arthrostraca

from The Century Dictionary.

• In Gegenbaur's system of classification, one of two prime divisions of malacostracous crustaceans (the other being Thoracostraca), corresponding approximately to the edriophthalmous or sessile-eyed crustaceans of other authors, and divided into the three orders Amphipoda, Læmodipoda, and Isopoda.
• In Burmeister's system of classification, one of three orders of Crustacea (the other two being Aspidostraca and Thoracostraca), divided into nine lesser groups.
• An order of Arthropoda, including those malacostracous crustaceans which have the first, and sometimes the second, thoracic segment fused with the head and bearing maxillipeds, while the remaining seven are free and bear legs, the eyes usually being sessile. The order includes the Amphipoda and the Isopoda.

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

• noun plural (Zoöl.) One of the larger divisions of Crustacea, so called because the thorax and abdomen are both segmented; Tetradecapoda. It includes the Amphipoda and Isopoda.