Entry 1 sense Webster, 1913 Pectostraca /(pĕk*tŏs"trȧ*kȧ)/ · Pec·tos·tra·ca · IPA 01 prop. n. pl. A degenerate order of Crustacea, including the Rhizocephala and Cirripedia. 1. A degenerate order of Crustacea, including the Rhizocephala and Cirripedia.(Zool.)