ORGANS OF EXCRETION. NEPURIDIA. 



59 



suiting; from tlie breaking down of living tissue. This does not 



O O O 



mean the passing away of the refuse of digestion through the 

 anus (defaecation, p. 53), for such matters have never been 

 absorbed and therefore have never really been within the body 

 proper. Excretion means the removal from the body of matter 

 which has really formed a part of its substance, but lias been 

 used up and is no longer alive. In higher animals this function 

 is performed chiefly by the kidneys, the lungs, and the skin, the 

 waste matters passing off in the urine, the breath, and the sweat. 

 In the earthworm it is principally performed by small organs 

 called neplti'i^'ia^ of which here are two in each somite, except- 

 ing the first three or four (Fig. 29). 



Each nephridium (Fig. '27) consists of a long convoluted 

 tube, attached to the hinder face of a 

 dissepiment, and lying in the cceloin at 

 the side of the alimentary canal. At 

 one end the tube passes through the 

 body-wall and opens to the exterior by a 

 minute pore situated between the outer 

 and inner rows of setre (p. 46). The 

 other end of the titbe passes through the 

 dissepiment very near to the point 

 where this is penetrated by the nerve - 

 cord (p. 66), and opens by a broad, 

 funnel-like expansion into the cavity of 



FIG. 28. A nephrichal funnel 

 the next SOmite in front (f. Fig. 27). much enlarged, showing the 



The margins of the funnel and the inner 

 ~ 



surface of the upper part of the tube are 

 densely covered with powerful cilia (Fig. 28), whose action tends 

 to produce a current setting from the coelom into the funnel and 

 through the nephridium to the exterior. 



The coils of the nephridium are disposed in three principal loops (I, II, 

 III in Fig. 27). The tube itself comprises five very distinct regions, as 

 follows : 



1. The funnel or nephrostome ; much flattened from above downwards, 

 with the opening reduced to a horizontal chink. It is composed of beau- 

 tiful ciliated cells set like fan-rays around its edge. It leads into 



2. The " narrow tube " (n.t.}, a very delicate thin-walled contorted tube 

 extending from the nephrostome through the first loop and a part of the 

 second. In certain parts of its course (a to 6, at c, and from d to e) this 



cilia, the beginning of the 

 ciliated canal (c), and the 

 outer sheath (s). 



