Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/451

P L Y Z A 431 viduation of the zoarium of a similar kind. The most remarkable example is afforded by the locomotive zoarium or colony of Cristatella (fig. 3) ; and another very striking instance is that of the stalked zoaria of Kinetoskias (fig. 14) and Adeona, The horny consistence of the cells which are produced by Paludicella is very usual in other Polyzoa ; but we find frequently that the substance which forms the cells is gelatinous and soft instead of being horny, or again may be strongly calcareous. The term ccenoecium is applied to the mass of cells belonging to a colony or zoarium when considered apart from the living polypides which form it. Often such coencecia are found retaining form and structure when the soft living polypides have decomposed and dis appeared. A single cell of the coenoecium, corresponding to a single polypide, is called by the special students of the Polyzoa a zooecium. If we examine a single cell or zooecium of Paludicella more carefully whilst its polypide is alive, we discover that the horny cell is nothing more than the cuticle of the polypide itself, to which it is absol utely adherent. At the so-called " mouth " or spout of the cell the cuticle suddenly changes its character and becomes a very delicate and soft pel licle instead of being- thick and horny. There is no real discontinuity of the cuticle at this region, but merely a change in its qualities. This gives to that por tion of the body of the polypide which lies beyond the spout a mobility and capacity for folding and pleat ing which is entirely denied to that part where the cuticle is more dense (fig. 2, A). Accordingly we find that the anterior por tion of the body of the polypide can be pulled into the hinder part as the finger of a glove may be tucked into the hand. It is, in fact, an "introvert" (for the use of this term see MOLLTJSCA, vol. Xvi. FIG. 2 A. Polypide of Patutttcella Ehrenbergii, seen as a transparent object in optical section and highly magnified (from Gegonbaur, after Allman). For natural size see fig. 1, E. a, anus; br, peristomial circlet of ciliated ten tacles; /, thickened cuticle of the body-wall, forming the horny cell or zocecium ; m, median retractor muscle of the introversiblc part of the body ; r , anterior retractor of the same; mr, great retractor muscle of the same; o, ovary, passing from which to the stomach is the anterior mesentery or funiculus ; t, testis ; ce . ossophagus ; v, stomach; .r, posterior mes entery or funiculus; x , anterior mesentery or funiculus. Observe at the right upper corner of the figure the base of a second polypide and the "rosette-plate" of separation. B. Diagram of a polypide of Plumatella. Letters as above. p. 652). This arrange ment is universal in the Ectoproctous Eupolyzoa, but does not obtain either in the Entoprocta, the Pterobranchia, or the Yermiformia. In Phoronis, Rhabdopleura, and Cepha- lodiscus the anterior part of the body can not be tucked or telescoped into the hinder part as it can in typical Eu polyzoa, On the other hand it is very important to note that the Sipunculoid Gephyraeans are all pre-eminently characterized by possessing identically this arrangement. The introversion is effected in Paludicella (as in other Eu polyzoa) by a series of long detached retractor muscles of considerable power (fig. 2, A, mr, r , m) the same is tru3 of Sipunculus. The view has been advanced by Allman (4) that the re tractile part of the polypide is to be considered as a distinct individual budded from the basal portion, which is regarded as an equivalent individual. It does not appear to the present writer that such a theoretical conception tends to facilitate the understanding of the structure and relations of these animals. An "ectocyst" and "endocyst" have also been distin- guished in former treatises, and these terms form part of a special " polyzoarial " nomenclature, but do not appear to be any longer needful. Equally undesirable is the misap- [ plied term "endosarc" lately introduced by Jolliet (5) to denote a certain portion of the Polyzoon structure which will not be referred to here by that name. The retractile or introversible portion of the body of the polypide of Paludicella is terminated by a crown of sixteen stiff non-contractile tentacles (fig. 2, A, br) which form a circle around a central aperture the animal s mouth. These tentacles are hollow and beset with vibratile cilia. The beating of the cilia causes a powerful current in the water by which food is brought to the animal s mouth. Each tentacle is also muscular, and can be bent and straightened at will. The tentacles not only serve to bring food into the mouth, but they are efficient as gill- filaments, being possibly homologous with (as well as func- tionally similar to) the gill-filaments of Lamellibranch Molluscs. They also serve as delicate tactile organs, and are the only sense orgaas possessed by the Eupolyzoa. In Paludicella the platform around the mouth from which the tentacles arise, or lopliophore, as it is termed, is circular. This is the case in all members of the large group of Gymnolsema and in the Entoprocta. But in the Phylactolaema the lophophore is drawn out on each side, right and left, so as to present a horse-shoe shape (fig. 2, B), and in some forms, notably Lophopus and Alcyonella, the two arms or diverging rami of the horse shoe are very strongly developed. In the Pterobranchia the tentacles are confined in one genus (Rhabdopleura) to the two arm-like outgrowths of the lophophore, and are not simply hollow but contain a well-developed cartilaginoid skeleton (fig. 7). In the allied genus Cephalodiscus there are not merely a single pair of such arm-like processes, each bearing two rows of tentacles, but the lophophore is developed into twelve arm-like pro cesses (fig. 9), which form a dense tuft of filaments around the anterior extremity of the animal. In the Yermiformia (Phoronis) we again meet with a very perfect horse-shoe-shaped lophophore (fig. 4). The tentacles upon the crescentic or otherwise lobed circumoral region of the Sipunculoids are the representatives of the tentacles of the Polyzoa ; whilst the tentaculiferous "arms" of the Brachiopoda appear to be the equivalents of the Polyzoon s lophophore much drawn out and in most cases spirally rolled. Just below the circular crown of tentacles in Paludicella we find an aperture which the study of internal anatomy proves to be the anus. In all Polyzoa the anus has this position near the mouth ; and in this respect we again note an agreement with Sipunculus and the other so-called Gephyreea inermia. In one division of the Polyzoa alone is there any noteworthy variation in the position of the anus, namely, in the Entoprocta (sub-class of the section Eupolyzoa). In these forms the anus, instead of lying just below the lophophore or platform from which the tentacles spring, is included like the mouth within its area (fig. 15, C). Passing now to the deeper structure of Paludicella, we

find that it is a Ccelomate animal : that is to say, there