesenteries the muscle-banners have the same position as in the Alcyonaria, namely, on the sulcar faces; but in the two remaining mesenteries, namely, those which are attached on either side of the sulcus, the muscle-banners are on the opposite or sulcular faces.