Pronunciation: (u-pis'thu-brangk"),[key] —n. any gastropod mollusk of the order Opisthobranchia, as the sea slugs, sea butterflies, and sea hares, characterized by a vestigial or absent mantle and shell and two pairs of tentacles.
—adj. of or pertaining to the opisthobranches. Also,o•pis•tho•bran•chi•atePronunciation: (u-pis"thu-brang'kē-it, -āt").[key]