CIRCULATlOx"^, AQUIFEROUS SYSTEM. 89 



ciall}^ in the anterior part of tlie body these are replaced by 

 hicunoe, as in heteropods and pulmonates, equivalent to both 

 venous and arterial capillaries. 



The ventricle is short, conical, 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 orio-in. 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 the rectum, resembling the lamellibranchs in this 

 arrangement. 



The auricle always lies in front of the ventricle in the proso- 

 branchiates (as in the pulmonates and heteropods), 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 by Milne-Edwards to give 

 names to these two orders of branchiferous gastropods. 



In prosobranchiates with spiral shells the heart 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; in Chiton and other non-spiral genera it is 

 in the median line of the body. 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 cephalica 

 which gives off many branches forming a plexus over the 

 stomach, oesophagus, mantle, etc. 



Venous capillaries are wanting, as already observed, and the 

 arteries discharge into the body-spaces surrounding the oesoph- 

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

 in the mollusca is t3^pically uncolored ; but sometimes bluish and 

 transparent ; it is red in Planorbis. Its globules are rounded 

 and finel}^ granulated. 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 siJinses, anterior and pos- 

 terior, from which venous branches collect the blood into two 

 veins, which finally unite in the branchial artery. 



The number of heart-beats per minute varies greatl}^ in diflferent 

 species : the marine species have not been extensively investi- 

 gated. Alder counted 120 pulsations in Vitrina, whilst 20 to 90 

 pulsations are registered for several terrestrial and fluviatile 

 pulmonates. As excitement or other unusual conditions, as well 

 as the approach of the hibernating period would largely modify 

 the rapidity of the pulse, the mere statement of comparative 

 figures for various mollusks is scarcely scientifically valuable. 



