14 THE MOLLUSCA 
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 
mol Dine: 
Wt 
a 
Fic. 5vis, 
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 ; f, Prorhipidoglossum ; g, some Rhipido- 
glossa (Emarginula, etc.); h, specialised Gastropod ; 7, Protobranch Lamellibranch ; j, Pecten ; 
k, 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 Lamellibranchs. 
