In other solitary Polyps—for example in the marine Actinkv—the folded bag formed by the stomach is separated from the mouth by an cesophageal constriction. It is at the same time attached to, and isolated from, the general wall of the animal, by radiating mus cular bands ; which extend vertically down the whole depth of the organ, so as to resemble the septa of a poppy capsule as seen in a transverse section.
The compound Polyp appears chiefly to vary from this type by virtue of its individuals possessing a common stem, the proper nutri tion of which requires it to be closely con nected with the organ of digestion. Thus, in some of the Anthozoa which possess a stomach very similar to that of the Sea-ane mones just described, an orifice of' small size at the bottom of the gastric sac seems to admit the results of digestion into the general cavity of the animal, within which they experience a kind of circulation.
In the Tubularian Polyp, the canal is modi fied by the addition of a structure which may be reg,arded as a pharyngeal proboscis. It is a globular projection, surrounded by tentacles at its free extremity, and by its other end received immediately within a circle of simi lar organs;—the place of its attachment being marked by an internal constriction, through which the cavity of this appendix communi cates with that of the stomach.
In many of these Polyps, the stomach has been seen to possess a ciliated lining ; and there are grounds fbr presuming the ex istence of a similar structure in several other species. Some of the circulatory movements observed in their ingesta are perhaps accom plished by the aid of such an apparatus. While their vigorous and almost peristaltic character in other instances is due to structures, the voluntary and powerful contractions of which entitle them to rank as muscles.
The cilio-brachiate Polyp possesses an ab dominal cavity occupied by fluid, in which the alimentary canal is free to move. The canal itself has a mouth and anus, which are both situated at the free extremity of the animal ; —the former orifice being within, the latter without, its whorl of tentacles. The mouth opens into a pharyngeal dilatation, from which a narrow tube leads into an organ analogous to a gizzard. This organ possesses radiating muscular fibres, and rhomboidal teeth, that are capable of crushing its contents. Immediately
beneath it is the stomach, in shape like a two necked flask, and having its blind extremity fixed to the attached base of the animal by a retractory muscle. The pyloric aperture is guarded by cilia, which rotate, and thus delay, the food. The intestine is narrow and simple, and its excrementitious contents are expelled from the anus, to be immediately hurried away by the current arising from the action of the neighbouring cilia.
The body of the Acalepha• generally con stitutes a disc with a fringed margin. It is convex above, and concave below, with large dependent processes. And it swims by what seems to be an alternate preponderance of contraction in these two surfaces.
The condition of the alimentary canal is here very remarkable. The Entozoa have already .offered us a ramified tube, that could scarcely be regarded as strictly diges tive. But these Sea-nettles further com plicate this branched state by the posses sion of a central cavity. This is sometimes placed between a convergent and a divergent set of anastomosing canals ; and sometimes approaches the stomach of the Distoma in possessing the latter set only. In the latter case, the so-called stomach communicates, by a short and simple tube, with the centre of the lower or concave surface. And in one species it also radiates unbranched tubes which open on the margin of the disc. The movements of the contents of these canals seem to be effected by cilia. The ramifications of the canals chiefly occupy the under surface of the animal.
The large order of Echinodermata again presents us with an iniportant advance of development in passing from its lowest to its highest members.
Thus the alimentary canal of the Asterias has a single aperture on the under surface of the animal. This leads by a short tube to a central cavity, which divides into two pro cesses for each ray. These processes give off secondary branches at right angles to them selves, and the latter end in tertiary, cwca. In Comatula the cwca disappear, and the canal acquires a distinct mouth and anus, which open near each other. In all, the canal is muscular, is enclosed in a ciliated peritoneum, and has its primary divisions attached by a kind of mesentery.