Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/452

Rh 432 POLYZOA exists between the body-wall and the wall of the aliment ary tract a distinct space termed &quot; perigastric space,&quot; &quot; body-cavity,&quot; or &quot;coelom.&quot; This is true of all Polyzoa, though it has been erroneously stated by G. O. Sars that llhabdopleura does not possess such a ccelom. In Eu- polyzoa (excepting the Entoprocta) the coelom is very Capacious ; it is occupied by a coagulable haemolymph in which float cellular corpuscles, and also the generative products, detached, as is usual in Ccelomata, from definite &quot;gonads&quot; developed on its lining membrane (fig. 2, A, o, t). This lining membrane or &quot; coelomic epithelium &quot; is ciliated in the Phylactohema, but its characters appear not to have been definitely determined in other Eupolyzoa. The coelomic space and the tissues bounding it are continuous throughout the colony or zoarium of a Polyzoon either directly without any constriction marking off one polypide from another, or through perforate septum-like structures as in Paludicella (see right-hand upper process of fig. 2, A), which form incomplete barriers between juxtaposed zooecia, and are termed &quot;rosette-plates&quot; or &quot;communication-plates.&quot; The coelomic cavity is continued in Paludicella and probably in all Polyzoa into the tentacles, so that these organs expose the haemolymph fluid to a respiratory action, and hence may be called branchial. The body-wall of Paludicella consists, alike in the anterior introversible region and in the posterior region, of an outer cuticle which has already been spoken of as thickened around the base of the polypide so as to become there the hard tube-like zooecium. Beneath this is the delicate layer of living epidermic cells which are the mother-cells or matrix of that cuticle. Beneath this again are a few scattered annuli of muscular fibre-cells arranged ring - wise around the cylindrical body ; more deeply placed than these are five large bundles of longitudinally placed muscular fibre-cells which are attached at three different levels to the soft introversible portion of the body, and by their retraction pull it in three folds or tele scopic joints into the capacious hinder part of the body. In some Polyzoa the muscular fibre-cells present trans verse striations. These folds are shown in fig. 2, A; but when the longitudinal muscles are completely con tracted the tentacular crown would be pulled down far out of sight into the midst of the body by the great longitu dinal muscle mr. Deeper than the longitudinal muscles, and clothing them and everything else which projects into the coelom, is the ccelomic epithelium, not easily observed, and sufficiently known only in the Phylactolaema. Part of it gives rise to the generative products (fig. 2 A, o, t). Other Eupolyzoa have a similar but not identical arrange ment of the longitudinal muscles acting essentially as retractors of the &quot; introvert &quot; or soft anterior region of the body and a similar structure of the body-wall which is in essential features identical with that of t .ie Sipunculoid worms, the Cluutopod worms, and other typical Ccelomate animals. The alimentary canal of Paludicella forms a closely com pressed U-shaped loop depending from the closely approxi mated mouth and anus into the capacious coelom. It is clothed on its coelomic surface (in Phylactohema at any rate) with ccelomic epithelium, and beneath this are extremely delicate muscular layers. Within it is lined, except in the immediate region of the mouth (which is lined by the in-pushed outer cell-layer), by the enteric cell- layer the digestive cells derived from the archenteron of the embryo. We can distinguish in Paludicella a contrac tile pharyngo-cesophagus (fig. 2, A, #), a digestive stomach v (the lining cells of which have a yellow colour), and an intestine which forms that arm of the loop connected with the anus. This simple form of alimentary canal is uni formly present in Polyzoa. In Howerbankia and its allies a muscular gizzard with horny teeth is interposed between oesophagus and digestive stomach. The alimentary canal of Paludicella does not hang quite freely in the ccelomic cavity, but, as is usually the case in other classes where the ccelom is large, mesenteries are present in the form of fibrous (muscular 1) bands clothed with ccelomic epithelium and suspending the gut to the body-wall. In Paludicella there are two of these mesen teries, an anterior (x) and a posterior (,r). The presence of two mesenteric bands is exceptional. Usually in the Eupolyzoa we find one such mesentery only, corresponding to the hinder of the two in Paludicella. The special name funiculus (Huxley) is applied to this mesenteric band, and it is noteworthy that the cells of the coelomic epithelium, either upon its surface or at its point of insertion into the body-wall, are modified as reproductive elements, forming either the testis or ovary; in the Phylactohema they form here also special asexual reproductive bodies, the stato- blasts. The nervous tissue and organs of Paludicella have not been specially investigated, but in many Eupolyzoa an oval mass of nerve-ganglion cells is found lying between the mouth and anus, and there is no doubt that it is present in this case. In Plumatella nerve-fibres have been traced from this ganglion to the tentacles and other parts around the mouth (fig. 11, ^v, oc, y). A &quot;colonial nervous system &quot; was described some years ago by Fr. Miiller in Serialaria ; but modern histologists do not admit that the tissue so named by Miiller is nerve-tissue. The ganglion above mentioned is the only nervous tissue at present known in Polyzoa (but see fig. 17, of). No heart or blood-vessels of any kind exist in Paludi cella nor in any of the Eupolyzoa or Pterobranchia. On the other hand the isolated vermiform genus Phoronis presents a closed contractile system of longitudinal vessels (dorsal and ventral) which contain nucleated corpuscles coloured red by haemoglobin (figs. 4, 5). No excretory organs (nephridia) or genital ducts have been observed in Paludicella, nor have such organs been detected in the majority of the Polyzoa which have been studied. In the Entoprocta, however, a pair of minute ciliated canals are found in the nearly obliterated body- cavity opening to the exterior near the tentacular crown in both Pedicellina and Loxosoma, which represent the cephalic nephridia of worms. A definite pair of nephridia occur in Phoronis. A similar significance is perhaps to be attributed to the &quot; intertentacular organ &quot; of Farre a ciliated pas sage opening between two tentacles of the lophophore in Membranipora, Alcyonidium, and other forms through which Hincks has observed the spermatozoa to escape in large numbers. This organ occurs equally in female speci mens of Membranipora, and is not therefore simply a sper matic duct.