HALANYCH, K.M.*; McHUGH, D.; Dahlgren, T.G.: Evidence that some lesser-known “phyla” are annelids

In traditional schemes, the Annelida has been classified as the Polychaeta, Oligochaeta, and Hirudinea (the last two comprising the Clitellata). However, recent analyses indicate that annelids are much more diverse than traditionally believed, and that polychaetes are a paraphyletic group. Specifically, some lesser-known taxa (previously regarded as “phyla”) appear to fall within the annelid radiation. Abundant molecular, developmental, and morphological data show that the Siboglinidae, which includes the formerly recognized Pogonophora and Vestimentifera, are derived annelids; recent data from the Elongation Factor – 1a (EF-1a) gene also suggest that echiurids are of annelid ancestry. In contrast, two other lesser-known groups of marine worms, the Myzostomida and Sipuncula, do not appear to be closely related to the annelids. With advances in our understanding of annelid phylogeny, our perceptions of body plan evolution within the Metazoa are being profoundly changed. For example, the evolution of segmentation is probably more plastic than traditionally believed.