PHYLOGENETIC POSITIONS OF THE ABERRANT BRANCHIOBDELLIDANS AND APHANONEURANS WITHIN THE ANNELIDA AS DERIVED FROM 18S RIBOSOMAL-RNA GENE-SEQUENCES

Different hypotheses have been proposed on the phylogenetic relationsh ips of branchiobdellidans and aphanoneurans among the Annelida based o n the anatomical and embryological characters. The 18S ribosomal RNA g ene sequences have been analyzed from representatives of the three maj or taxa of the Annelida plus the branchiobdellidans and aphanoneurans to assess their phylogenetic relationships to each other. In this prel iminary study, all of the phylogenetic analyses show the branchiobdell idans as a sister group to the leeches, rather than the oligochaetes. The position of the aphanoneurans is stable as an independent taxon th at evolved after the polychaetes branched from the evolutionary stem, but before the ancestral oligochaetes emerged.