NEPHRIDIA 



331 



in correspondence with the complexity of the animal itselfi 

 they are very different from 

 the simple contractile vacu- 

 oles of Paramoecium or 

 Vorticella, and are more 

 nearly comparable with 

 those of the frog (p. 

 146). 



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. 78, 

 79, and 80 neph). You 

 will remember that in the 

 frog all the nephridia are 

 connected together to form 

 a pair of kidneys, each with 

 a single duct communi- 

 cating with the cloaca (p. 

 14s). In the earthworm 

 each nephridium is a lonf 

 and extremely delicate tub' 

 arranged in three main 

 loops (Fig. 80), opening at 

 one end into the ccelome 

 by a ' nephros/ome and at 

 the other communicating 

 with the exterior directly (Fig, 



Fig. 80. — A nephridium of Lumbricus, 

 showing the three main loops into which 

 the different parts of the tubule are ar- 

 ranged, as well as the diffeient portions 

 of the tubule. 

 ll nephrostome ; b. l>. b. slender portion 

 of the tubule into which the nephrostome 

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

 glandular portion ; c. muscular portion ; 

 e'. end of e at which the nephridiopore 

 opens. (From Gegenbaur.) 



78). 1 The tubes are 



' In tnc frog the nephrostomes lose their connection with the ne- 

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



