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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 palHal 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. Ketraction 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. 



