Century Dictionary and Cyclopedia - Same as Opisthobranchiata.
GNU Webster's 1913 - n. (Zoöl.) A division of gastropod
Mollusca, in which the breathing organs are usually situated
behind the heart. It includes the tectibranchs and nudibranchs.
- n. gastropods having the gills when present posterior to the
heart and having no operculum: includes sea slugs; sea
butterflies; sea hares “The mollusk taxon that is best
represented on the Internet is probably the Opisthobranchia, the
sea slugs.” ‘Opisthobranchia’ hasn't been added to any lists
yet. Looking for tweets for Opisthobranchia.