P L T Z A 



161 



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 ccenaecium 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 cceno3cium, corresponding 

 to a single polypide, is called by the special students of 

 the Polyzoa a zocedum. 



If we examine a single cell or zocecium 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 

 thickandhorny. 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 

 MOLLUSCA, vol. xvi. 

 p. 652). This arrange- 

 ment is universal in the Ectoproctous Eupolyzoa, but does 

 not obtain either in the Entoprocta, the Pterobranchia, or 

 the Vermiformia. la 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 Gephyrasans 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 



FIG. 2. A. PolypWe of Paludicella Ehrmbergii, 

 seen as a transparent object in optical section 

 and highly magnified (from Gegcnbaur, after 

 Allman). For natural size see fig. 1, E. a, 

 anus; br, peristomial circlet of ciliated ten- 

 tacles; i, thickened cuticle of the body-wall, 

 forming the horny cell or zooecium ; m, 

 median retractor muscle of the introversible 

 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 ; /, testis ; 

 o?, oesophagus ; r, stomach ; x, posterior mes- 

 entery or funiculus; x f . 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. 



considerable power (fig. 2, A, mr, r', m) ; the same is true 

 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 organs possessed by the Eupolyzoa. 



In Paludicella the platform around the mouth from 

 which the tentacles arise, or lophophore, as it is termed, is 

 circular. This is the case in all members of the large 

 group of Gymnolaema and in the Entoprocta. But in 

 the Phylactolaama the lophophore is drawn out on each 

 side, right and left, so as to present a hprse-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 Vermiformia (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 

 Gephyrsea 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 



X 



