SYNOPSIS. The 18S rRNA database is continuously growing and new tree construction methods are being developed. The present study aims at assessing what effect the addition of recently determined animal 18S rRNA sequences and the use of a recently developed distance matrix calculation method have on the results of some previously published case studies on metazoan phytogeny. When re-assessing three case studies, part of their conclusions was confirmed on the basis of the present 18S rRNA data set: 1) the monophyly of Arthropoda; 2) the monophyly of the Vestimentifera-Pogonophora and their protostome character; 3) the doubt about the monophyletic origin of Echiura-Sipuncula and 4) the coelomate characterof Nemertea. Yet, it is also demonstrated that some of the previous results are no longer warranted when updating the analyses: 1) the monophyly of both the Annelida and the Eutrochozoa; 2) the sister-group relationship of Echiura to Vestimentifera-Pogonophora and 3) the polyphyly of the Mesozoa and their close relationship to Myxozoa and Nematodes. In addition, some new very preliminary evidence is provided for: 1) a common ancestry of Platyhelminthes and Mesozoa and the monophyly of the latter group and 2) the monophyly of Clitellata, Hirudinida and Oligochaeta. Finally, doubt is casted on the monophyly of the Polychaeta and the polychaete orders Spionida, Phyllodocida, and Sabellidae. Of course, these hypotheses also need further testing.

Author notes

1From the symposium Evolutionary Relationships of Metazoan Phyla: Advances, Problems, and Approaches presented at the Annual Meeting of the Society for Integrative and Comparative Biology 3–7 January 1998, at Boston, Massachusetts