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918 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
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 RENAL ORGAN. 
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 cecal divisions (fen., 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 1s, 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 
