340 



THE EARTHWORM 



CHAP. 



The excretory organs of the earthworm consist of 



little tubes called nephridia, 

 of which each metamere 

 except the first three and 

 the last possesses a pair, 

 one on either side (Figs. 

 81-84). You will remem- 

 ber that in the frog all 

 the urinary tubules are 

 connected together to form 

 a pair of kidneys, each with 

 a single duct communi- 

 cating with the cloaca. 

 In the earthworm each 

 nephridium is a long and 

 extremely delicate tube, 

 arranged in three main 

 loops (Fig. 84), opening at 

 one end into the ccelome 

 by a nephrostome and at 

 the other communicating 

 with the exterior directly 

 (Fig. Si). 1 The tubes are 

 attached to the posterior 

 faces of the septa. Each 

 nephrostome (a) is ciliated, 

 and projects through the 

 corresponding septum so 

 as to communicate with 

 the segment of the body- 

 which the main part of 



FIG. 84. A nephridium of Lumbricus, 

 showing the three main loops into which 

 the different parts of the tubule are ar- 

 ranged, as well as the different portions 

 of the tubule. 



. nephrostome ; b. b^b. slender portion 

 of the tubule into which the nephrostome 

 opens ; c. c. second ciliated portion ; d. 

 glandular portion ; e. muscular portion ; 

 e'. end of e at which the nephridiopore 

 opens. (From Gegenbaur.) 



cavity next in front of that in 



1 In the frog the nephrostomes lose their connection with the ne- 

 phriclia, and open in the adult into the renal veins (Fig. 47, p. 146). 



