THE UROGENITAL SYSTEM 



291 



Lower Metazoa. While specialized excretory organs are not present 

 in coelenterates, they do occur in flatworms, where they assume the form 

 of branched tubules which carry liquid wastes to the exterior. In many 

 flatworms, their apertures, usually paired, open on the dorsal side of the 

 body near its anterior end. One peculiarity of the excretory system of 

 flatworms is the flame cells located at the terminations of the branched 

 tubules. (Fig. 260) These are relatively large hollow cells containing 

 numerous long cilia which extend into the cavity of the cell and which 

 sometimes unite to form a single long flagellum. The function of the cilia 

 appears to be to drive the secretions of the cell towards the excurrent canal 

 and the exterior. Many invertebrates besides flatworms possess flame 



C^i!!_ /-PROTONEPHRIDIUM 



COELOMOSTOME 



ELOMODUCT 

 ■ORE 



NEPHRIDIOPORE 



COELOMOSTOME 

 ■COELOMODUCT 



PORE 

 ■4-NEPHRIOIOPORE 



NEPHRIDIUM 

 ^NEPHROSTOME 



COEUDMOSTOME 

 COEUDMODUCT 



NEPHRIDIOPORE 



coelomostome 



degenerate 

 coelomoduct 

 coelomoduct 



nephpostome 



nephridium 

 ephridiopore: 



NEPHRIDIOPORE' 



Fig. 261. — Diagrams illustrating the varying relations of coelomoducts and nephridia 

 in different annelids. Nephridia like those seen in annelids occur also in cephalo- 

 chordates. The coelomoducts are homologized with the renal tubules of vertebrates, 

 although the latter are mostly excretory and not — as in annelids — reproductive in 

 function. (Redrawn after Goodrich.) 



cells in connexion with their excretory systems. The flame cells of flat- 

 worms are probably the prototypes of those of the highly specialized 

 solenocytes of annelids and Amphioxus. 



Amielids. Two sorts of excretory organs occur in anneUds, proto- 

 nephridia and metanephridia, the former, as their name suggests, con- 

 sidered the more primitive. (Fig. 261) 



Protonephridia. The protonephridia of annelids and molluscs are 

 branched or unbranched excretory tubules which end blindly within the 

 body-cavity or in the connective tissues. In the annelids they are seg- 

 mentally arranged and are ectodermal in origin. Their inner blind ends 

 are beset with numerous solenocjrtes. Each solenocyte is prolonged into 

 a tubular capillary containing an elongated flagellum, which extends 

 throughout the length of the capillary into the cavity of the nephridial 

 tubules. The solenocytes gather liquid wastes from the surrounding 

 tissues, and pass them to the nephridium and thus to the outside. Thus 



