ORGANS OF EXCRETION. NEPllRIDIA. 



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suiting from the breaking down of living tissue. Tliis docs not 

 mean the passing away of the refuse of digestion throiigli tlie 

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

 absoi'bed and therefore have never really been witliin tlic IkkIv 

 proper. Excretion means the removal from the body of matter 

 which has really formed a part of its substance, l)ut lias been 

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

 is performed chieily 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 nephridia^ of which here are two in each somite, excej)t- 

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



Each nepln-idium (Fig. 27) consists of a long convoluted 

 tube, attached to the hinder face of a 

 •dissepiment, and lying in the coilom at 

 the side of the alimentary canal. At 

 one end the tube passes through the ^>^:/;}::;'lii^v;;:- .; 

 body -wall and opens to the exterior by a ^^B^;^:^^^ 

 minute pore situated between the outer * ' "**" 

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

 other end of the tube passes through the 

 dissepiment very near to the point 

 where this is penetrated by the nerve - 

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

 funnel-like expansion into the cavity of ^^^ ^^ _.^ nephridiai funnel 



the next somite in front (/", Fig. 27). much enlarKed, showing the 

 rr^i • J* j^i J" 1 1 j_i • cilia, the betfinnin^; of tl>o 



The margms of the funnel and the nmer ^.^^^,^^^^^ ^,^,^^^ (,)^ ,,,,j the 

 surface of the upper part of the tube are o^ter sheath («). 

 densely covered with powerful cilia (Fig. 28), whose action tend.«=i 

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

 through the nephridium to the exterior. 



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

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

 follows : 



1. 1\\Q funnel ov nej^hrostome ; 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 tubo 

 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 



