38 VASCULAR SYSTEM 



lacunae, as in heteropods and })ulmonates, equivalent to both 

 venous and arterial capillaries. 



The ventricle is short, conicid. with the aorta at its apex with 

 two valves at its origin ; and at its opposite blunt end, the 

 rounded auricle, separated by a constriction and like the aorta 

 with two valves at its origin. Sometimes the auricle is divided, 

 a division lying on either side of the heart and receiving each 

 the blood of one branchia. When there are two auricles they 

 surround tlie rectum, resembling tlie lainellibranchs in this 

 arrangement. 



The auricle always lies in front of the ventricle in the ])roso- 

 branchiates (as in the |)ulmonates and heteroi)ods), and therefore 

 the blood flows from before backwards to the heart, whilst in the 

 opisthobranchiates the reverse is the case ; and this difference 

 was deemed of sufficient importance b}- Milne-Edwards to give 

 names to these two orders of branchiferous gasteropods. 



In prosobranchiates with sj)iral shells the lieart lies behind 

 and below the apex of the respiratory cavity on the left side of 

 the animal, between the anterior portion of the liver and right 

 border of the kidney. The aorta, which arises from the apex of 

 the ventricle, soon divides into two branches, the aorta visceralis 

 which supplies the posterior and coiled portion of the animal, 

 its liver and sexual organs, and the aorta cepbalica which gives 

 ofl" many branches forming a j^lexus over the stomach, esoph- 

 agus, mantle, etc. 



Venous capillaries are wanting, as already observed, and the 

 arteries discharge into the body spaces surrounding the cesoph- 

 agus, the stomach, the hepatic lobes, the intestine. The blood 

 in the mollusca is typically bluish and transparent in color, but 

 red in exceptional cases (not in the prosobranchiata), which 

 will be noted hereafter. By breaking away the shell of a Helix, 

 the circulation of the blood is faintly visible through the thin 

 skin of the body. 



Usually there are two large venous sinuses, anterior and pos- 

 terior, from which venous branches collect the blood into two 

 veins, which finally unite in the branchial artery. The aqueous 

 vascular system of the foot forms an important connection be- 

 tween the venous sinus and the external world. This pedal 

 aqueous system includes a pore on the pedal disc, with direct 



