228 ‘I'RANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
behind the ventricle and at the opening of the posterior 
pedal into the renal sinus. Since it has been proved 
that there is no entrance of water from without, either 
into the blood stream or into a closed water vascular 
system, the mechanism of the erection of the foot and 
siphon must be sought for in the intrinsic muscula- 
ture of those parts or-in arrangements whereby the 
blood pressure in localised regions of the animal’s body 
can be varied at will. Since there appears to be no 
arrangement of muscles which can possibly bring about 
the protrusion of the siphons, the only other evident cause 
of this must be their distension with blood which is forced 
in from the heart through the posterior pallial arteries. 
The condition of ‘‘turgescence”’ in the siphons is probably 
initiated by the simultaneous relaxation of the constrictor 
muscle fibres at their bases, and of the retractor muscles. 
The lacunar spaces are thus enlarged and become distended 
with blood. The radial muscles and, to a certain extent, 
the constrictor muscles must function in preventing 
lateral expansion so that the blood pressure is distri- 
buted towards the tips of the siphons and is directed 
largely towards increasing their length. The valve behind 
the ventricle prevents the reflux of blood back into the 
heart. Retraction is abundantly provided for by the con- 
strictor fibres of the siphonal walls and by the retractor 
muscle bundles. 
Similarly the turgescence of the foot is initiated by the 
relaxation of the posterior and anterior retractores pedis. 
The entrance of the posterior pedal sinus into the renal 
organ is guarded by an arrangement of muscle fibres 
which is in effect a valve, and the closure of this is most 
probably the first effect of the relaxation of the posterior 
retractor of the foot, since in sections through this region, 
In an extended specimen, the opening is difficult to find, 
