52 



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 



