Abstract
Traditionally, the Annelida has been classified as a group comprising the Polychaeta and the Clitellata. Recent phylogenetic analyses have led to profound changes in the view that the Annelida, as traditionally formulated, is a natural, monophyletic group. Both molecular and morphological analyses support placement of the Siboglinidae (formerly the Pogonophora) as a derived group within the Annelida; there is also evidence, based on molecular analysis of the nuclear gene elongation factor-1α, that the unsegmented echiurids are derived annelids. While monophyly of the Clitellata is well-supported by both molecular and morphological analyses, there is no molecular evidence to support monophyly of the polychaete annelids; the Clitellata fall within a paraphyletic polychaete grade. Relationships among groups of polychaete annelids have not yet been resolved by molecular analysis. Within the Clitellata, paraphyly of the Oligochaeta was indicated in a phylogenetic analysis of cytochrome c oxidase I, which suppo...
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