Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/450

Rh 430 P L Y Z A ad interim as forming with the Brachiopoda and Sipuncu- loids an isolated group, to which the name &quot; Podaxonia&quot; may be applied, pending the decision of their affinities by the increase of our knowledge of the embryology of import- ant members of the group. l The forms included at the present day in Thompson s class of &quot; Polyzoa may then be thus classified : PHYLUM PODAXONIA. Cuss 1.SIPUNCULOIDEA. CLASS II. BRACHIOPODA. bud, the latter because the buds become detached from their parent as soon as formed, as do the buds of the Hydrozoon Hydra. On the whole Paludicella presents us with a very simple form of Polyzoon-colony (technically termed a &quot;zoarium&quot;), in which the aggregate of budded persons, each of which Section 1. VERMIFORMIA. Sole genus : Pkorotris (figs. 4 ami ;&quot;). Section 2. PTEROBRANCHIA. Genus 1 : Rhabdopleura (tig. 7). Genus 2: Cephalodiscus (figs. 8, 9, 10). Section 3. EUPOLYZOA. Sub-class L Ectoprocta. Order 1. PHYLACTOL.EMA. Examples : Lophopus, Plumatdla (fig. 2; B). Cristatelln (fig. 3), Fredericella. Order 2. GYMNOL.EMA. Sub-order 1. Cyclostoma. Examples: Crisia (fig. 13, A), Hornern, Tubulipora, Discoporella. Sub-order 2. Ctenostoma. Examples: Alcyonidium, Vesicularia, Serialaria, Bower- bankia (fig. 1, A), Paludicella (fig. 1, E and fig. 2, A). Sub-order 3. Chilostoma. Examples : Cellular in, Scrupocellaria, Kinetoskias (fig. 14), Bugula, Biccllaria, Flustra (fig. 1, G), Mucro- nella (fig. 1, C, D, F), Membranipora, Lepralia, Eschara, Ccllcpora, Retepora. Sub-class 2. Entoprocta. Genera: Pcdicellina (fig. 15), Luxosoma (fig. 16), Urna- tclla, Ascopodaria. We shall most readily arrive at a conception of the essential structure of a Polyzoon, and of the variations to which that essential structure is subject within the class, by first examining one member of the group in detail and subsequently reviewing the characters presented by the divergent sub-classes, orders, fcc., above indicated. The most convenient form for our purpose is Paludicella Ehrenbergii (fig. 2, A), belonging to the typical section of the class (the Eupolyzoa) and to the order Gymnolsema. The organism occurs as minute tree-like growths (figs. 2, A and 1, E) attached to stones in freshwater streams and canals. The branches of the little tree are rarely more than an inch in length, and are regularly swollen and jointed at intervals. Each of the very numerous joints is about one- fifth of an inch long, and is in reality a tubular horny box attached above and below to the preceding and succeeding joints, and having on one side of it a spout-like aperture from which a crown of tentacles can be protruded. Each joint is thus inhabited by a distinct animal which is more or less completely shut off from the one in front of it and the one behind it, although it originated from the hinder and has given rise to the fore-lying individual by a process of budding, and retains a continuity of substance with both. A single cell or joint with its contained animal is repre sented in fig. 2, A. Paludicella produces an arboriform colony, the main trunk or stolon being adherent to some stone or piece of wood. The substance of the wall of the cells is formed by a chemical body allied to chitin. Other Polyzoa may form mat-like expansions the cells being placed in one plane, side by side (fig. 1, C, D, F, G), as well as in linear series ; others again form solid masses, whilst many agree with Paludicella in the simple linear arrangement of their units. Phoronis and Loxosoma, on the other hand, do not Jorm^colonies at all the former because it does not 1 The research of Hanner (18) on Loxosoma is published too late for due notice in this article. Jt tends to the conclusion that the Eu]&amp;gt;olyzoa are after all degraded Mollusca, and have no connexion with the Vermiform in, Pterobranc hia, Brachiopoda, and Sipuwuloidea. Tin- reader is referred to Mr Manner s memoir. FIG. 1. Various forms of zoaria of Eupolyzoa. A. Bowerbankia pustalosa, one of the Ctenostoma; natural size. B. A cluster of polypidcs of Bowerbankia pustulosa, some with expanded tentacles ; more highly magnified. C. Zocecia of Mucronclla pavonella (Chilostoma); highly magnified. I). Zoarium of Afucronella paconel/a, forming a disk-like encrustation on n piece of stone; natural size. 1C. Zoarium of Paludicella Ehrenberyii (Ctenostoma), natural size. F. Zoobcia of Mticronel/a Prachii ; highly magnified. Compare with C in order to note specific characters. G. Zoarium of flwstra securifronr, natural size. is called a &quot;polypide,&quot; does not exhibit any marked indi- viduation, but is irregular and tree-like. But, just as in the Hydrozoa we find the Siphonophora presenting us with a very definite shape and individuality of the aggregate or colony, so in the Polyzoa we find instances of high indi-