NEPHRIDIAL TISSUES 



353 



of the tubule. All the cells are equal in size, and each is ciliated over the 

 whole of the centrally directed face, the other face being covered by 

 a few ccelomic epithelial cells" (Fig. 



FlG. 316. Section through the long axis 

 of a nephrostome of Polygordius. 

 (After GOODRICH.) 



In Lumbricus herculeus the nephro- 

 stome has become most complex. The 

 wall of the lumen of the preseptal tu- 

 bule spreads upon one side to become 

 fan-shaped; on the opposite side it 

 thins out to become cleft. From this 

 cleft on each side there diverges a 

 row of grooved cells which are called 

 the centrifugal cells (Fig. 318, cf.\ 

 These cells meet a second set of cells 

 known as the centripetal cells (Fig. 318, 

 cp.}. The centripetal cells, as they are 

 removed from the cells leaving the lu- 

 men of the tube, become large and 

 form nearly a complete ring of cells 

 about the end of the tubule. The 

 cells of the ring have been called the marginal cells. These marginal 

 cells are columnar. Their cytoplasm is slightly granular and supports 

 a nucleus near its middle. Between the marginal cells and the cen- 

 trifugal cells there lies a large, clear, crescent-shaped cell with a very 

 large nucleus lying at its middle. This cell has been called the 

 "central cell" (Fig. 318, c.c). Between its inner 

 margin and the nephridial tubule the opening into 

 the nephridial tubule is found. The marginal and 

 grooved cells form an expanded collecting appa- 

 ratus over which many cilia are distributed. 



In Ammocotes each tubule is provided with a 

 ciliated nephrostome. The cilia are directed to- 

 ward the nephridial tubule. The sides of the 

 nephrostome are nearly parallel, so that the shape 

 of the nephrostome is but slightly like a funnel. 

 The cells forming its wall are small. ' They de- 

 crease in size as the lumen of the tubule is ap- 

 proached. Likewise the cilia decrease in size 

 (Fig. 319). 



Examples of structures of excretion acces- 

 sory to tubules. In the ccelomic epithelium of 

 Lumbricus certain cells are found which have elaborated within their 

 cytoplasm a substance which in picro-sublimate material has a coarse, 



FIG. 317. End of a neph- 

 rostome of Perichata 

 malamaniesis . (After 

 BENHAM.) 



