The Collaborative International Dictionary of English v.0.48:

Arthrostraca \Ar*thros"tra*ca\, n. pl. [NL., fr. Gr. 'a`rqron
   joint + ? a shell.] (Zool.)
   One of the larger divisions of Crustacea, so called because
   the thorax and abdomen are both segmented; Tetradecapoda. It
   includes the Amphipoda and Isopoda.
   [1913 Webster]