VI 



CIRCULATORY SYSTEM 



169 



the other Azygobranchiate Prosobranchs (Trochidae, Neritidae, 

 etc.). In the last case the right auricle exists, as well as the 



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

 the Mollusea: A, In Chiton; B, Pelecypoda ; C, Dibranchiate Cephalopoda; D, 

 Tetrabranchiate Cephalopoda; E, Prosobranchiata Zygobranchiata ; F, Proso- 

 branchiata Azygobranchiata ; G, Prosobranchiata Monotocardia ; H, Opistho- 

 branchiata Tectibranchiata : 1, Ventricle; 2, Auricle; 3, Aorta; 3«, Cephalic 

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



left, but is simply a closed sac, the coalescing of the two gills 

 on the left side having thrown all the work upon the left 

 auricle. Compare Fig. 79, F, G, H. 



Circulatory System 



All Mollusea, without exception, possess a circulatory system 

 of more or less complexity. The 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 Sca- 

 phopoda alone there appears to be no distinct heart. 



The heart may consist simply of a single auricle and ven- 

 tricle, 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 blood back to the breathing organs, 

 after passing over which it returns by the branchial or pulmonary 

 vein to the heart, thus completing the circuit. 



