Source : Webster's Revised Unabridged Dictionary (1913)

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