352 MOORE. [Vol. XIII. 



each side by valve-like structures. In the connecting section 

 of the efferent duct the tubule (in Bdellodrilus illuminatus) is 

 smaller (of .02 mm. diameter) and the lumen larger (.009 mm.) 

 than in the intermuscular section. In the terminal section 

 the lumen winds more from side to side, having, as shown 

 in Fig. 47, PI. XXIII, a quite wavy course. Shortly before 

 entering the pulsatile vesicle the lumen of the efferent duct 

 forms a chamber, sometimes of twice the diameter of the rest 

 of the lumen ; it opens by a minute pore guarded by a valve 

 into the posterior chamber (Figs. 47, 48). 



The living efferent tubule presents much the same appear- 

 ance as the shorter loop, but is more transparent, and the radial 

 striations are much more distinct. The free coelomic section 

 and the enlargement terminating it are more highly granular, and 

 in sections (Fig. 23) the walls are seen to be differentiated into 

 deeply staining radial strands, forming a coarse reticulum, and 

 a granular, less intensely stained portion, filling the meshes. 

 Nuclei are distributed as follows in B. illuminatus : in the con- 

 necting tubule, 4 ; in the intermuscular tubule, 3 or 4 ; in the 

 coelomic tubule, 2. In the latter, which is the shortest of 

 the three, they are nearest together. 



Although all descriptions^ of B. parasita state that the efferent 

 duct is ciliated throughout, in B. illuminatus cilia are entirely 

 absent, except very rarely two small patches opposite to the 

 nuclei of the coelomic section. B. instabilia has these always 

 normally and strongly developed. In this species the coelomic 

 tubule is longer and forms quite a wide loop beneath the intes- 

 tine (in part Fig. 9). 



In B. instabilia, B. pulcherrima, and especially in B. philadel- 

 phicus, the intermuscular tubule is exceedingly conspicuous, 

 being, as described for B. parasita, much larger than the looped 

 tubules. The lumen is disproportionately large, and the walls 

 thin and not obviously striated (PL XXIII, Figs. 43,44). The 

 diverticula of the lumen are conspicuous, but short, and the 

 lumen shows a ring-like constriction from which they arise. 

 Before entering the terminal vesicle the lumen is greatly con- 

 stricted (Fig. 43). The connecting tubule in these species is 



1 Except that of Voinov. 



