The molluscan rhogocyte (pore-cell, Blasenzelle, cellule nucale), and its 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 connective tissue. Rhogocytes closely resemble cyrtocytes and podocytes in having slit-like diaphragms with an encoating extracellular matrix at their surface which probably acts as a molecular sieve. However, rhogocytes are solitary cells, whereas cyrtocytes and podocytes form epithelia. Occurrence, variability, naming, and possible functions of the rhogocyte among the Mollusca are reviewed and discussed. In general, rhogocytes play an important role in metal ion metabolism of molluscs, they are possibly involved in the recycling of respiratory pigments and may also act in detoxification. Up to now direct homologues of the molluscan rhogocyte have not been described in any other bilaterian phylum, so-called 'pore cells' of other phyla show entirely different fine-structures. However, at least partial (serial) homology can be accepted between rhogocytes (free cells), arthropod nephrocytes (cell aggregates 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. Implications of this postulated 'cell-family' with slit-diaphragms and their respective ultrafiltration systems for theories on the evolution of filtration nephridia are discussed.