EXCRETORY ORGAN OF FRESH-WATER POLYZOA. 34 j 



of sedentary annelides and upon the stems of hydroids, were care- 

 fully removed from the substratum and then fixed with saturated 

 solution of corrosive sublimate in sea-water, used h(jt Although 

 some authors have stated that it is impossible to study the nephridia of 

 Endoproctous Polyzoa by means of sections, I find it not only pos- 

 sible but also necessary in investigating their iniuute structure. My 

 sections, 5/^ in thickness, show the structure of the nej)hridial tubes 

 pretty welJ, and by reconstruction, the entire figure of the organ can 

 be obtained without much difficulty. 



For the specimens of Plioronis Koivalevskii Caldwell I am in- 

 debted to Dr. CoHi, who kindly sent them to me together with 

 specimens of some other f)rms. They had been fixed with chromo- 

 acetic acid and were really in a state of excellent preservation, as 

 judged from the appearance of sections. 



Historical Account. 



Verworn (1<S,S7) who was the first to give any account of this 

 organ in Phylaetohematous ]^Jlyzoa, states that there are two ciliated 

 tubes, situ:ited on the anal side of the «esophagus directly beneath the 

 external layer of the integument. He describes them as follows : 

 "Die beiden Ivanälchen, die eigentlich ihrer Kürze wegen kaum 

 diesen Xamen verdienen, werden von einer einzigen Lage kubischer 

 Mesodermzellen gebildet, welche mit, Wimpern besetzt sind. An der 

 inneren Ötfuung setzen sie sich unmittelbar in das Mesoderm epithel 

 der Leibeshöhle fort, aussen grenzen sie au das Ektoderm des Lopho- 

 phors. ?)eide Kanälchen vereinigen sich kurz vor ihrer äusseren 

 Mündung zu einem einzigen." He thus left the endings of the 

 tubes undetermined, confining his attention to the median portion 

 where they are most conspicuous. After making the above remark, he 



