374 MOORE. 



EXPLANATION OF PLATE XX. 



Fig. I. An entire anterior nephridium of B. illuminatus, dissected, and the 

 plexus region slightly teased apart. Drawn from a living specimen and details 

 added after staining with methylene blue and comparison with many similar 

 preparations. Represented in greater part as an optical section. X 195. 



ap, ap\ accessory plexus lobes. 



cc, ciliated chambers of longer tubule loop. 



cct, tubule connecting longer and shorter tubule loops. 



ct, connecting tubules of plexus region. 



ctl, connective-tissue investment. 



ctn, nests of connective tissue with nuclei. 



e, external compartment of nephridial vesicle. 



ep, epidermis and cuticle. 



et, efferent tubule; 1-2, connecting section; 2-3, folded coelomic 

 section ; 3-3, intermuscular section. 



etc, enlargement of efferent tubule at point of entry into the inter- 

 muscular space. ^ 



/, funnel. 



ts, funnel stalk. 



i, internal compartment of nephridial vesicle. 



Id, diverticula of the lumen of the efferent tubule. 



Id}, ditto of the short tubule loop. 



//, //, large tubule lobes. 



It, It, It, long tubule loop. 



mp, mp', tnp", main plexus lobe. 



ti, nucleus in long tubule loop. 



nn, nephrostome. 



np, external nephridial pore. 



/, enlarged peritoneal cells investing tubule loops. 



pft',pn", plexus nodules. 



sc, ciliated points in short tubule loop, i is always and 2 sometimes 

 ciliated. 



si, si, small tubule lobe. 



st, st, short tubule loop. 



Fig. 2. Diagram of the same nephridium. The arrows show the direction of 

 the currents. 



Fig. 3. Diagram of a cell element (modified drain-pipe cell) of the plexus 

 region of B. illuminatus. 



Fig. 3°'. Diagram of a drain-pipe cell element of the tubular region of the 

 same. 



Fig. 4. B. instabilia. A small portion of the apex of the large tubule lobe, 

 showing the ampulla-like enlargements of the long tubule as it bends around the 

 short loop ; also the peritoneal sheet connected with the body-wall, st, short 

 tubule. It, long tubule, h, peritoneum, b, body-wall. 



