ORDER III. Alcyonaria (Octocoralla) comprise tl families Alcyonidae, Tubiporidm, Pennatulid; Gorgenidw, and Flelioporidae.
The Tubiporidle are known as the organ-pit corals ; the sea-pens and sea-rods form tl Pennatulidm ; and the Gorgonidm are known ; sea-shrubs.
The simplest corallum is most commonly cylix drical, conical, or turbinate (Caryophyllia, Tui binolia, Balanophyllia, etc.), but it may be ma or less compressed (Flabellum) ; also arborescei or dendroid (Madrepora, Dendrophyllia, Loph( helia, etc.), or in clusters of branches springin from a common base (Mussa, Caulastrlea, man species of Porites, Madrepora, etc.) ; others al foliaceous, as species of Madrepora, Poeillopor; Manopora, etc.
Amongst the more important reef-buildin corals may be mentioned the Astrceithe general] (Astrcea, Astrangia, Cladocora, Diploria, Moear drina, etc.) ; also the Madreporidte (Madrepon etc. ) ; the Poritidm (Porites, Goniopora, Mont.
porn, etc). ; many of the Oculinithe (Orbicella, Stytaster, I'ocillopora, etc.) ; the majority of the Fungidm and the Millepores. Various alcyonoid corals (Heliopora, Tubipora) also help with many of the Gorgonidm and the calcareous algtu (Nullipores and continues).
The structures raised by them are known as barrier reefs, fringing reefs, and atolls. Fringing reefs are in the immediate neighbourhood of land, in shallow water, either surrounding islands or skirting the shores of continents. Barrier reefs also may either encircle islands or skirt continents, but far from land. The barrier reefs occasionally surround islands. An atoll is a ring-shaped reef enclosing an open lagoon. óDarwin, Encyc. Britan. See Corals.