The focus of this EOL project is a group of mollusks called sea slugs that comprise the “traditional” subclass Opisthobranchia, placed within gastropods (phylum Mollusca), a group that exhibits a wide range of body forms, from shell-bearing species, externally similar to caenogastropods, to highly derived shell-less slugs. The loss of the shell is correlated with the evolution of many additional external structures that are present in this peculiar and interesting group, such as cerata, sensory tentacles, and rhinophores. These structures serve a variety of sensory, defensive, and respiratory functions. Opisthobranchs are mostly marine organisms, with the only exception being a few freshwater acochlidians. These gastropods are defined by having a gill posterior to the heart. Almost all species are hermaphroditic. According to recent phylogenetic analyses (Grande et al. 2004, 2008, Vonnemann et al. 2005, Wägele and Klussmann-Kolb 2005; Klussmann-Kolb and Dinapoli 2006, Malaquias et al. 2009, Dinapoli and Klussmann-Kolb 2010) the opisthobranch subgroups, were confirmed as monophyletic in most analyses, whereas the monophyly or polyphyly of Opisthobranchia is unresolved. All phylogenetic hypotheses show a sister-group relationship between Nudibranchia and Pleurobranchoidea (Nudipleura). In most phylogenetic hypotheses, sister-group relationships occur between Cephalaspidea and Anaspidea, and between Nudipleura and Acteonoidea. However, some hypotheses reject the monophyly of the Nudibranchia.