ANTHOZOA (i.e., "flower-animals"), a group of animals belonging to the phylum Coelenterata (q.v.). Many of them form a strong skeleton known as "coral" (q.v.) . The term "coral" is applicable to the hard parts of any Coelenterate which secretes a firm skeletal support, but most of these forms are Anthozoa, and amongst them the term "true coral" is applied to the group Madreporar5a only. The flower-like shape and brilliant colouring of the soft parts of many of these creatures has attracted atten tion for hundreds of years, and the discovery that they are in deed animals instead of being plants, minerals, or intermediate organizations, did not gain acceptance for some time of ter it was first made, in connection with the precious red coral of corn merce, by J. A. de Peyssonel in 1727.
Not all the Anthozoa produce a skeleton. A hard support is quite lacking in the Sea Anemones, and among the other kinds of Anthozoa although it is present more often than not it does not necessarily form a mass sufficiently compact to retain its shape after the death of the soft parts.
An Anthozoan may consist of a single polyp (this term is de fined in the article COELENTERATA), as it does in the case of a Sea Anemone; but more frequently a colony is formed, containing a number of polyps permanently united together; and it is the skeletons built up by certain of these colonies which constitute the most characteristic "coral." The Anthozoa as a class are distinguished from the other groups of the Coelenterata not only by the structure of the individual polyps, but also by the fact that none of them at any time dur ing their life-history assumes the form of medusae ("jellyfish," see COELENTERATA). The Anthozoan individual or colony there fore corresponds to the polyp-generation of any Coelenterate which exhibits both polyp and medusa alternately during its life cycle. The Anthozoan polyps themselves, though of ten small, tend to be more muscular and substantial than those of other Coelenterates. They are characterized by the possession of a body which is in principle a cylinder, closed in above and below by two discs of tissue. The upper disc or peristome is encircled by a corona of hollow tentacles, and is perforated in the centre by a more or less slit-shaped mouth. Leading inwards from the mouth is a tube, the throat, which opens directly into the main cavity of the body (coelenteron). The latter cavity is partially subdivided into alcoves by a series of radially ar ranged membranous partitions, the mesenteries, some or all of which are inserted along the upper part of their inner edge into the outer wall of the throat. The mesenteries bear along their free edge a marginal thickening of epithelium known as the mesenterial filament, and also carry reproductive organs and muscles. The number and arrangement of the mesenteries as well as the structure of throat, mesenteries, and filaments, varies from one group of Anthozoa to another.
The symmetry of the Anthozoan body presents to the casual observer a radial appearance, in common with that exhibited by the Coelenterata in general. Underlying this radial symmetry, there becomes revealed on closer inspection a fundamental bilat eral symmetry, which is so definite that it is actually possible to divide a polyp into two perfectly equivalent halves along one plane of cleavage only. Much discussion has been aroused by this fact since there is no obvious reason why such a symmetry should exist at all. The probable explanation of the matter is that the An thozoa were not always the sedentary radially arranged creatures which they are to-day. There may have been a phase in their history when they were small creeping animals with definite head and tail ends, upper and lower sides, and general bilateral symmetry. When such animals adopted a sedentary life and assumed a radial symmetry in correlation therewith, the former bilateral condition would remain as a vestige of the former state of affairs. The bilateral arrangement of parts is very prominent during the early development of a polyp.
An important fact in the development of many Anthozoa is that the wall of the cylindrical body appears to become marked out of ter the early stages are passed, into vertical strips of which some are zones in which active differentiation of new parts takes place, others being zones in which, after a stated amount of struc ture has been formed, no new parts are added (fig. I). In various groups of Anthozoa the arrangement of the mesenteries in the adult polyp depends on the relationship which the zones of active growth in the body-wall bear to those in which only a certain amount of differentiation of parts will take place. In other cases no new growth takes place after the early stages, and here a simpler plan of structure consequently prevails.
The axis of symmetry sometimes possesses a distinct signifi cance with regard to the regeneration of parts of an adult polyp which have been separated from the whole. In the Sea Anemones this axis coincides with a non-growing zone, and if in a suitable anemone a fragment of reasonable size be cut away from the edge of the animal's base, in such a way that the lower ends of the two "directive" mesenteries which flank the axis lie in its centre, the piece will in many cases regenerate an animal with two heads instead of one. A similar piece containing no direc tives will regenerate an ordinary one-headed adult.
The living Anthozoa fall into six principal series, the Alcyonaria, Actiniaria, Madreporaria, Zoanthinaria, Ceriantharia, and Anti patharia. These sub-classes are distinct from one another. Space forbids consideration of the three last groups mentioned, which contain relatively few and aberrant representatives. The three large groups will now be described.