prosobranchiata

from The Century Dictionary.

• An order or subclass of gastropods, having the gills anterior to the heart, generally breathing water, more or less completely inclosed in a univalve shell, and sexually distinct: opposed to Opisthobranchiata.

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

• noun plural (Zoöl.) The highest division, or subclass, of gastropod mollusks, including those that have the gills situated anteriorly, or forward of the heart, and the sexes separate.