HIRUDINEA. 



521 



remainder is intracellular, and is described as consisting of a row 

 of tubular cells fitted end to end like drain-pipes. On account of 

 the long convoluted character of the nephridium, different parts of the 

 cell-rows are in contact, and where this happens the cells of the 

 different rows are not distinguishable save by their nuclei. At 

 some parts of its course the main tube of the nephridium gives off 

 branching ductules in the protoplasm of the nephridial cells. Just 

 before the internal opening is reached, the nephridium often becomes 

 dilated into the cap- 

 sule (Oka), which con- SI^B& a 

 sists of a protoplasmic 

 wall with embedded 

 nuclei and an internal 

 mass of cells, which 

 are partially, sometimes 

 completely, fused with 

 one another (Fig. 421). 

 On reaching the cap- 

 sule the intracellular 

 tube of the nephridium 

 branches out into a 

 number of branching 

 and anastomosing fine 

 ductules, which open 

 into a space between 

 the wall and central 

 mass ; this space leads 

 into the funnel, and 

 is most developed near 

 the point, where that 

 structure is connected 

 to the capsule. The 

 funnel itself and its 

 stalk consist of protoplasm without cell-limits, but containing a few 

 nuclei ; it is ciliated, and the cilia are directed towards the capsule. 



In PontobdeUa (Fig. 423) and Branchellion the nephridial system consists of 

 a network of hollow cells extending from segment to segment, and across the 

 middle line. In each of the segments in which the nephridial network is 

 found there are connected to the network two internal openings (of the usual 

 structure, a syncytial ciliated funnel and neck, and a capsule), and two ventral 

 external openings. 



In Clepsinc (Fig. 423 C) and Nirudo (Fig. 423 B) the nephridial tube is much 



FIG. 422. Diagram of a nephridium of Hirudo (from Perrier, 

 after Bourne), a capsule and funnel ; 6-c testis lobe ; c-d 

 main lobe ; e-g recurrent lobe ; f-g apical lobe ; h-j the 

 duct ; j-fc the vesicle ; I the external opening. 



