Opisthobranchia

{ ‖O‐pis′tho‐bran″chi‐a (?), O‐pis′tho‐bran′chi‐a″ta (?), } n. pl. [NL., from Gr. ὄπισθεν behind + � gills.] (Zoöl.) A division of gastropod Mollusca, in which the breathing organs are usually situated behind the heart. It includes the tectibranchs and nudibranchs.