i 7 6 



AN INTRODUCTION TO ZOOLOGY 



Leading from the third tube to the external aperture is a large 

 muscular duct. The structure of the nephrostome and of the 

 thin tube just behind it is of particular interest. The nephro- 

 stome (Fig. 89) consists of a large crescentic central cell (c.c.) sur- 

 rounded by a layer of 

 ciliated marginal cells 

 (cp.) ; these join a num- 

 ber of grooved cells (cf.) 

 which lead to the thin 

 ciliated tube. The cen- 

 cf- tral cavity of this cil- 

 iated tube is not 

 .~cp. surrounded by cells, 

 that is, intercellular, 

 but is intracellular, pass- 

 ing directly through the 

 cells. The tube thus 

 resembles a drainpipe, 

 the sections of which 

 are represented by sin- 

 gle cells placed end to 

 end. 



FIG. 89. Nephrostome of the earthworm, c.c., EXCRETION. A ne- 

 central cell ; cf., grooved cells ; cp., ciliated phridium functions in 

 marginal cells. (From Dahlgren and the flowing manner. 



Kepnen) The cilia on the neph- 



rostome and in the thin tube and middle tube create a current 

 through the muscular duct which leads to the exterior. Solid 

 waste particles which may be floating about in the coelomic fluid 

 are drawn into the nephridium by this current, and pass to the 

 outside. Waste matter in solution is taken from the blood by 

 the glands of the " wide tube " and stored in the large sac-like 

 muscular duct until excreted. 



Nervous System. The nervous system differs from that of 

 Planaria and Ascaris in being more concentrated. At the an- 



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