No. 3.] THE DISCODRILID NEPHRIDIUM. 343 



regularly from side to side in a zigzag shoe-lacing fashion, the 

 angles becoming the thickened nodules, the straight limbs the 

 connecting simple tubules. Thus is produced an essentially 

 two-ranked arrangement of the nodules, more or less irregular, 

 it is true, especially as many of the nodules bend out of the 

 common plane and conform themselves more closely to the 

 position of their neighbors. It will be seen, however, that a 

 more or less sinuous groove passes along one face (the mesial) 

 of this lobe, its floor corresponding very nearly with the posi- 

 tions of the connecting tubules, which mostly lie side by side, 

 and it is in this groove that the efferent duct is accommodated. 

 The number of nodules in the main lobe is nearly constantly 

 sixteen or seventeen. With one terminal plexus the nephro- 

 stome communicates by a short canal (ts)\ the other passes 

 into communication with the accessory plexus lobe {ap). 



Third: That the accessory plexus lobe consists of four or 

 five plexus nodules with simpler internal passages, but other- 

 wise resembling the nodules of the main lobe. The nodules of 

 this lobe lie side by side in a single series, and the connecting 

 tubules are usually arched. The plexus region of B. illuminatus 

 is thus seen to have a remarkably open structure, in which it 

 differs from all other discodrilids examined, and which is prob- 

 ably the more primitive condition. 



In the massive region of the nephridia of B. philadelphicus, 

 B. instabilia, and B. pulcherrima (and B. parasita seems to belong 

 to this group), the course of the tubule is more difficult to trace. 

 At one point is a tangled group of tubules very conspicuous in 

 the living worm, and corresponding to the small tubule lobe of 

 B. illuminatus. From this three tubules extend around about 

 three-fourths of the entire margin of the compact opaque region. 

 The latter in B. philadelphicus and B. instabilia, the only species 

 which I have examined critically while alive, is not uniform, 

 but, as Lemoine (25) has mentioned for B. parasita, consists of 

 a dull yellowish portion and a more deeply colored brownish- 

 yellow one. The former is less opaque, contains larger and 

 simpler passages, and is closely related to the smaller tubule 

 lobe; it consequently corresponds to the accessory plexus lobe 

 of B. illuminatus. The latter is thicker, more granular, and so 



