ULTRASTRUCTURE OF THE RENOPERICARDIAL COMPLEX OF THE INTERSTITIAL GASTROPOD PHILINOGLOSSA-HELGOLANDICA HERTLING, 1932 (MOLLUSCA, OPISTHOBRANCHIA)

The renopericardial complex of the Mollusca consists of an endothelial ly lined pericardium which encloses the heart, and a pair of excretory renopericardial ducts. The heart represents a primary body cavity and the pericardium a secondary body cavity or coelomic cavity. In order to study this complex in an opisthobranch gastropod and to obtain data for a comparison of excretory organs and coelomic cavities between mo lluscs and annelids, the interstitial gastropod Philinoglossa helgolan dica Hertling, 1932 was investigated ultrastructurally. The lens-shape d pericardium lies dorso-laterally between the midgut gland and the ep idermis and is surrounded by an extracellular matrix (ECM) and muscle cells. The epicardium of the ventricle and the auricle consists of epi thelio-muscle cells, while the side of the pericardium facing the kidn ey is predominant lined with podocyte-like cells. These cells are pres umed to allow an ultrafiltration of blood fluid into the pericardium. The remaining pericardial surface is lined with non-muscular cells. Th e renopericardial ducts consist of a ciliated proximal section and a d istal aciliated section. The cells of the latter section have enlarged basal and apical surfaces and morphologically reveal transcytotic act ivity. The nephropore lies ventro-laterally. A comparision of the moll uscan renopericardial complex with the coelom and the nephridia of the annelids reveals that the pericardium and the renopericardial ducts m ust have evolved in the stem lineage of the Mollusca and, thus, repres ent an autapomorphy of this taxon.