ON THE NEPHRIDIUM OF PHYLACTOLAEMATOUS POLYZOA. 27 



that the lophophoral cavity is separated frora the rest of the body-cavity by a 

 sort of diaphragm on the oral side, while on the anal side this l)oundary is by 

 no means so sharp. The dotted hnes in fig. 1 show the limit of the diaphragm. 

 An anal diaphragm, snch as was described by Cori, seems not to exist in this 

 genus. 



Thus, a renewed investigation convinces me of the correctness of nw 

 former statement, as well as that of Braem, that the cavity of the nephridial 

 tubes stands in open communication with the lophophoral cavity. I can not 

 understand why Cori describes the nephrostomes as opening into the 

 " Metasomhohle," for the communicatioii between the lophophoral cavity 

 and the cavities of the nephrial tubes is too large to escape attention. How- 

 ever, This difference in our statements might have arisen from the fact that 

 the boundary between the lophophoral cavity and the rest of the body-cavity 

 on the anal side is necessarily arbitrary and we may have fixed on different 

 levels for it. 



After having thus determined the form and extension of the body-cavity, 

 we may now proceed to the discussion of the position which the nephridium 

 occupies in a polypide. From the section represented in fig. 0, we see that 

 the short ciliated tubes are closely enveloped on the anal side l)y the outer 

 (ectodei-mal) layer of the body-vv-all and on the oral side ])y the lining (meso- 

 dermal) ei^ithelium of the epistomial cavity. In other words, the nephridium 

 is situated between the two layers of epithelium that constitute the body- 

 wall. This Avas already pointed out by Cori, who writes, " In die Lei))es- 

 wand ist die Niere in so fern eingelagert, als sie zvi'ischen der Peritoneal- 

 schicht and der Epithelschichfc, also retroperitoneal zu liegen kommt." 

 Braem's account of the position of the nephridium is exactly the same, 

 although the word retroperitoneal can not be used here, for this author 

 regards the cavity inside the ciliated tubes as a part of the peritoneum itself. 

 At any rate, the statements of all the writers agree as to the actual position 

 of this oi-gan, which may be summed up as follows. In Phylactolaematous 

 Polyzoa, the nephridium is situated on the anal side of the jjody, between the 

 anus and the bases of the median tentacles of the inner i-ow, ])ounded on the 

 aiial side by the ectodermal layer, and on the oral side by the mesodermal 



