Ultrastructure of cerebral eyes in Oweniidae and Chaetopteridae (Annelida) - implications for the evolution of eyes in Annelida.

Background: Recent phylogenomic studies have revealed a robust, new hypothesis of annelid phylogeny. Most surprisingly, a few early branching lineages formed a basal grade, whereas the majority of taxa were categorized as monophyletic Pleistoannelida. Members of these basal groups show a comparatively simple organization lacking certain characters regarded to be annelid specific.