The animal has been bisected vertically: and one of the halves thus obtained has been suspended with the peristome or mouth-disc pointing to the right, and the base of attachment or pedal disc to the left. The preparation shows the chief features characteristic of the class Anthozoa, as well as of the Actiniaria, to which suborder the Sea-Anemone belongs.
The animal creeps about upon the base, which is here much contracted, so that the free edge or limbus is scarcely discernible. At the limbus the base passes into the wall or column, which is naturally more or less straight but owing to muscular contraction is convex in this specimen. The wall has a distinct margin where it passes in its turn into the peristome which supports a marginal series of tentacles and has the mouth in its centre. The tentacles are contracted to short conical stumps. In the living animal they appear to be arranged in four circles, but are really dis-posed in five. Of these the innermost contains forty tentacles; the fourth twenty; the third ten, and the two external circles five apiece. The area of the peristome is naturally flat or even slightly concave. It has a slight furrow concentric with the mouth, which may be seen on the surface of the section to correspond with a round spot, the cut surface of a strong cir-cumoral sphincter muscle. The mouth is naturally a long slit, the edges of which are kept apposed, save at the two extremities or oral angles.
The margins of these angles are prominent, and the open spaces they border are the pharyngeal or oesophageal grooves - the gonidial canals of Gosse - or si-phonoglyphes of Hickson. These grooves or furrows are lined with long cilia which create currents of water to, and perhaps from, the interior. They cannot be seen here. The tube which leads inwards from the mouth is the oesophagus or stomodaeum. Its lower edge is at some little distance from the base, and is prolonged into two lappets which correspond to the oesophageal grooves. Food passes down it into the central region of the coelenteric space. This space extends outwards and upwards between the oesophagus and the wall, and into the tentacles, at the tips of which it opens by a pore. But the space in question, as may be seen here, is not a simple space: it is broken up into a series of radial chambers by radial mesenteries or sarco-septa, one of which is reflected at the lower part of the preparation. These mesenteries are attached above to the peristome, below to the pedal disc, and externally to the wall, of which they are really processes. A certain number of them are complete, i.e. are attached to the stomodaeum; they are sometimes termed primary. The remainder fail to reach the stomodaeum and are hence incomplete.
Some fall short of reaching it by a little distance; others by a greater, etc.: hence they are often termed secondary, tertiary, etc. The mesentery exposed in the upper part of the preparation is complete: the one reflected in the lower part is incomplete. As the greater portion of the substance of the wall has been removed, the external edges of the mesenteries are clearly visible and it may be seen how great is their number. They are really grouped in pairs, and the space between the two members of a pair is known as the intra-septal, that between two adjacent pairs as the inter-septal, chamber. The pairs are also so grouped that between two primary pairs there is one secondary pair; between a primary and a secondary pair one tertiary, and so on.
If the complete mesentery in the upper part of the preparation is carefully examined it will be seen to have two perforations. One of these, the inner septal stoma, lies about 1/4 inch from the circumoral sphincter and is found universally among Sea-Anemones piercing the primary or complete mesenteries. The other perforation or outer septal stoma occurs in very few instances. It pierces all the mesenteries and lies just within the little curve made by the margin external to the outermost tentacle. Its presence indicates the existence of a very well developed marginal sphincter,'Rotteken's ring-muscle,' which contracts the margin over the tentacles when the peristome is retracted, but it is, however, not visible in this preparation.
The incomplete mesentery which has been reflected has a thickened inner free margin. This thickening is the mesenterial filament or craspedon; it is slightly convoluted. The free edges of all the mesenteries, complete and incomplete alike, are similarly bordered; and the convolutions of the filaments are very visible on the inner surface of the coelenteric space or stomach below the free edge of the stomodaeum.
All Anthozoa possess a wall, a peristome with marginal tentacles, a stomodaeum, and a coelenteric space subdivided by mesenteries. The Actiniaria or Malacodermata possess simple non-pinnate tentacles; the number of their mesenteries is usually some multiple of the number six; and there is no hard skeleton. But some corals, e.g. Caryophyllia cyathus and Madrepora variabilis (Koch, M. J. v. 1880), possess the same paired mesenterial arrangement as do the Actiniae.
The oesophagus or stomodaeum is formed as an invagination of the oral disc, and is consequently lined by ectoderm. In the development of a Hexactinian (in Hertwig's sense) twelve mesenteries appear, which group themselves in pairs. These are, properly speaking, the primary mesenteries. The remaining mesenteries of the adult appear in pairs in the primary inter-septal chambers. A certain number of these mesenteries which are really secondary generally fuse with the stomodoeum, thus becoming as it were primary. In Sagartia the really primary mesenteries are alone thus connected.
If the surface of a mesentery be carefully examined, - and it is always with the exception of the 'directive septa' (infra) the surface turned towards the intra-septal chamber - a more or less pronounced ridge may be seen traversing it from peristome to base. This ridge indicates the position of the retractor or tentacular muscle of the peristome. In the case of two pairs of primary mesenteries (in the strict sense) this muscle runs on the inter-septal surface. The two pairs in question correspond one to each oral angle, and are known as 'directive septa.' They divide the animal into a right and left half, and extend downwards along the lappets of the oesophageal groove.