The mouth conducts by an oesophagus into a dilated stomach. In some cases a pharnyx may be present, and in others there is in front of the stomach a muscular proventriculus, or gizzard. From the stomach proceeds the intestine, which shortly turns forward to open by a distinct anus close to the mouth. As the nervous ganglion is situated on that side of the mouth towards which the intestine turns in order to reach its termination, the intestine is said to have a "neural flexure;" and this relation is constant throughout the entire class.
Respiration in the Polyzoa appears to be carried on by the ciliated tentacles, and by the "perigastric space," which is filled with a clear fluid, containing solid particles in suspension. A kind of circulation is kept up in this "perigastric fluid" by means of the cilia lining the inner surface of the endocyst. Beyond this there is nothing that could be called a circulation, and there are no distinct circulatory organs of any kind.
The nervous system in all the Polyzoa consists of a single small ganglion (fig. 198, 2), placed upon one side of the oesophagus, between it and the anal aperture, and apparently really of a double nature. Besides the single ganglion which belongs to each polypide, there is also in some of the Polyzoa, a "colonial nervous system;" that is to say, there is a well-developed nervous system, which unites together the various zooids composing the colony, and brings them into relation with one another. It is probably in virtue of this system that the avicularia are enabled to continue their movements and retain their irritability after the death of the polypides; but high authorities deny that the so-called "colonial nerve-system" is really of a nervous nature at all.
The muscular system is well developed, and consists of various muscular bands, with special functions attaching to each. The most important fasciculi are the retractor muscles (fig. 198, 2, g), which retract the upper portion of the polypide within the cell. These muscles arise from the inner surface of the endocyst near the bottom of the cell, and are inserted into the upper part of the oesophagus. The polypide, when retracted, is again exserted, chiefly by the action of the "parietal muscles," which are in the form of circular bundles running transversely round the cell.
As far as is known, all the Polyzoa are hermaphrodite, each polypide containing an ovary and testis (fig. 198, 2). The ovary is situated near the summit of the cell, and is attached to the inner surface of the endocyst. The testis is situated at the bottom of the cell, and a curious cylindrical appendage, called the " funiculus," usually passes from it to the fundus of the stomach. There are no efferent ducts to the reproductive organs; and the products of generation - i.e., the spermatozoa and ova - are discharged into the perigastric space, where fecundation takes place; and the impregnated ova escape by special openings in the body-wall, by dehiscence of the cell, or in some manner not as yet thoroughly understood.
As already mentioned, continuous gemmation occurs in all the Polyzoa, the fresh zooids thus produced remaining attached to the organism from which they were budded forth, and thus giving rise to a compound growth.
A form of discontinuous gemmation, however, occurs in many of the Polyzoa, in which certain singular bodies, called "statoblasts," are developed in the interior of the polypide. The statoblasts are found in certain seasons lying loose in the perigastric cavity. In form "they may be generally described as lenticular bodies, varying, according to the species, from an orbicular to an elongated-oval figure, and enclosed in a horny shell, which consists of two concavo-convex discs united by their margins, where they are further strengthened by a ring which runs round the entire margin, and is of different structure from the discs. . . . When the statoblasts are placed under circumstances favouring their development, they open by the separation from one another of the two faces, and there then escapes from them a young Polyzoon, already in an advanced stage of development, and in all essential respects resembling the adult individual in whose cell the statoblasts were produced " (Allman). The statoblasts are formed as buds upon the "funiculus" - the cord already alluded to as extending from the testis to the stomach - upon which they may usually be seen in different stages of growth. They do not appear to be set free from the perigastric space prior to the death of the adult, and when liberated they are enabled to float near the surface of the water, in consequence of the cells of the marginal ring, or "annulus," being spongy and filled with air. They must be looked upon as "gemmae peculiarly encysted, and destined to remain for a period in a quiescent or pupa-like state " (All-man).
As regards the development of the Polyzoa, the embryo upon its emergence from the ovum presents itself as a ciliated, free-swimming, sac-like body, from which the polypide is subsequently produced by a process of gemmation. In the singular Rhabdopleura the primitive bud is enclosed between two fleshy lobes or valve-like plates, attached along their dorsal margin, and giving exit in front to the rudimentary lophophore. As the development proceeds, these plates cease to keep pace in their growth with the rest of the bud; till ultimately they appear as a peculiar shield-like organ on the haemal side of the lophophore. These lobes have been compared by Dr Allman with the mantle-lobes of the Lamellibranchiata; and according to the most recent researches, the whole shield-like organ is a specially modified zooid, and in no way corresponds with the "epistome" of the Phylactolaemata.