348 ZOOLOGY. 



In the earthworm we find nephridia which satisfy the 

 first two of the above conditions but not the third, and it 

 is held that a true nephridium may develop a coelomic 

 opening, but that this is a secondary modification, not a 

 primitive character. The generative ducts in the earth- 

 worm are true coelomic duets, not modified nephridia, and 

 this explains why there are nephridia in the same segments 

 as the oviducts and sperm ducts. 



