MORPHOLOGY OF ANIMALS 

 funnels and no longer open into the body-cavity, the fluid 

 which passes through the tubule being transuded plasma 

 from the glomerulus; at the same time the single pair of 

 tubules originally present in each somite may give rise 

 to others by budding, so that several may be found in each 

 somite. This second edition of the nephridial system of 

 vertebrates is known as the mesonephros^ and it is the per- 

 manent excretory organ of iishes and amj^hibians (Fig. 

 21, C), while it is only an embryonic organ in reptiles, 

 birds and mammals (Fig. 21, B, mes). 



Metanephros. Finally, in the last-named classes, the 

 definitive kidney or metanephros appears in several of the 

 somites posterior to the mesonephros (Fig. 2 1, D, E, met^. 

 Its tubules, while similar to those of the mesonephros, are 

 still more complex, having no trace of a ciliated funnel, 

 and by budding, very many of them are formed in each 

 somite. The duct into which they open is called the 

 ureter (Fig. 21, D, E, 2/), and it is an outgrowth from 

 the segmental duct. It is thus to be seen that the very com- 

 plex excretory system of vertebrates can be derived, step 

 by step, from the simple nephridial system of such in- 

 vertebrates as the annelids. 



The stage of development of the kidney system in 



C 59 3 



