The second great division of living Adinozoa is that of the Alcyonaria, defined by the possession of polypes with eight pinnately -fringed tentacles, the mesenteries and somatic chambers being also a multiple of four (eight). The corallum, when present, is usually sclerobasic, or spicular; if "thecae" are present, as is rarely the case, septa are wanting or rudimentary.
The Alcyonaria or "Asteroid Polypes" differ numerically from the Zoantharia in having their soft parts arranged in multiples of four, instead of five or six, as in the latter, whilst the septa are not in pairs. Their tentacles, too, are pinnate, and are not simply rounded. Numerically, the Alcyonaria agree with the extinct order Rugosa ; but the latter invariably possess a well-developed sclerodermic corallum, the thecae of which exhibit either septa or tabulae, or both combined.
With the exception of two genera (Haimeia and Hartea), both of which are possibly founded upon immature forms, the Alcyonaria are all composite, the tubular polypes being united by a coenosarc, and their body-cavities being placed in communication by means of anastomosing canals, which ramify in the coenosarc, and permit of a free circulation of nutrient fluids. The form of the colony differs greatly in different cases, but none possess the power of independent locomotion, most being rooted to foreign objects, or sunk in the mud, whilst some float freely in the sea. The polypes, in most of the essential points of their organisation, agree with those of the Zoantharia, the mouth opening into a tubular stomach, which in turn communicates freely with the body-cavity, and the stomach-sac being connected with the body-wall by means of a series of vertical membranous laminae or "mesenteries." The mesenteries, however, are only eight in number, and are not paired, one of the tentacles corresponding with and opening into each intermesenteric chamber. A corallum may be wanting, and when present its structure varies. In some cases, lastly, it has been shown that the actinosoma normally consists of two kinds of polypes - one sexual, the other sexless and permanently rudimentary. The Alcyonaria are divided into five families - viz., the Alcyonidae, the Tubiporidae, the Pennatulidae, the Gor-gonidae, and the Helioporidae.