No. 3-] THE DISCODRILID NEPHRIDIUM. 351 



level of the ventral gland pores, and the efferent duct perforates 

 the longitudinal muscle layer directly opposite to this point. 



Bdellodrilus illuminatus exhibits another peculiar feature at 

 the point where the coelomic tubule passes into the inter- 

 muscular. Here the diameter of the tubule about doubles (PI. 

 XXII, Fig. 27), and its walls become very granular; the lumen 

 develops curious and complicated enlargements, which are 

 sometimes in the form of lateral diverticula of greater or less 

 complexity (Fig. 28), or irregular chambers of various forms 

 (Fig. 29), or loops and rings of divers shapes (as Fig. 30), or 

 there may be combinations of all these. 



The complexity of these structures increases with the growth 

 of the animal, the original diverticula enlarging, and new and 

 more complex ones appearing. In worms of about two milli- 

 meters in length there is usually only one or two small diver- 

 ticula of the lumen, with perhaps a simple ring (Fig. 30^). In 

 those of three millimeters the ring may have become double 

 (8-shaped), and the lumen dilated at the position of the diver- 

 ticula of the younger stage. In examples of three and one-half 

 to four millimeters the dilatations become highly irregular and 

 coecal, and the loops, if present, more complex ; but there is 

 no constancy in these stages, and the same individual may pre- 

 sent marked differences on the two sides. This arrangement 

 would doubtless be useful as a means of preventing the collapse 

 of the tubule owing to pressure of the muscles to which it is 

 here subjected. In the posterior nephridia similar complexi- 

 ties of the lumen are developed on the efferent duct where it 

 penetrates the thicker masses of vertical muscle fibres. In B. 

 philadelphicus (PI. XXII, Figs. 32^ 32^) the tubule is simply 

 enlarged, and the capacity of the lumen greatly increased. 

 This simpler condition also exists in Branchiobdella. 



Throughout the remainder of the efferent duct the lumen 

 remains simple, as in the short tubule loop, and as in that loop 

 is provided with paired or whorled coecal diverticula at some- 

 what regular intervals, that is, midway between each two nu- 

 clei. The diverticula are more prominent in this than in any 

 other section of the tubule, and an additional feature is empha- 

 sized, namely, that the lumen is more or less constricted on 



