THE EARTHWORM 



161 



Excretory System. Most of the excreted matter (with the 

 exception of gases) is passed to the exterior by a series of tubes 

 known as nephridia (Gr. nephros = kidney), one pair in each 

 segment. Each of them (see Fig. 78) consists of a long tube, 

 which begins in a segment of the body cavity as a minute 

 funnel-shaped opening, i, and then passes through the septa, s, 

 to the segment immediately behind. In the posterior segment, 

 the tube is coiled back and forth in three distinct loops that 

 differ in structure and function. Eventually the distal end 

 passes through the walls of the body to the exterior, by a 

 lateral opening, e, in each 

 segment. These nephridia 

 are very delicate organs and 

 can only be made out by 

 very careful study with a 

 magnifying glass. Their 

 function in excretion is as 

 follows: The funnel opening 



in the anterior segment is __ 



.... ... FIG. 78. A NEPHRIDIUM, COMPLETE 



guarded with cells provided 



with cilia, and some of the 

 coils are also lined with cilia. 

 The movements of these cilia produce currents in the liquid 

 in the tube and force the liquids through the tube to the 

 exterior. As a further result of the action of these cilia, solid 

 particles of waste material, which may be floating in the 

 ccelomic fluid, are forced into the tube and then through 

 the tube, passing through its coils and finally reaching the 

 exterior through its opening. The coiled walls of the tube 

 are made up of thick active cells which are well supplied with 

 blood vessels. These are secreting cells and resemble gland 

 cells. They have the power of extracting waste products from 

 the blood and excreting them into the tube which they surround. 

 The materials enter the duct of this nephridium and are slowly 

 forced along by the ciliary current, and finally carried to the 



i, incurrent opening; 

 e, excurrent opening; 

 s, septa. 



