from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Any of various marine gastropod mollusks of the subclass Opisthobranchia, characterized by gills, a shell that is reduced or absent, and two pairs of tentacles.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. Any of a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia, characterized by two pairs of tentacles and a single gill behind and to the right of the heart.
from The Century Dictionary and Cyclopedia
- n. A member of the Opisthobranchiata.
- Having posterior gills; specifically, of or pertaining to the Opisthobranchiata.
From New Latin Opisthobranchia, order name : Greek opistho-, behind; see opisthognathous + Greek brankhia, gills.(American Heritage® Dictionary of the English Language, Fourth Edition)