No. 3.] THE DISCODRILID NEPHRIDIUM. 347 



I have been thus particular in describing the integrity of the 

 tubule in all its parts, because Bourne (11) and others have 

 shown that in Clepsine, etc., the tubules in their recurrent 

 courses pass through the same cells, and that consequently the 

 drain-pipe cells in some regions are perforated by two passages, 

 in which the currents run in opposite directions. 



The tubules of the longer loop have a fairly constant diame- 

 ter of about .02 mm., showing only slight increase or decrease 

 here and there. The thickness of their walls is, however, more 

 inconstant (Figs, i, 36, 37), but is nowhere great. The proto- 

 plasm is fairly transparent and homogeneous (Fig. 36). Sec- 

 tions stained with haematoxylin (Figs. 22, It, 37, 38) exhibit a 

 reticular arrangement, and show a not very great affinity of the 

 protoplasm for the stain. In living tubules of B. illuminatus 

 the inner surface of the wall shows peculiar rod-like markings, 

 which have proved to be a thin coating of bacilli (Fig. 54). 

 These bodies are dense and very refringent, and their general 

 arrangement is much like the bricks on a sidewalk — in zigzag 

 parallel lines. They are most numerous at the ampulla-like 

 enlargement at the point of turning of the long tubule, but may 

 be seen in other regions also. None are apparent in any of 

 my sections. Bourne (10, 11) has described similar markings 

 in the living tubules of Hirudo and Pontobdella. 



The lumen is throughout of somewhat irregular calibre, con- 

 stricted slightly here, widened there; and it winds so that it is 

 sometimes nearer the surface on one side, sometimes on the 

 other. Its average diameter is .012 mm. Midway between 

 two nuclei slight diverticula of the lumen are sometimes devel- 

 oped, but this peculiarity is more prominent in following regions 

 of the tubule. 



At points opposite to the nuclei (PI. XX, Fig. i) the lumen 

 usually enlarges slightly to form more or less well-marked 

 chambers (Fig. 36). These are sometimes very prominent in 

 B. instabilia, in which the whole tubule may be expanded at 

 these points. In B. illuminata they are frequently bounded on 

 one or both ends by more or less prominent collars or dia- 

 phragms, sometimes developed excentrically, sometimes of equal 

 height all around (Figs, i, 36). On the thickest part of the 



