On the other hand, when the branchia is single, or when both branchiae are on the same side, and one is aborted and functionless, the auricle is single too, and on the same side as the branchia. This is the case with the Tectibranchiate Opisthobranchs (_Philine_, _Scaphander_, etc.), all the Pectinibranchiate Prosobranchs (Rachiglossa, Taenioglossa, and Ptenoglossa), and the other Azygobranchiate Prosobranchs (Trochidae, Neritidae, etc.). In the last case the right auricle exists, as well as the 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. [Illustration: FIG. 79.--Diagram illustrating the relations between branchiae, heart, and aorta in the Mollusca: =A=, In Chiton; =B=, Pelecypoda; =C=, Dibranchiate Cephalopoda; =D=, Tetrabranchiate Cephalopoda; =E=, Prosobranchiata Zygobranchiata; =F=, Prosobranchiata Azygobranchiata; =G=, Prosobranchiata Monotocardia; =H=, Opisthobranchiata Tectibranchiata: 1, Ventricle; 2, Auricle; 3, Aorta; 3_a_, Cephalic aorta; 3_b_, Visceral aorta; 3_c_, Posterior aorta. (From A. Lang.)] =Circulatory System= All Mollusca, 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 Scaphopoda alone there appears to be no distinct heart. The heart may consist simply of a single auricle and ventricle, 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. As regards position, the heart is situated within the pericardium, a separate chamber which in the Pelecypoda, Cephalopoda, and the bilaterally symmetrical Gasteropoda lies on the median line, while in the asymmetrical Gasteropoda it is on one or other of the sides of the body, usually the right. The veins connected with the branchiae, and consequently the auricle into which they open, are situated _behind_ the ventricle in the Opisthobranchiata (whence their name), while in the Prosobranchiata they are situated _in front of_ the ventricle. The number of auricles corresponds to the number of branchiae. Thus there is only one auricle in the great majority of Prosobranchiata (which are accordingly classified as _Monotocardia_), and also in the Opisthobranchiata, while the Pulmonata have a single auricle corresponding to the pulmonary chamber. There are two auricles in the Amphineura, in a small group of Gasteropoda, hence known as _Diotocardia_, in all Pelecypoda, and in the Dibranchiate Cephalopoda. In the Tetrabranchiate Cephalopoda alone there are four auricles corresponding to the four branchiae.