THE MOLLUSC A 



In cases in which a single pair of renal sacs is present, one 

 member of the pair is often rudimentary or absent. This condi- 

 tion is found in the majority of Gastropods, where the ctenidium 



in 



in 



Fin. 5">. 



Transformations of the genital duct in the Mollusca : diagrammatic dorsal views of the peri- 

 cardium, gonad, and kidneys, a, ancestral hypothetical form ; b, Cephalopod ; c, stock form 

 of Amphineura ; d, Aplacophora ; e, Polyplacophora ; /, Prorhipidoglossum ; g, some Rhipido- 



?loss& (KmcirqinHla, etc.); A, specialised Gastropod ; t, Protobranch Laniel li branch ; j, Pecten ; 

 , Eulamellibranch. I, pericardial part of coelom ; II, gonad ; III, kidney ; IV, genital duct of 

 Cephalopoda and Polyplacophora ; V, duct leading from the gonad into the pericardium, into 

 the reno- pericardial duct, or into the kidney ; VI, reno-pericardial duct ; VII, secondary genital 

 duct in specialised Gastropoda and Lainellibraiirhs. 



