No. 3.] THE DISCODRILID NEPHRIDIUM. 345 



between windings of what constitutes the lumen of the original 

 tubule. The entire organ is much shortened and relatively 

 smaller than in B. illuminatus, and the passages in all of the 

 species mentioned relatively larger and the plexuses simpler. 

 Cf. Figs. 13, 14, 15. 



In living specimens the ciliary action in the simple lumena, 

 so far as it can be followed, indicates a less regular folding than 

 in the type nephridium. Nuclei and cilia have the same appear- 

 ance as in B. illuminatus, but the former are larger in B. 

 philadelphicus. 



5. The Tubule Loops. 



Starting at the last plexus nodule of the accessory lobe and 

 extending to the intermuscular portion of the efferent duct, 

 indeed, including this if the single enlargement be omitted, the 

 nephridium consists of what is practically a hose-pipe tubule, 

 more or less complexly looped and twisted, and differing some- 

 what in size and structure in different parts. The general 

 arrangement of the loops has already been described, and a 

 longer distinguished from a shorter. The two tubules of the 

 long loop lie everywhere in close contact (except here and there 

 where small groups of connective-tissue cells have pushed 

 between) and extend for the entire length of both the large 

 and small tubule lobes, passing in the former a short distance 

 beyond the end of the shorter loop, around which they bend 

 (Fig. I). 



As shown in Fig. i , the paired tubules of this loop are thrown 

 in B. illuminatus into three conspicuous folds, of which one, 

 developed at the base of the larger lobe, is always more promi- 

 nent and complex than the other two, and of these the second 

 is larger than the third. In the small lobe also, just after the 

 outer limb arises from the plexus, and before the inner recurves 

 toward the shorter loop, the two become twisted about one 

 another more complexly than is shown in the figure. It is at 

 this point that in B. philadelphicus and B. instabilia, and, 

 according to Lemoine, in B. parasita, the tubules become inter- 

 twined to form a prominent and intricate mass, which it is 

 difficult to unravel satisfactorily. 



