Fluid Dynamics of Excretion in Anodonta

Abstract

URINE is produced in the bivalve mollusc Anodonta by the filtration of blood plasma into the pericardium and the pericardial fluid so formed is directed into the renal organs by the cilia of the reno-pericardial funnels¹. The output of urine from the external urinary pore was until recently considered to be caused by filtration pressure and ciliary action. Trueman², however, confirms the hypothesis of Ramsay³ and Krijgsman and Divaris⁴ that a negative pressure tends to occur in the pericardium at systole and suggests that the suction produced favours auricular filling. This suction would also oppose the flow of urine through the renal organs. Using transducers to record pressures in the pericardium and haemocoel of various bivalves, Trueman^2,5 has shown that there are considerable hydrostatic pressures corresponding to adduction of the shell and has established that there is a pressure gradient between the pericardium and the mantle cavity immediately after adduction. The possibility of these pressure gradients being involved in the mechanism of urine flow in Anodonta prompted this investigation.