ARTHROSTRACA, crustacea of the subclass Malacostraca in which the first, sometimes the second thoracic segment is fused with the head and bears maxillipedes; the remaining seven being free and bearing legs. The eyes are usually sessile. The group is divided into the Amphipoda (q.v.) and Isopoda (q.v.) Common examples are pill-bug, wood-louse, etc.