ORIGINAL ARTICLE

Abstract Comparative morphology currently permits no unambiguous decision on the primary homology of the nephridia of Annelida and Arthropoda. In order to obtain additional information on this subject, ultra-structure of morphogenesis and further differentiation of nephridia was studied in the onychophoran Epiperipatus biolleyi (Peripatidae). In this species, the nephridial an-lage develops by reorganization of the lateral portion of the embryonic coelomic wall that initially gives rise to a ciliated canal. All other structural components, includ-ing the sacculus, merge after the nephridial anlage has been separated from the remaining mesodermal tissue. The nephridial sacculus does not represent a ‘persisting coelomic cavity’, since it arises de novo during embryogenesis. There is no evidence for ‘nephridioblast‘ cells participating in the nephridiogenesis of Onycho-phora, which is in contrast to the general mode of nephridial formation in Annelida. Available data on nephridiogenesis in euarthropods (Chelicerata, Myria-poda, Crustacea, and Hexapoda) also provide no evi-dence for nephridia of Annelida and Arthropoda being a synapomorphy of these taxa. These findings accordingly weaken the traditional Articulata hypothesis