THE MOLLUSCAN RHOGOCYTE (PORE-CELL, BLASENZELLE, CELLULE NUCALE), ANDITS SIGNIFICANCE FOR IDEAS ON NEPHRIDIAL EVOLUTION

The rhogocyte (Leydig's cell, cellule nucale, Blasenzelle, pore cell) is a specific molluscan cell type that occurs throughout the animal's primary body cavity, i.e. free in the haemocoel or embedded in connect ive tissue. Rhogocytes closely resemble cyrtocytes and podocytes in ha ving slit-like diaphragms with an encoating extracellular matrix at th eir surface which probably acts as a molecular sieve. However, rhogocy tes are solitary cells, whereas cyrtocytes and podocytes form epitheli a. Occurrence, variability, naming, and possible functions of the rhog ocyte among the Mollusca are reviewed and discussed. In general, rhogo cytes play an important role in metal ion metabolism of molluscs, they are possibly involved in the recycling of respiratory pigments and ma y also act in detoxification. Up to now direct homologues of the mollu scan rhogocyte have not been described in any other bilaterian phylum, so-called 'pore cells' of other phyla show entirely different fine-st ructures. However, at least partial (serial) homology can be accepted between rhogocytes (free cells), arthropod nephrocytes (cell aggregate s or solitary cells) and podocytes (true epithelial cells), and there is also a cytological and functional continuum of both rhogocytes and podocytes to protonephridial cyrto- respectively solenocytes. Implicat ions of this postulated 'cell-family' with slit-diaphragms and their r espective ultrafiltration systems for theories on the evolution of fil tration nephridia are discussed.