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.


of or relating to the opisthobranches.

Also o·pis·tho·bran·chi·ate [uh-pis-thuh-brang-kee-it, -eyt] /əˌpɪs θəˈbræŋ ki ɪt, -ˌeɪt/.

Origin of opisthobranch

1850–55; < New Latin Opisthobranchia the subclass; see opistho-, branchia
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

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British Dictionary definitions for opisthobranch



any marine gastropod of the class Opisthobranchia (or Opisthobranchiata), in which the shell is reduced or absent: includes the pteropods, sea hares, and nudibranchs

Word Origin for opisthobranch

via New Latin from Greek opisthen behind + -branch
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012