Evolution of the Multicellular Animals

SYNOPSIS. Molecular sequence analysis is providing new insights into the study of metazoan relationships. The use of ribosomal RNA sequences is revising many of the metazoan phylogenies that have been established traditionally with anatomical and embryological data. Four new findings that seem to be well supported by molecular data, both from the authors' laboratories and from others, are described and discussed. First, the arthropods are members of a deep primary clade within the protostomes and are not the sister taxa of either the annelids or the mollusks. Second, the lophophorate animals are clearly protostomes and are contained within a lophotrochozoan superclade including the mollusks, annelids, and many other phyla. Third, the arthropods together with all other molting animals comprise a second monophyletic superclade within the protostomes, the ecdysozoa. Fourth, the platyhelminthes are contained within the lophotrochozoan superclade.