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the primary gills of mollusks. Each ctenidium is a bipinnate projection in the mantle cavity; it consists of an axial plate or lamella from the flat sides of which project numerous filaments covered with ciliated epithelium. Along one narrow side of the plate there is a blood vessel that brings venous blood, and on the other side there is a blood vessel that carries away oxygenated arterial blood. The number of ctenidia ranges from many pairs (in Amphineura) to a single pair (in Gastropoda and Cephalopoda), and even a single ctenidium (in many Gastropoda). The lamellar gills of bivalve mollusks are a pair of modified ctenidia.