vii MOLLUSCATHE CIRCULATORY SYSTEM 203 



The blood passes out of the large venous sinus back into the heart by three 

 channels. 



1. A large part of it flows through lacunte or vessels into the paired or unpaired 

 branchial artery (afferent branchial vessel). In the course of branchial respiration 

 the blood becomes arterial, and collects in an efferent branchial vessel (cf. section on 

 the respiratory organs, p. 84), which, as branchial vein, conducts it to the auricle 

 of the heart. Where there are two gills, there are naturally two branchial arteries 

 and two branchial veins, the latter conducting the arterial blood to the two auricles. 



2. Another part of the venous blood flows through the kidney, then again 

 collects in lacunae or vessels which lead to the gills, and finally reaches the heart 



FIG. 169. Circulatory system of Paludina vivipara (after Leydig). The animal is seen from 

 the left side. 1, Eye ; 2, cerebral ganglion ; 3, efferent branchial vessel (branchial vein) ; 4, gill 

 (ctenidium) ; 5, afferent branchial vessel ; >, kidney ; 7, aorta visceralis, winding up close to the 

 columella ; 8, ventricle ; 9, auricle ; 10, aorta cephalica ; 11, venous sinuses of the body ; !_', 

 auditory vesicle ; 13, pedal ganglion. 



through the branchial veins. Less frequently, the venous blood, after passing 

 through the kidney, enters the auricle more or less directly, i.e. without passing 

 through the gills, and there mixes with the arterial blood coming from the gills. 



3. A certain part of the venous blood, passing by both the kidney and the gill, 

 flows direct into the branchial veins leading to the auricle. 



The arterial blood in the heart is thus mixed with venous blood. 



(b) Pulmonata. (Examples : Helix pomatia, Limax, Figs. 170, 171, 95, p. 100). 

 The blood vascular system is like that of the Monotocardia. The only important 

 deviation is caused by the occurrence of pulmonary, respiration. Various veins col- 

 lect the venous blood out of the large body sinus and the lacunar system, and unite 

 to form one large vein, which accompanies the hind-gut, and, as vena circularis, 



