polybranchia

from The Century Dictionary.

• In J. E. Gray's classification (1821), one of two orders (the other being Pygobranchia) of nudibranchiate gastropods, having lamellar or plumose gills on the upper surface of the mantle, and containing the families Tritoniadæ, Scyllsæidæ, and Tethyadæ.
• In latersystems, a suborder or superfamily comprising the same forms, but subdivided among numerous families: same as Polybranchiata, 1.

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

• noun plural (Zoöl.) A division of Nudibranchiata including those which have numerous branchiæ on the back.