VII 



MOLLUSGA—THE OIRGULATORY SYSTEM 



199 



the gills, where it becomes arterial and flows back through the auricles 

 (atria) into the heart. 



There is, typically, one pair of auricles, one on each side of the 

 ventricle. This is the case in all Molluscs provided with two sym- 

 metrical gills. The arterial blood flows out of the left gill into the left 

 auricle and thence into the ventricle, and out of the right gill into the 

 right auricle and thence into the ventricle (Diotocardia, Zeugobranchia, 

 Lamellih-ancMa, Cephalopoda JDibranchia). Again, where a longitudinal 



1- 



7— - 



Fia. 168.— A-H, Diagrams illustrating the relation between tlie otenidia, the heart, and 

 the aorta. A, OUton ; B, LameUihranchia ; C, Dibranohiate Cephalopoda ; D, Tetra- 

 branchiate Cephalopoda ; E, Prosobranohia Diotocardia Zeugobranchia ; F, Prosobranohia 

 Diotocardia Azygobranchia ; G, Prosobranohia Monotooardia ; H, Opisthobranchia Teoti- 

 branchia. 1, Ventricle ; 2, 3, 2a, 2b, 3a, 3b, auricles ; 4, vena braiicliialis = efferent branchial vessel ; 

 5, aorta ; 5a, aorta cephalica ; 5&, aorta visceralis ; 6, aorta posterior vel superior ; 7, ctenida. 



row of numerous gills is found on each side in the mantle furrow 

 (Chitonidce), the heart lies posteriorly above the hind-gut, and has one 

 auricle on each side of the ventricle. This fact appears quite as much 

 to support the view that one pair of gills and one pair of auricles w^ere 

 present in primitive Molluscs, as does the arrangement in Nautilus 

 {Cephalopoda Tetrahranchia) the other view, that there were two pairs 

 of gills and also two pairs of auricles. 



In the majority of Gastropoda, where one of the two original gills 

 has disappeared, the auricle belonging to it has usually also disappeared. 



