ORGANS OF EXCRETION. 



133 



coelom 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 setse 



(p. 122.) The other end of the tube passes through the dissepi- 



ment very near to the point where this is 



penetrated by the nerve-cord (p. 136), and 



opens by a broad, funnel-like expansion 



into the cavity of the next somite in front 



(/, Fig. 78). The margins of the funnel 



and the inner surface of the upper part 



of the. tube are densely covered with pow- 



erful cilia (Fig. 79), whose action tends 



to produce a current setting from the 



coelom into the funnel and through the 



nephridmm to the exterior. 



The nephridium is covered with a net- 





WOrk Of blood-VeSSels, and its Walls COn- FlG - 79. A nephridial funnel 

 , i -, i -,-, 1-11 much enlarged, showing the 



tain glandular cells which have the power ci i ia , the beginning of the C n- 

 of removing from the blood a substance iated canal (c) ' and the outer 



. . . *,' . . . sheath (s). 



(urea) which is the most important con- 

 stituent of the urine of higher animals. This substance is 

 passed into the cavity of the nephridium, and so out to the ex- 

 terior. 



Several distinct parts may be distinguished in the nephridium, as 

 follows : 



1. The funnel; this is flattened with an oblique opening, and is com- 

 posed of beautiful ciliated columnar cells set like a palisade around its 

 edge. It leads into 



2. The ciliated (urimferous) tubule (c.p, Fig. 78), which is a very 

 long, thin-walled tube bending back and forth and lined by long active 

 cilia. This suddenly enlarges to join 



3. The glandular part (y.p) ; a stouter thick-walled tube, whose walls 

 contain numerous granular gland-cells in which the urea is probably 

 secreted. This enlarges in turn to join 



4. The muscular part (m.^j); much larger and shorter than the pre- 

 ceding, its walls containing numerous muscular fibres crossing one another 

 in all directions. This part forms a kind of sac or reservoir like a bladder, 

 in which the excreted matter may accumulate and be passed out to the 

 exterior. 



The various parts of the nephridium are held together by connective 

 tissue (p. 155), and are covered with a rich network of blood-vessels. 



