No. 3-] THE DISCODRILID NEPHRIDIUM. 33 1 



the body-wall. Again turning sharply, the longer loop being 

 anterior, they cross the alimentary canal transversely, still on 

 the dorsal side, to the right anterior angle of the segment, 

 where the longer loop turns backward around the end of the 

 shorter one, and the peritoneal covering extends as a sheet 

 (with the apparent exception of B. illuminatus) to become con- 

 tinuous with the parietal peritoneum. At this point the two 

 tubules of each pair become continuous with one another to 

 form the two complete loops. The figure formed by the 

 tubules of the anterior nephridium in the case described is seen 

 to be a nearly complete quadrangle, extending around the four 

 sides of the second somite (the anterior transverse leg frequently 

 crowds the septum '/„ far forward), and terminating nearly at 

 the point of apparent origin from the granular mass. 



When the anterior nephridium lies on the left side, which 

 is the more usual condition in B. illuminatus, the tubules are 

 generally disposed in a different way. The tubule group first 

 extends backwards from the anterior margin of the opaque mass 

 to the septum "/iii> then forward dorsally on the left side to the 

 anterior septum, and transversely to the right, so that the tubule 

 loops terminate at the same place as before. This terminal 

 point is not, however, an absolutely fixed one, for rarely the 

 case is found of a left anterior nephridium with the tubules 

 arranged in quadrangular form, but reversed so that the loops 

 begin and terminate on the left side. We find, then, two plans 

 of disposal of the tubules — the quadrangular, occurring usually 

 when the right one is anterior, and the zigzag, seen usually 

 when the left one is anterior. 



Comparing the published figures of nephridia of B. parasita, 

 we find that Dorner (14) shows a left with the zigzag, while 

 Lemoine figures a right with the quadrilateral, so that this 

 species also appears to be liable to the same peculiar variation, 

 the full significance of which is not now apparent, but which 

 appears to be related to the position of the heart loop. 



In the second nephridium, when on the left side, the tubules 

 extend from the anterior border of the opaque mass along its 

 ventral margin, pass through the neural arcade of the septum 

 "'/iv, then ventrally by the side of the alimentary canal to the 



