THE ANATOMICAL AND ULTRASTRUCTURAL BASIS OF PRIMARY URINE FORMATION IN BIVALVE MOLLUSKS

This paper examines the anatomical and ultrastructural basis of filtration in bivalves, and proposes a mechanism by which the filtrate is derived by a combination of hydrostatic pressure and suction generated by auricular and other muscles, assisted by cilia in the walls of the renopericardial canals. It is proposed that the primitive auricular site of filtration becomes less efficient with increasing size and may impair the contractility of the auricle. The migration of the filter, including blood supply and muscles, to a separate pericardial site is indicated by the discovery in two species, including Scrobicularia plana, with functional pericardial glands, of vestigial glands in the auricle. The development of a filtration site in the pericardial wall, where constraints on size do not exist, made possible the increase in the rate of urine formation which must accompany colonization of estuarine and freshwater habitats. In freshwater genera such as Anodonta the problem of creating a sufficient pressure gradient for a high rate of filtration is overcome by the separation of that part of the pericardial cavity drained by the renopericardial canal (the Nebenhohle) from the chamber housing the heart. Contraction of predominantly transverse muscles in the wall of the Nebenhohle is believed to be responsible for expelling urine from it along the renopericardial canals to the kidneys. The fine structure of the filtration site in ten species of bivalve is described.