Phylogeny of Opisthobranchia. Cladogram based on 18S and 28S gene, after Vonnemann et al. (in press). Positions of Acteonoidea, Tylodinoidea and Acochlidacea are marked by bold lines, because they differ from those on the morphology-based tree (Fig. 3). Grey arrows indicate the internalisation, black arrows the loss, of the shell.