MORPHOLOGY OF ANIMALS 



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Fig. 20. Diagrams of Excretory System in Annelid (A) and in primitive Vertebrate (B) ; 

 /, ciliated funnel ; g, glomerulus ; np, nephropore ; sd, segmental duct ; cl, cloaca, cut off 

 from the intestine at z ; a, anus. (After Semper). 



"head kidney") and is usually a purely embryonic organ 

 (Fig. 21, A). 



Mesonephros. In all vertebrates above the cyclostomes 

 longer and more complicated tubules are formed in the 

 somites behind the pronephros, which also open into the 

 segmental duct at one end and into the body-cavity at the 

 other. Near the ciliated funnel a knot of blood vessels forms 

 on the side of the tubule and infolds its wall; this 

 is the glomerulus or Malphigian corpuscle (Fig. 19 C, <7). 

 Many of the tubules in this region then lose their ciliated 



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