pygobranchia

from The Century Dictionary.

• In J. E. Gray's classification (1821), one of two orders (the other being Polybranchia) of gymnobranchiate gastropods, having plumose or branching gills surrounding the anus on the middle of the hinder part of the back, and the skin more or less spiculous.

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

• noun plural (Zoöl.) A division of opisthobranchiate mollusks having the branchiæ in a wreath or group around the anal opening, as in the genus Doris.