Haemodynamics in Tresus nuttallii and certain other bivalves.

Molluscs, having a number of relatively large species, offer an opportunity to study general circulation dynamics in an open type of circulatory system. Certain aspects of their cardiac function, such as filtration through the heart wall, are found only in molluscan hearts. Other features, such as the myogenic nature of the heart and the pressure dynamics, are of more general interest. Few investigators, however, have looked at bivalves in terms of their internal pressures, filling mechanisms, the presence and structure of valves, or the function of the aortic bulb. None has done so with modern electronic equipment. In their comprehensive review of molluscan hearts Krijgsman & Divaris (1955) suggest that ventricular filling requires functional auricles and that auricular filling requires an intact pericardium. They infer that in clams with an immovable pericardium, ventricular contraction creates a negative pressure which in turn causes the expansion and filling of the auricle. Then the auricular contraction, according to their hypothesis, is required for ventricular filling. We used isolated ventricles to test this hypothesis. The aortic bulb—a hollow, contractile organ attached to the posterior aorta just inside the pericardium—is found in some bivalves, but its role has not been satisfactorily explained. We anticipated that measurements of its pumping activity would correlate with some other measurable function and thereby provide clues to its functional significance. A few secondary questions also attracted our attention. Does the heart provide pressure for the extension of the foot or siphons? Is there increased venous return during sudden bodily contractions? What valves, if any, control the direction of blood flow? Such problems have been widely examined in many vertebrates, but scarcely considered for bivalves.