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sac and the outer wall is the blood sinus surrounding the 

 secretory tubules. The renal sinus communicates with 

 the pedal sinus by perforations in the muscular wall of 

 the viscero-pedal mass, as the pedal retractors originate 

 from the latter. The muscles lie actually in the blood 

 sinus. In front of them the sinus is a wide central 

 cavity lying beneath the flattened renal sac, with renal 

 tubules dipping into it on all sides except dorsally. 

 The body-wall is gathered up into a pair of lateral folds, 

 which take part in the formation of the bases of the 

 ctenidia, and the blood from the central cavity filters 

 through the spaces between the tubules into these 

 lateral folds, and so into the gills. 



The reno-pericardial canals (Ben.per., fig. 4, PI. II ) are 

 a pair of large tubules which take origin on the lateral 

 internal walls of the lateral posterior divisions of the renal 

 sac. In sections they may be found on the part of the 

 wall lying on the outside of each retractor muscle. They 

 open into the renal sac by wide fimbriated mouths. Their 

 walls near tliese openings are often peculiarly modified, 

 and are produced out into several small diverticula, 

 resembling the renal tubules. The canals pass down- 

 wards and forwards along the ventral wall of the renal sac, 

 and come to open into the pericardium on the floor of the 

 latter underneath the ventricle, by a pair of prominent 

 slits. All along their course the tubules carry a lining of 

 long cilia, and on the openings of the canals into the peri- 

 cardium these are very prominent, especially in young 

 specimens (0'5 to 1 cm.), where they sometimes form a 

 ciliated fringe projecting into the latter. 



The form and course of the renal tubules are very 

 irregular, and in sections through the posterior margin of 

 the renal organ it is at first sight difficult to distinguish 

 between the tubules themselves and the blood sinus. This 



