The Actinozoa are defined as Coelen-terata with a differentiated digestive sac opening below into the somatic cavity, but separated from the body walls by an intervening "perivisceral space" which is divided into a series of compartments by vertical partitions or "mesenteries," to the faces of which the reproductive organs are attached.
The Actinozoa (fig. 69), therefore, differ fundamentally from the Hydrozoa in this, that whereas in the latter the digestive cavity is identical with the somatic cavity, in the former there is a distinct digestive sac, which opens, indeed, into the somatic cavity, but is, nevertheless, separated from it by an intervening perivisceral space. As a result of this, the body of a typical Actinozoon (fig. 68) exhibits on transverse section two concentric tubes, one formed by the digestive sac, the other by the parietes of the body; whereas the transverse section of a Hydrozoon exhibits but a single tube, formed by the walls of the combined digestive and somatic cavity.
Histologically, the tissues of the Actinozoa are essentially the same as those of the Hydrozoa, consisting of the two fundamental layers, the "ectoderm" and the "endoderm." In the Actinozoa, however, there is a much greater tendency to a differentiation of these into specialised structures, and in some members of the class muscular fibres are well developed. Thus, the Sea-anemones have a well - developed series of longitudinal and circular muscular fibres, of which the former become radial in the disc and base. Cilia are often present, especially in the interior of the somatic cavity, where they serve to promote a circulation of the digestive fluids contained therein. The sole digestive apparatus in the Actinozoa consists of a tubular stomach-sac, which communicates freely with the outer world by means of the mouth, and opens inferiorly directly into the general body-cavity. In most, the "perivisceral space" between the body-walls and the digestive sac is subdivided into compartments by a series of vertical lamellae, which are called the "mesenteries" (fig. 68, m). Upon the faces of these are borne the reproductive organs in the form of band-like ovaria or spermaria. There are no differentiated respiratory organs as a rule. Some forms, however, which live half buried in sand or mud, have lobed and crimped organs attached to or near the tentacles, which have been supposed to act as breathing-organs; whilst structures supposed to be gills are developed in some Zoanthids on either side of the primary mesenteries.
Fig. 68 - A, Transverse section of a Hydrozoon, showing the body-cavity in the form of a single tube enclosed by the body-walls. B, Transverse section of an Actinozoon: s Digestive sac ; m One of the primary mesenteries, dividing the body-cavity into vertical compartments. Between the six primary mesenteries are seen the secondary and tertiary mesenteries, which fall short of the walls of the stomach. a Ectoderm: b Endoderm.
Thread-cells, often of very complicated structure, are almost universally present, some few forms having been asserted to be without them; and some of the Actinozoa are able to sting severely.
A nervous system has not yet been proved to exist in the Actinozoa generally, except in the Ctenophora, and in none are there any traces of a vascular system. Some Actiniae are said to have short optic nerves distributed to the pigment-masses at the bases of the tentacles, and these masses are clearly of a sensory nature; whilst the same animals are affirmed to have a general nervous system as well.
Distinct reproductive organs occur in all the Actinozoa, but these are internal, and are never in the form of external processes as in the Hydrozoa. Sexual reproduction occurs in all the members of the class, but in many forms gemmation or fission constitutes an equally common mode of increase. Some Actinozoa, therefore, such as the common Sea-anemones, are simple organisms; whilst others, such as the reef-building corals, are composite, the act of gemmation or fission giving rise to colonies composed of numerous zooids united by a coenosarc. In these cases the separate zooids are termed "polypes," the term "polypite" being restricted to the Hydrozoa. In the simple Actinozoa, however, the term "polype" is employed to designate the entire organism. In other words, the "actinosoma," or entire body of any Actinozoon, may be composed of a single "polype," or of several such, produced by a process of continuous gemmation or fission, and united by a common connecting structure, or coenosarc.
Fig. 69. - A, Actinia mesembryanthemum, one of the Sea-anemones (After Johnston); B, section of the same, showing the mouth (a), the stomach (A), and the body-cavity (c).
Most of the Actinozoa are permanently fixed; some, like the Sea-anemones, possess a small amount of locomotive power; and one order, the Ctenophora, is composed of highly active, free - swimming organisms. Some of the Actinozoa are unprovided with any hard structure or support, as in the Sea-anemones and in all the Ctenophora; but a large number secrete a calcareous or horny, or partially calcareous and partially horny, framework or skeleton, which is termed the "coral," or "corallum."
The Actinozoa are divided into four orders - viz., the Zoantharia, the Alcyonaria, the Rugosa, and the Ctenophora; but the last is sometimes placed amongst the Hydrozoa, and it has been recently proposed to remove the Rugosa also to the same class.