THE DISCODRILID NEPHRIDIUM. 375 



EXPLANATION OF PLATE XXL 



Figs. 5, 6, 7. Living nephridial funnels of B. illuminatus; 5, optical section 

 as seen from the dorsal side (cUia of the central cell omitted) ; 2, profile view, 

 optical section, dorsum toward the left, the relation of the efferent duct is indi- 

 cated ; 3, view of nephrostomal end. All X 500. 



mn, nuclei of marginal cells. 



en, nucleus of central cell. 



tn, nucleus of funnel stalk. 



«, nephrostome, the arrows by the side of Fig. 7 indicate the direction of the 



ciliary action. 



mn, uneven ciliated surface of the marginal cells. 



cc, cilia of central cell. 

 tc, cilia of stalk cell. 



et, in Fig. 6, outline of efferent tubule, in Fig. 5, its position. 



Fig. 8. B. philadelphicus, funnel stained with methylene blue. The nucleus 

 of the central cell is seen through the lumen. X 500. Lettering as above. 



Fig. 9. B. instabilia, funnel showing relation to the efferent duct, which is 

 looped around it. The cilia had just ceased to beat. X 500. Lettering as 

 above. 



Fig. 10. B. philadelphicus. A longitudinal section of a funnel, reconstructed 

 from two successive sections. Showing the continuity of the marginal cells with 

 the peritoneum, and the inclusion by the latter of the efferent tubule. Delafield's 

 haematoxylin. X 880. 



/, peritoneum. 



pm, place where peritoneum passes into the marginal cells. 



ct, nests of connective-tissue nuclei. 



Other letters as in the foregoing figures, a-a and b-b respectively indicate the 

 planes of the sections represented in loa and lob. 



Figs, io^, lob. B. philadelphicus. Two transverse sections of the funnel at 

 the levels a-a and b-b of Fig. 10 ; a is through the marginal cells, b through the 

 central cell. The connective-tissue investment of this funnel {ct lob) extended 

 nearer to the nephrostome than in Fig. 10. The cilia are of course represented 

 diagrammatically, as in the actual sections they appeared as a close bunch of 

 oblique sections.. X 500. 



Fig. II. B. instabilia. Horizontal section near the body-floor of postcephalic 

 somites II, III, and IV to show the normal position of the nephridial funnels. 

 The somites and septa are numbered. X 112. 



a, alimentary canal. 



et, efferent tubules at points of entrance into the body-walls. 



t', i", funnels. 



im, intermuscular spaces of somite III. 



Im, longitudinal muscle fibres. 



tl, tubule loops. 



vm, dorso-ventral muscle fibres. 



