pseudobranchia

from The Century Dictionary.

• noun A false gill. See the quotation.
• A suborder of scutibranchiate gastropods, with the gills developed as a branching vessel on the inner surface of the mantle, the body and shell spiral, the lateral central teeth of the odontophore large and irregular, and no operculum. The group was instituted by J. E. Gray for terrestrial forms belonging to the family Proserpinidæ.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) A rudimentary branchia, or gill.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun marine biology A small patch of gill-like filaments on the inner surface of the opercle near the junction of the preopercle.