The protonephridial system of the tusk shell, Antalis entalis (Mollusca, Scaphopoda)

The development and microanatomy of the protonephridial system in larvae an d postmetamorphic juveniles of Antalis entalis (Dentaliidae) have been exam ined by means of a semithin serial sectioning and reconstruction technique. One late larval stage has been additionally examined by transmission elect ron microscopy. The protonephridium appears during larval development and i s reduced in the juvenile approximately 13 days after metamorphosis. This i s the first unambiguous evidence of a protonephridium in a postlarval mollu sc. When fully developed the protonephridium is unique in consisting of two cells only, a terminal cell (=cyrtocyte) and a duct-releasing cell with gl andular appearance. The polyciliary terminal cell has several distinct ultr afiltration sites, resembling conditions in bivalve protonephridia. The lar ge duct-releasing cell shows a very large nucleus probably reflecting polyp loidy. Its basal infoldings and many mitochondria suggest metabolic activit y, the cytoplasm is characterised by many distinct granules. The unique fea tures of the scaphopod protonephridial system are compared with available d ata on the protonephridia of other molluscan classes. The finding gives add itional evidence that protonephridia belong to the ground pattern of the Mo llusca.