Marine Coelenterata either free or fixed, simple or colonial. The mouth is an elongated slit in the centre of an oval disc or peristome which bears one or more circles of hollow tentacles. It leads into an oesophagus or stomo-daeum of some length which projects into the gastric cavity, and is united to the body-wall by radial lamellate mesenteries. The generative organs are situate on the mesenteries and are of endodermic origin. There are no organs of special sense except isolated sensory cells.
The. body is usually columnar, rarely disc-like, terminated by the peristome at one end, and in many simple Anthozoa by a pedal disc or base at the other. But in most colonial forms its true shape is masked, the oral part only remaining free. It has a distinctly radiate appearance, but the axis of the mouth indicates a line of bilateral symmetry which divides it into two halves. The two ends of this line may be anatomically dissimilar owing to the structure of the corresponding mesenteries, hence leading to a distinction of a dorsal or axial from a ventral or abaxial aspect. The mouth is rarely circular. It is usually closed with the exception of one end, or of both ends which remain open, leading into well marked grooves strongly ciliated known as gonidial grooves or siphono-glyphes. The oesophagus is an ectodermic involution. It leads into a central cavity the size and length of which vary. The part of the cavity which surrounds the oesophagus is divided into a series of radial chambers, completely or incompletely isolated by the mesenteries. These structures are covered by endoderm supported by a mesoglaeal lamella continuous with that of the body-wall, and with that of the oesophagus when they are, as some always are, complete.
The mesenteries which do not reach the oesophagus are termed incomplete. When the mesenteries are paired, the two members of every pair inclose a space which is known as intra-septal, the spaces between adjacent pairs being termed inter-septal1. Their free edges are usually bordered by thickenings or mesenterial filaments, more or less convoluted. They are ciliated, and are sometimes entirely, sometimes partly, glandular, and usually contain nema-tocysts. They are applied to the food when swallowed and their glands secrete the digestive fluid. The tentacles are evaginations of the oral disc, simple or compound, contractile, sometimes invaginable. The peristome and tentacles, and sometimes the fore-part of the body, are retracted by special longitudinal retractor muscles developed in the endoderm on one aspect of all or some of the mesenteries.
Nematocysts are always present on the tentacles and in the mesen terial filaments, in the stomodaeum, and in the Zoantharia Actiniaria also on the body-wall and peristome. It appears to be rare for them to occur on the surface of the colony as they do in the Alcyonarian Heliopora. Their size varies, and they have usually simple threads: but in some Actiniaria, and in the Madrepore Caryophyllia the thread is beset with spirally arranged barbs of large size near its base.
The colonial Anthozoa are produced by gemmation or fission from a solitary form. The individuals or zooids may originate from a basal stalk or stolon, or from a lamella, in which case they usually remain isolated. But they often spring from the wall or side of the first zooid, and then usually form a massive colony in which the individuals are united by a plentiful common basis or coenosarc. They are generally connected by canals variously arranged arising from their gastric cavities. The latter, however, ane sometimes imperfectly separated. In some instances the zooids become completely isolated.
A skeleton is present in many simple and nearly all colonial forms.
1For these spaces Mr. Fowler has suggested (Q. J. M. xxv. p. 578) the terms 'entocoele' and 'exocoele' respectively.
It may be (I) discontinuous in the shape of calcareous spicules, or (2) continuous and then (i) organic and horny, (ii) both organic and inorganic, i. e. calcareous, or (iii) solely calcareous. The discontinuous skeleton coexists with the continuous either (i) or (ii) stcpra, but never with the lamellate form of (iii) supra. When the skeleton forms calycles, or tubes lodging the zooids of a colony, the deeper parts of the tubes or calycles may be successively closed off by calcareous partitions, usually horizontal and known as tabulae. Tabulae occur in Tubipora, and Heliopora among Alcyonaria, in many extinct Rugosa and a few living corals among Zoantharia1. The skeleton-producing cells appear to be derived from the ectoderm (except in Pennatulidae ?); they may remain in continuity with it or be detached from it, in this case becoming mesoglaeal. The individual or colony may be either of separate sexes, or hermaphrodite2. Fission and gemmation may occur in solitary forms. There are two perfectly distinct sub-classes, the Alcyonaria and Zoantharia.