CIRCULATORY SYSTEM 



169 



cliiate rrosol)ra.iichs (Trochidae, Neritidae, etc.). In the last case 

 the right auricle exists, as well as the left, but is simply a closed 



Fig. 79. — Diagram illustrating the relations between branchiae, heart, and aorta in the 

 Mollusca : A, In Chiton ; B, Pelecypoda ; C, Dibranchiate Cephalopoda ; D, Tetra- 

 branchiate Cephalopoda ; E, Prosobrauchiata Zygobranchiata ; F, Prosobrauchiata 

 Azygobranchiata ; G, Prosobranchiata Monotocardia ; H, Opisthobranchiata Tecti- 

 branchiata : 1, Ventricle ; 2, Auricle ; 3, Aorta ; 3a, Cephalic aorta ; '3b, Visceral 

 aorta ; 3c, Posterior aorta. (From A. Lang. ) 



sac, the coalescing of the two gills on the left side having thrown 

 all the w^ork upon the left auricle. Compare Fig. 79, F, G, H. 



Circulatory System. 



All Mollusca, without exception, possess a circulatory system 

 of more or less complexity. Tlie centre of the system is the heart, 

 which receives the aerated blood from the breathing organs, and 

 propels it to every part of the body. In the Scaphopoda alone 

 there appears to be no distinct heart. 



The heart may consist simply of a single amnele and vent- 

 ricle, and an aorta opening out of the ventricle. From the 

 aorta the blood is conveyed to the various parts of the body by 

 arteries. Veins convey the l)lood back to the breathing organs, 

 after passing over which it returns by the branchial or pulmonary 

 vein to the heart, thus completing the circuit. 



