ON THE NEPHETDIUiM OF PHYTiACTOLAF-MATOrS FOTA'ZOA. 85 



cretor}' products are thrown ofit by the action of the cihated tubes. That 

 there are sucli floatinp^ cells in Endoproctous Polyzoa can he very easily 

 observed by examining them in living state. However, our knowledge on 

 this subject is still too limited to admit of an}' definite comparison between 

 the modes of excretion in various groups of Polyzoa. 



After thus considering the structure of the nephridinm in various groups 

 of Polyzoa, it might perhaps be worth while to discuss the phylogenetic 

 relationship of the two main divisions of this group of animals. This is all 

 the more interesting as there are now zoologists who regard them as being 

 geneticahy wholly unconnected, and consequently pi'opose to place them 

 widely apart in the system. For instance, Hatschek^ places the endoproctous 

 Polyzoa somewhere in the neighbourhood of Scolecida in his group Proto- 

 nephridozoa, while he puts Ectoproct in his Metanephridozoa. 



That the Ectoprocta is provided with a real body-cavity with mesodermal 

 epithelium, incontrast to the Endoprocta which is destitute of such, might 

 seem at first for a theoretical zoologist to constitute a difference, that makes 

 the union of these forms in one group impossible. But on studying various 

 representatives of the group, we find that, in the case of Polyzoa, this dif- 

 ference ought not be regarded as to ])e of primary importance in discussing 

 their phylogeny. In several species of marine Ectoprocta, belonging to the 

 genera Scri/jyoceUaria, B/rgnla, &c., I could i:iot find a continous mesodermal 

 lining of the body-cavity throughout the w^hole animal, especially in the 

 body-wall. Eather the mesodermal cells are placed in the perigastric cavity, 

 spanning the space between the ectodermal layer of the integument and the 

 digestive canal. Even in Phylactolaematous Polyzoa in which the mesoder- 

 mal fining of the body-M^all is best developed, there are places where the 

 latter is loosened into separate cells ; for instance, in the epistome. In my 

 opinion the existence or non-existence of the lining epithelium of the body- 

 cavity should not be regarded as signifiying the descent of the two forms from 

 two distinct ancesters with and without such epithelium respectively. I am 

 rather inchned to believe that in the case of Polyzoa the existence or non- 

 existence of the lining epithelium is in "a certain relation with the size of the 



1. Lehrlmch der Zoolo<^ie. 



