SEA-FISHERIES LABORATORY. 999 
Blood which is forced into the viscero-pedal mass by the 
increased action of the heart, is now retained there since 
there is no other exit than that into the renal sinus, and, 
as in the case of the siphons, an area of increased pressure 
is established. This, of itself, must tend to straighten out 
the two limbs of the foot, and this is in fact observed in 
cases of extreme distension. But the action of the in- 
trinsic circular and transverse fibres also aids in the 
protrusion of the foot, since by their correlated contraction 
both diameters can be reduced and the increased pressure 
distributed towards the tip. Within the distal limb the 
courses of the intrinsic fibres are so various that no reliable 
deduction as to the result of their contraction can be 
made. 
Retraction of the foot is provided for by the contraction 
of the posterior retractor pedis. This, first of all, opens 
the passage leading into the renal sinus and allows the 
blood in the foot to enter the latter space. Then both 
anterior and posterior retractors operate by their contrac- 
tion in reducing the length of the proximal limb, and 
waves of contraction passing upwards from the tip chase 
the blood in the whole foot into the renal sinus. It does 
not appear from the disposition of the muscle, regarded 
here as the homologue of the “ protractor pedis”’ in other 
Lamellibranchs, that its contraction can have any appre- 
ciable effect in the protrusion of any part of the viscero- 
pedal mass. 
It follows from the above that in the condition of 
turgescence, the large blood spaces in parts of the body, 
other than the viscero-pedal mass, are relatively emptied 
of blood; and that conversely, in the contracted condition, 
those spaces are gorged. In sections made from the tur- 
gescent animal the empty condition of the renal sinus in 
particular is evident. In the contracted condition, blood 
