MICROANATOMY, ULTRASTRUCTURE, AND SYSTEMATIC SIGNIFICANCE OF THE EXCRETORY SYSTEM AND MANTLE CAVITY OF AN ACOCHLIDIAN GASTROPOD (OPISTHOBRANCHIA).

The microanatomy and ultrastructure of the excretory system of an undescribed mesopsammic gastropod of the genus Hedylopsis have been examined by means of semithin serial sections, reconstructions, and transmission electron microscopy. The functional metanephridial system comprises a monotocardian heart with a single ventricle and auricle in a spacious pericardium as well as a single, large kidney. Podocytes in the auricular epicardium represent the site of ultrafiltration and formation of the primary urine, whereas the flat epithelium of the kidney with extensive basal infoldings, large vacuoles and the apical microvillous border indicates modification of the primary filtrate. Solitary rhogocytes (pore cells) represent additional loci of ultrafiltration with an identical fine-structure as those of the podocytes (meandering slits with diaphragms covered by extracellular matrix). The presence of podocytes situated in the epicardial wall of the auricle is regarded as plesiomorphic for the Opisthobranchia and is confirmed for the Acochlidia for the first time. Kidney and rectum both open into a small, yet distinct mantle cavity. Within the Acochlidia this condition represents a plesiomorphic character only known from one further Hedylopsis species until now. Special cells (here termed microvillous pit-cells) with a presumed absorptive function are interspersed between the epithelial cells of the mantle cavity. They are mainly characterized by a prominent invagination of the apical border with densely arranged, very large microvilli. The presence of a mantle cavity that has been lost in all other acochlidian genera supports the systematic placement of the Hedylopsidae at the base of the Achochlidia.