Annelida, mainly consisting of marine Polychaeta and in faunal and partly parasitic Clitellata, is one the most significant metazoan taxa. Its more than 20.000 described species invade nearly all habitats and play a central role in marine benthic systems as well as in terrestrial soil communities. Annelids include all soft-bodied segmented worm-like organisms and have been recognized as a separate "phylum" for almost 200 years.

Recently, evidence has been accumulated which shows that some of the groups formerly regarded as independent "phyla" such as Pogonophora (now recognized as...