EXCRETORY OßGAX OP FRESH-WATER POLYZOA. 349 



of these tubes is exactly the same, although the word 'retroperitoneal' 

 can not be used here, for the author regards the cavity inside the ciliated 

 tubes as a part of the peritoneum itself. On this latter point I am of 

 the same opinion with Braem, from reasons to be given afterwards. At 

 any rate, the statements of all the writers agree as to the actual posi- 

 tion of the organ, which may be summed up as fallows. In Fhylac- 

 tolaematous Polyzoa, there are two extremely short ciliated tubes 

 situated on the anal side of the body, between the auus and the base 

 of. the median tentacles of the inner row, bounded on the anal side by 

 the ectodermal and on the oral side by the mesodermal layer of the 

 body-wall. The walls of these tubes are directly continuous with the 

 lining epithelium of the body-cavit}'. 



With regard to the lower end of these tubes the existing- accounts 

 are not so harnionious. CoRi, who names the bouudary between the 

 thickened and ciliated ])art of the lining epithelium of the body-cavity, 

 which, by-the-by, con^^titutes the so-called nephridium, and the ordinary 

 thin portions of the same layer, as the Nephrostome, states that it opens 

 into the " Metasomhohle," or the lower division of the body-cavity. I, 

 on the other hand, mentioned in my former paper that it opens into the 

 lophophoral cavity. . Renewed investigation enables me to maintain 

 the correctness of my former statements, as may most easily be under- 

 stood from the figures. Woodcut 4 shows decidedly that, contrary to 

 CoRi's account, the ciliated tubes at the lower end open into the 

 lophophoral cavity. 



Form and Structure. In describing the form and structure of the 

 ciliated tubes I have thought it best (1) to figure and describe the actual 

 sections which are rather small in number, and (2) to illustrate the 

 relations of these tubes wdth other portions of the body by means of 

 diagrammatic figures. To those who are not themselves occupied in 

 studying the subject, schematic representations of a highly diagram- 



