We determined the complete sequence of the small ribosomal subunit RNA of the pulmonate snail Onchidella celtica. This sequence and the one recently determined for the chiton Acanthopleura japonica were added to an alignment of 25 18S rRNA sequences of Metazoa, including three other Mollusca. The data set was used to assess certain aspects of molluscan phylogeny by distance matrix and character state methods. The trees obtained were tested for effects of random and systematic errors. The results of our analyses support: (a) molluscan monophyly; (b) gastropod monophyly; (c) bivalve monophyly; (d) a sister group relationship of Gastropoda and Polyplacophora. The position of the phylum among other Metazoa remains uncertain due to a lack of representatives of many invertebrate phyla in our data set. Most of our results are congruent with existing hypotheses.
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