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ELEMENTS OF BIOLOGY 



no structures in common. In the lower vertebrate animals there is a 

 common receptacle that receives the fxcal material and the excre- 

 tory wastes, known as the cloaca; it is emptied through the anus. 

 Vertebrate Excretory Systems. In vertebrate animals three 

 different excretory systems appear, the head, middle, and hind 

 kidneys, or more properly, the pro-, meso-, and metanephros. The 

 first to be formed in all vertebrates is the pronephros (Fig. 129), 



MESONEPHRIC 

 TUBULES 



\^ PRONEPHRIC 

 TUBULES 



PRONEPHRIC 

 DUCT 



CLOACA 



Fig. 129. — Diagram of the vertebrate pronephros. (After Patten: Embryology of the 

 Chick,, published by P. Blakiston's Son and Co.) 



which may be roughly compared with the series of nephridia found 

 in the earthworm, modified so that a common tube, the pronephric 

 DUCT, receives each segmental duct and opens into the extreme pos- 

 terior end of the digestive canal. The pronephros and the nephridia 

 of the annelids are not homologous, however, for the vertebrate 

 pronephros is of mesodermal origin, while the nephridia of the 

 earthworm are derived from ectoderm. 



The pronephros is the only type of excretory system developed in 



