No. 3-] THE DISCODRILID NEPHRIDIUM. 357 



(Figs. 27, 32). Fibrous strands and sheets with nuclei 

 scattered here and there anchor the tubule to both muscular 

 walls, and small muscle fibres situated at intervals along its 

 sides serve to adjust the tubule to the body movements. These 

 are best developed in B. philadelphicus, and are shown in 

 Fig. 44. 



The parietal and septal peritoneal endothelium is a thin but 

 dense protoplasmic layer of very uniform thickness of about 

 .001 mm. in all of the species. Here and there, separated 

 by considerable intervals, are marked convex thickenings con- 

 taining conspicuous nuclei. A single transverse section seldom 

 shows more than one of these, and four or five successive 

 sections may not infrequently be searched before even a single 

 peritoneal nucleus is evident. In sections conspicuous striae 

 connected by interfibrillar meshes are seen passing from the 

 neighborhood of the nucleus, and as these show equally well in 

 longitudinal and cross sections, they probably radiate in all 

 directions. The peritoneal cells must consequently be very 

 large but excessively flattened, and slope upon all sides to a 

 central nucleated thickening. Cell boundaries have not, how- 

 ever, been demonstrated, even with silver impregnations. 



At the point where the efferent duct enters the body-wall 

 the parietal peritoneum is reflected inward over the tubule 

 (Figs. 27, 32) and the entire nephridium, from the plexus region 

 of which (this refers only to the anterior nephridia) two broad 

 but very thin double suspensory sheets pass to the body-wall, 

 one from the funnel end, the other from the opposite end 

 (Fig. 33). At the termination of the longer tubule lobe, the 

 peritoneum leaves the nephridium (except in B. illuminatus) as 

 a double fold and passes again into the parietal layer, at the 

 base of the adjacent septum. In B. philadelphicus muscle 

 fibres are occasionally seen in the supporting sheets of the 

 peritoneum. 



Except that the nucleated thickenings are densely filled with 

 conspicuous granules and have more the appearance and color 

 of chlorogogue cells, the nephridial peritoneum does not differ 

 in B. philadelphicus from the parietal. In a full-grown speci- 

 men of this species nine or ten nuclei can be counted, of which 



