ON THE NEPHRIDIA OF THE P0LY0H2ETA. 191 



wall is composed of loose fibrous-looking tissue (an appear- 

 ance probably due to the presence of a large quantity of water 

 in the living cells). A denser layer surrounds the lumen 

 (figs. 24 and 25, neph. t.). Nuclei are seen in the wall here 

 and there, but the lumen is probably intra-cellular. As in 

 Nereis, the cilia are disposed along one side of the tube only 

 (occasionally, however, in two opposite rows), and in a nephri- 

 dium freshly dissected out, the characteristic undulation pro- 

 duced by such an arrangement of cilia is very marked. 



The path of the canal as it passes up behind through the 

 substance of the ciliated organ is shown in fig. 19 by the dotted 

 line, p. neph. t,, and in the sections drawn in figs. 23, 24, and 25, 

 neph. t. On reaching the upper edge of the ciliated organ the 

 tube, with its lumen, divides into three, four, or five branches, 

 more generally four, which float freely in the ccelom. One or 

 more of the branches usually has a T-shaped extremity, and 

 the multiplication of the branches (to the maximum number 

 of five) would appear to take place by the splitting of such a 

 T-shaped branch to its base. The lumen of the nephridial 

 canal ends blindly at the tip of each branch (as far as I have 

 been able to make out). But the chief interest lies in the 

 minute structure of the branches themselves. 



Roughly speaking, each branch may be said to consist of a 

 double row of cells, with swollen bases containing the nuclei, 

 enclosing the lumen of the canal (figs. 26, 27, and 28). Each 

 cell tapers off" into a long narrow neck, we., which stretches out 

 at right angles to the axis of the branch on which the cells are 

 set. At its distal extremity the neck-like process becomes 

 slightly swollen, and sharply bent round towards the corre- 

 sponding cells of the other side. This sort of crook bears at 

 its extreme end a long narrow tube, tu., which runs down 

 parallel to the neck towards the nephridial canal at the base of 

 the cell. Piercing the wall of the canal the delicate tube 

 leads directly to its lumen, even projecting slightly into it. 

 A very long and slender flagellura undulates freely in the tube. 

 Attached by its base at the distal end of the tube, the flagellum 

 passes downwards and out from the tube into the lumen of the 



