No. 3.] THE DISCODRILID NEPHRIDIUM. 349 



The tubule of the shorter loop is of smaller diameter (.016 

 mm.), and its wall relatively thicker than the longer. The 

 lumen is consequently very much narrower (.006 mm.), and is 

 more regular and generally less tortuous. The nodal diver- 

 ticula mentioned above as sometimes occurring in the longer 

 tubule are, however, more frequent and distinct (Fig. i, ld^)\ 

 two, three, or four such diverticula frequently occur halfway 

 between two nuclei, and may reach almost to the surface of the 

 tubule. They differ somewhat in form, but are usually pouch- 

 shaped, with constricted openings into the lumen. Cilia are 

 entirely absent from the inner, and most of the outer, limb of 

 this loop, only one group being present in B. illuminatus, near 

 its plexal end (Fig. i, sc^). These cilia are similar to those 

 already described, but, unlike those of the long loop, do not lie 

 in a chamber, although borne upon a prominence placed nearly 

 opposite to a nucleus. The nuclei are ten, placed at intervals 

 of .9 mm., and at corresponding points in the two limbs (Fig. 

 I, sc^, and Fig. 34). They resemble those of the longer 

 tubule. 



While the protoplasmic wall of the long tubule is only 

 slightly granular, that of the short one is very densely and finely 

 so, and stains very deeply. Cross sections show a dense struc- 

 ture with a very evident arrangement of the granules in radial 

 lines (Figs. 22, 35). In the living state its substance is 

 evidently more resisting than that of the long tubule. Its 

 walls may be seen to contain minute excreted granules (Fig. 34), 

 exactly similar to those found within the enlarged peritoneal 

 cells. Similar granules are found in large numbers within the 

 lumen of the short tubule, and sometimes the entire cavity of 

 the nephridium is crowded with them. 



6. The Efferent Ducts. 



While the tubule loops are closely similar in the different 

 species, the efferent ducts present considerable differences in 

 size and structure. In all we can conveniently distinguish the 

 three regions of connecting tubule, free coelomic tubule, and 

 intermuscular tubule. 



