EXCRETION 



185 



the coelom and the outer world, and affording a path of egress for 

 the waste matter in the coelomic fluid. But in addition, the blood 

 vascular system carries nitrogenous waste, inorganic salts in solu- 

 tion, etc., to the coiled part of the nephridial tube where special 

 cells take them from the blood and deliver them to the interior 

 of the tube to be passed out of the body. Thus the nephridia 

 of the Earthworm remove waste material from both the coelomic 



fluid and the blood. Artery Vein 



Although the primitive /* "x \* ^ 



segmentation of the coe- / \ . 



lorn has disappeared in ' 

 the Vertebrates, neverthe- 

 less there are good grounds 

 for believing that the 

 ancient, segmentally ar- 

 ranged nephridia are the 

 basis of the essential excre- 

 tory elements of the kid- 

 neys. Thus in the lowest 

 Vertebrates the primitive 

 type of kidney, or pro- 

 nephros as it is called, con- 

 sists of a series of segmen- 

 tally arranged nephridia in 

 the dorsal part of the ante- 

 rior end of the coelom. 

 These, however, instead of 

 opening independently to 

 the exterior, discharge their 

 products into a common 



tube (PRONEPHRIC DUCT) lit— Urethra 



which passes them to the Fig. 130. — Diagram of the human urinary 



outside. In higher forms system, posterior view. See Fig. 134B. 



the pronephros disappears, and its function is taken over by an- 

 other series of nephridia which appears in the coelom posterior 

 to the pronephros. This series constitutes the mesonephros, and 

 opens into the pronephric duct which accordingly now is called 

 the mesonephric duct. Finally, in still higher Vertebrates this 

 second urinary organ is replaced by a third, the kidney proper 

 (metanephros) and its special duct, the ureter. (Fig. 129.) 



Bladder 



