MORPHOLOGY OF ANIMALS 



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Fig. 19. Types of Excretory Organs. A, Protonephridium of flat-worm or rotifer (Acoelo- 

 mata) ; fc, flame-cells with excretory vacuoles and "flame" of cilia in lumen ; dp, "drain 

 pipe" cells of tubule; np, nephropore opening to exterior; int, integument. B, Metanephrid- 

 ium of annelid (Coelomata) ; /, ciliated funnel opening into coelom ; ini, integument. C, 

 Metanephridium (mesonephric tubule) of frog; /, ciliated funnel opening into coelom; g, 

 glomerulus, containing a plexus of capillaries, between the entering artery and the emerg- 

 ing vein ; the latter spreads out over the walls of the tubule ; the tubule opens into the 

 segmental duct (sd), which empties into the cloaca. 



nephridium consists of a tubule opening to the exterior at 

 one end and into the body-cavity (or some portion of it 

 such as the pericardium) at the other (Fig. 20, A). Where 

 it opens into the body-cavity the tubule is widened and 

 covered with long cilia and is known as the ciliated funnel 

 or nephrostome (Fig. 19, B and C, /). Following this is 

 the glandular portion of the tubule, consisting of a single 

 series of perforated ("drain pipe") cells, and following 

 this is an epithelium, composed of many cells, which form 

 the wall of the lumen. The latter is ciliated throughout, 

 and by the action of these cilia, together with those of the 

 ciliated funnel, coelomic fluid is drawn into the tubule 

 through the funnel and then driven to the exterior. Finally 



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