Development of the excretory system in a polyplacophoran mollusc: stages in metanephridial system development

The metanephridial system of Lepidochitona corrugata develops rapidly in the early juvenile phase. It is formed from a coelomic anlage that soon achieves endothelial organization. The pericardium and heart are formed from the central portion of the anlage. The nephridial components are formed by outgrowth from lateral differentiations of the anlage. Simultaneously with formation of the heart, podocytes appear in the atrial wall of the pericardium. In addition, renopericardial ducts, kidneys and efferent nephroducts, all showing downstream ciliation towards the internal lumen, become differentiated (specimen length: 0.62 mm). Further development consists of elongation of the kidney and reinforcement of filtration and reabsorptive structures.During development and in fully formed condition the metanephridial system of Lepidochitona corrugata shares many detailed traits (cellular and overall organization) with the protonephridia of the same species. Accordingly, we suggest a serial homology of various cell types and between the two excretory systems and the organs as a whole. The formation of the metanephridial system varies significantly within Mollusca, thus the mode of formation cannot be used as a homology criterion. Because of similarities in overall organization, we conclude that the molluscan metanephridial system is homologous with that of the annelids not only at the cellular but also at the organ level.Body cavities together with the functionally related nephridial systems are among the most discussed character complexes in the organization of molluscs [1,2]. Within bilaterians (lophotrochozoans in particular) two main types can be recognized: (1) Acoelomate and pseudocoelomate conditions are correlated with protonephridia. Here ciliary activity of the terminal cell (cyrtocyte or solenocyte) causes ultrafiltration from the primary body cavity through a basal membrane and an ultrafiltration weir into the lumen of the protonephridial duct, which releases the pr