160 



P O L Y Z A 



ad interim as forming with the Brachiopoda and Sipuncu- 

 loids an isolated group, to which the name " Podaxonia " 

 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. 1 



The forms included at the present day in Thompson's 

 class of " Polyzoa " may then be thus classified : 



PHYLUM PODAXONIA. 

 CLASS l.SIPUNCULOIDEA. 

 CLASS II. BRACHIOPODA. 

 CLASS III. POLYZOA. 



Section 1. VERMIFORMIA. 



Sole genus : Phoronis (figs. 4 and 51 

 Section 2. PTEROBRANCHIA. 

 Genus 1 : Rhabdopleura (fig. 7). 

 Genus 2 : Cephalodiscus (figs. 8. 9, 10). 

 Section 3. EUPOLYZOA. 

 Sub-class 1. Ectoprocta. 



Order 1. PHYLACTOL^MA. 

 Examples : Lophopus, Plumatdla (fig. 2, B), Cristatella 



(tig. 3), Fredericella. 

 Order 2. GYMNOL^MA. 

 Sub-order 1. Cyclostoma. 

 Examples : Crisia (fig. 13, A), ffornera, Tubulipora, 



Discopnrella. 

 Sub-order 2. Ctenostoma. 



Examples : A Icyonidium, Vesicidaria, Serialaria, Bower- 

 bankia (fig. 1, A), Paludicella (fig. 1, E and fig. 2, A). 

 Sub-order 3. Chilostoma. 



Examples : Celhdaria, Scrupocellaria, Kinetoskias (fig. 

 14), Bugula, Bicellaria, Flustra (fig. 1, G), Afucro- 

 nella (fig. 1, C, D, F), Membranipora, Lepralia, 

 Eschara, Cellepora, Retepora. 

 Sub-class 2. Entoprocta. 



Genera: Pedicelliiia (fig. 15), Loxosoma (fig. 16), Una- 



tella, 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, <fec., 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 Gymnolaema. 

 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 form colonies at all the former because it does not 



1 The research of Harmer (18) on Loxosoma is published too late 

 for due notice in this article. It tends to the conclusion that the 

 Eupolyzoa are after all degraded Mollusca, and have no connexion 

 with the Vermiformia, Pterobranchia, Brachiopoda, and Sipunculoidea. 

 The reader is referred to Mr Harmer's memoir. 



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 "zoarium"), 

 in which the aggregate of budded persons, each of which 



FIG. 1. Various forms of zoaria of Eupolyzoa. 



A. Bowerbankia pustulosa, one of the Ctenostoma ; natural size. 



B. A cluster of polypides of Bowerbankia pustulosa, some with expanded 



tentacles ; more highly magnified. 



C. Zocecia of Mucronella, pavonella (Chilostoma); highly magnified. 



D. Zoarium of Mucronella pavonella, forming a disk-like encrustation on a 



piece of stone; natural size. 



E. Zoarium of Paludicella Ehrenbergii (Ctenostoma) , natural size. 



F. Zocecia of Mucronella Peachii ; highly magnified. Compare with C in order 



to note specific characters. 



G. Zoarium of Flustra securifrons; natural size. 



is called a "polypide," 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- 



