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 p rotonephridium-like cyrtopodocyte from nearly undifferentiated (ento-) mesodermal cells is demonstrated. The ultrastructure of Hatschek's ne phridium in an early larval stage is described. The existence of a sec ond filtrational barrier around the rod-like microvilli of the cyrtopo docytes was confirmed. The mesodermal nephridium drains via an excreto ry canal which is possibly of ectodermal origin into the oral cavity. Cytotic vesicles in the canal cells suggest that the organ is function al in the earliest larval stages. The phylogenetic interpretation of t he cyrtopodocyte is clarified as an autapomorphy of the acraniates der ived from a podocyte with an apical cilium. The whole system is compar able to the pronephros of craniates and therefore represents a modifie d metanephridium. (C) 1998 The Royal Swedish Academy of Sciences. Publ ished by Elsevier Science Ltd. All rights reserved.