400 



COMPARATIVE ANATOMY 



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 pecuUarity of the excretory system of 

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

 tubules. 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 cells 

 in connexion with their excretory systems. The flame cells of flatworms 



^y~- — J-PROTONEPHRIDIUM 



NEPHRIDIOPORE 



COEUDMOSTOME 

 .COELX)M0DUCT 

 ■ORE 



COE1jOMOSTO« 

 ;OELOMODUCT 



&4— PORE 



i-NEPHRIDIOPORE 



NEPHRIDIUM 



COELOMOSTOME 



COELOMODUCT 



NEPHRIDIOPORE 



COEUDMOSTOME 



DEGENERATE 

 COELOMODUCT 

 COELOMODUCT 



NEPHROSTOME 



NEPHRIDIUM 

 lEPHRIDIOPORE 



"\NEPHROSTOME NEPHRIDIOPORE' 



Fig. 329. — Diagrams illustrating the varying relations of coelomoducts and nephridia. 

 in different annelids. Nephridia like those seen in annelids also occur 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.) 



are probably the prototypes of those of the highly specialized solenocytes 

 of annelids and amphioxus. (Fig. 328) 



Annelids. Two sorts of excretory organs occur in annelids, proto- 

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

 sidered the more primitive. (Fig. 329) 



Protonephridia. The protonephridia of annelids and molluscs are 

 branched or unbranched excretory tubules which end bhndly 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 solenocytes. 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 



