THE DISCODRILID NEPHRIDIUM. 377 



EXPLANATION OF PLATE XXII. 



Fig. 20. B. illuminatus. Oblique section through a long loop tubule, and its 

 investments. X 500. 



ct, connective-tissue investment, in some parts not distinguishable from the 



peritoneum. 

 *, slight intra-lumenal extensions of the walls. 



n, nucleus of tubule. 



/, peritoneal cell. 



Fig. 21. Same as Fig. 20, but a much thinner section ; the lumen contains 

 granules. X 500. 



pc, peritoneal (coelomic) corpuscle. 



Fig. 22. B. illuminatus. Transverse section across the large tubule lobe of 

 the anterior nephridium, showing the different structure of the long and short loop 

 tubules, and the peritoneal investment. X 500. This figure is taken from the 

 Journal of Morphology, vol. x, no. 2. 



It, long loop. 



st, short loop. 



Fig. 23. B. illuminatus. Transverse section of the anterior nephridium at 

 the region of the funnel stalk. X 500. 



ct, connective-tissue nuclei. 



ts, funnel stalk. 



et, efferent tubule. 



et^, thickening of the efferent tubule at its point of turning. 



pn, beginning of a plexus nodule into which the funnel stalk passes. 



Figs. 24, 25, 26. B. philadelphicus. Sections of the tubules of the anterior 

 nephridia ; 24 and 25 of the long loop, 26 of the short. X 500. 



Lettering as in Fig. 20. 



Fig. 27. B. illuminatus. Transverse section of a portion of the body-wall of 

 the 3d somite at the point of entrance into the intermuscular space of the 

 efferent duct. X 500. 



cm, layer of circular muscle fibres. 



ct, connective-tissue sheath. 



ep, epidermis. 



ets, section of the tubule. 



Im, longitudinal muscles. 



Ip, plexus of the lumen. 



p, peritoneum. 



r, radial marking around the lumen. 



Figs. 28, 29, 30. B. illuminatus. Outlines showing some of the peculiar 

 arrangements of the lumen within the enlargement of the efferent tubule. 



Fig. 30 is the form most frequently assumed. The arrows indicate the direc- 

 tion of the current. 



