Circulatory physiology ofNautilus

Summary Present information places the circulatory performance ofNautilus at the bottom of the cephalopod scale but shows it to be distinctly different from that of gastropods and bivalves. The feature which clearly places it closer to its coleoid relatives than to other groups of molluscs is the enhancement of the branchial circulation by the beating pericardial glands and renal appendages. These organs have their homologues in the coleoid pericardial chamber. Recently, studies of living embryos show that the development of a functional circulatory system is an early event during ontogeny and that this information may have important implications for our understanding of the evolution of a branchial circulation.