The ontogeny of the nephridial system of the larval amphioxus (Branchiostoma lanceolatum)

Three developmental stages of Branchiostoma lanceolatum were examined by means of transmission electron microscopy. The development of the protonephridium-like cyrtopodocyte from nearly undifferentiated (ento-) mesodermal cells is demonstrated. The ultrastructure of Hatschek's nephridium in an early larval stage is described. The existence of a second filtrational barrier around the rod-like microvilli of the cyrtopodocytes was confirmed. The mesodermal nephridium drains via an excretory canal which is possibly of ectodermal origin into the oral cavity. Cytotic vesicles in the canal cells suggest that the organ is functional in the earliest larval stages. The phylogenetic interpretation of the cyrtopodocyte is clarified as an autapomorphy of the acraniates derived from a podocyte with an apical cilium. The whole system is comparable to the pronephros of craniates and therefore represents a modified metanephridium. (C) 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved.