Ultrastructure of the heart and pericardium of an aplacophoran mollusc (Neomeniomorpha): evidence for ultrafiltration of blood

Transmission electron microscopy of the auricular wall in an undescribed mesopsammic mollusc of the genus Meiomenia (Class Aplacophora, Subclass Neomeniomorpha) has revealed podocytes interposed between the haemocoel and the pericardial coelom. These cells comprise the auricular epicardium and overlie a loose inner myocardium. Podocytes characterize sites of blood ultrafiltration in the Polyplacophora and most conch iferan moll use an classes, and are here described from the Aplacophora for the first time. This evidence for ultrafiltration suggests an excretory role for the neomenioid aplacophoran pericardial coelom and supports coelomic ultrafiltration of blood as a plesiomorphic molluscan trait.