As the names suggest, the subdivision of annelids into the Errantia and Sedentaria matches their overall lifestyles (Fig. 1). Members of the Errantia are free to move about, and crawl, swim or burrow.

Annelids are found throughout the world’s terrestrial, aquatic and marine habitats. They represent one of three major animal groups with segmentation, so understanding annelid body-plan evolution is ...