162 



P O L Y Z O A 



exists between the body-wall and the wall of the aliment- 

 ary tract a distinct space termed " perigastric space," 

 "body-cavity," or "coelom." This is true of all Polyzoa, 

 though it has been erroneously stated by G. 0. Sars that 

 Rhabdopleura does not possess such a coelom. In Eu- 

 polyzoa (excepting the Entoprocta) the coelom is very 

 capacious ; it is occupied by a coagulable hremolymph in 

 which float cellular corpuscles, and also the generative 

 products, detached, as is usual in Ccelomata, from definite 

 "gonads" developed on its lining membrane (fig. 2, A, o, t). 

 This lining membrane or " ccelomic epithelium " is ciliated 

 in the Phylactokema, 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 " rosette-plates " or " communication-plates. " 

 The coelomic cavity is continued in Paludicella and probably 

 in all Polyzoa into the tentacles, so that these organs expose 

 the hcemolymph 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 zocecium. 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 ; 



FIG. 3. The locomotive zoarium of the freshwater Phylactol.T.mous Polyzoon 

 Cristatella mucedo; magnified six times linear (after Allman). a, individual 

 polypides with their horse-shoe-shaped crown of tentacles exserted; 6, srato- 

 blasts seen through the transparent tissues ; c, the muscular foot or base of the 

 colony by means of which it crawls; d t portion of water-weed upon which the 

 Cristatella is crawling. 



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 ccelom, 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, u, t). 

 Other Eupolyzoa have a similar but not identical arrange- 

 ment of the longitudinal muscles acting essentially as 

 retractors of the " introvert " or soft anterior region of the 

 body and a similar structure of the body -wall which is in 



essential features identical with that of the Sipunculoid 

 worms, the Chaetopod 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 ccelomic surface (in Phylactolasma 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, of), 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 Bowerbankia 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 coelomic 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 coelomic epithelium and suspending the gut to the 

 body-wall. In Paludicella there are two of these mesen- 

 teries, an anterior (x) and a posterior (x). 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 ccelomic 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 Phylactol;ema 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. 1 1 , w, A; y). A " colonial 

 nervous system " 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, x). 



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 " intertentacular organ " 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. 



