n. pl. o·pis·tho·branchs
Any of a group of marine gastropods characterized by gills located behind the heart, a shell that is reduced or absent, and two pairs of tentacles, and including the sea slugs and the sea butterflies.
[From New Latin Opisthobranchia, former subclass name : Greek opistho-, behind; see OPISTHOGNATHOUS + Greek brankhia, gills.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.