VII 



MOLLUSGA—THE NEPHRIDIA 



217 



the diverging portions being directed backward, and the undivided portion 

 forward. These Y-shaped kidneys run longitudinally along each side of the body 

 through its whole length. One of the paired limbs of the Y opens outwar<l into 

 the posterior part of the mantle cavity, the other into the peiicardium, which also 

 lies in the posterior part of the body. In this way the pericardial and outer aper- 



FiG. ISO.— Nephridial and genital systems of CMton, diagramiuatic, from above, after tlie 

 figures and accounts of various authors. 1, Mouth ; 2, gills ; 3, unpaired-portion of the nephridiuni 

 which runs forward, with its lateral branches ; 4, gonad ; 5, efferent ducts of the gonad ; 6, portion 

 of the nephridium running to the outer aperture (10) ; 7, portion running to the reno-pericardial 

 aperture (9) ; 8, genital apertures ; 9, reno-pericardial funnel ; 10, nephridial aperture ; 11, peri- 

 cardium, indicated only in outline ; 12, anus. 



tures of the kidney lie near one another. The third limb of the Y ends blindly 

 anteriorly. Secondary lobules or lobed canals open into all the three parts of the 

 kidney, and are specially abundant in its anterior portion. Except in the terminal 

 portion of the efferent branch, the epithelium of the limbs as well as that of the lobes 

 is cubical and ciliated. 



B. Gastropoda. 



1. Prosobranchia. (a) Diotooardia. — Among all the Gastropoda, Fissurella 

 alone possesses a symmetrical excretory apparatus, in the sense of having two 



