• WordNet 3.6
    • n Opisthobranchia gastropods having the gills when present posterior to the heart and having no operculum: includes sea slugs; sea butterflies; sea hares
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Webster's Revised Unabridged Dictionary
    • n. pl Opisthobranchia (Zoöl) A division of gastropod Mollusca, in which the breathing organs are usually situated behind the heart. It includes the tectibranchs and nudibranchs.
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Century Dictionary and Cyclopedia
    • Opisthobranchia Same as Opisthobranchiata.
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Webster's Revised Unabridged Dictionary
NL., from Gr. 'o`pisqen behind + gills


In literature:

Such parapodia are common, though by no means universal, among Opisthobranchia.
"Encyclopaedia Britannica, 11th Edition, Volume 11, Slice 5" by Various