41 



ment of the muscles is such as to constitute a valve 

 regulating the flow of venous blood outwards from the 

 viscero-pedal mass. 



From the renal sinus the blood reaches the heart by 

 passing through the gills. The precise path taken will be 

 considered in connection with the structure of those 

 organs and of the kidney. 



The Eenal Oegan. 



The renal organ is a single median structure. As seen 

 from the ventral side it forms a crescentic mass with the 

 convexity facing posteriorly, and the two horns, which are 

 anterior, embracing the posterior retractor pedis (fig. 31, 

 PL VI.). It forms part of the lateral and the whole ventral 

 wall of that portion of the body lying between the viscero- 

 pedal mass and the posterior adductor. Its posterior wall 

 lies against the adductor. Its dorsal wall is applied to, 

 the ventral wall of the pericardium. 



In front the renal organ consists of a single wide sac 

 with a few secretory tubules opening into it along each 

 side, but the diverging retractor muscles of the foot 

 passing upwards through it on their way to their insertions 

 in the shell, break up the posterior portion of this sac into 

 three separate caecal divisions (Ben., fig. 7, PL II.). The 

 median posterior division passes backwards between the 

 diverging muscles, the right and left posterior divisions 

 pass to the outside of the right and left muscles respec- 

 tively. Each of these three divisions branches out behind 

 the muscles into a great number of irregular secreting 

 tubules, owing to which the mass of the organ is greatest 

 at its most posterior part, that is, at the convex margin of 

 the crescent. 



It is, of course, not the actual renal sac, but the outer 

 body-wall that is seen from the outside : between the renal 



