The Collaborative International Dictionary of English v.0.48:

Entomostraca \En`to*mos"tra*ca\, n. pl. [NL., fr. Gr. ? cut in
   pieces + ? burnt clay, the hard shell of Testacea.]
   (Zo["o]l.)
   One of the subclasses of Crustacea, including a large number
   of species, many of them minute. The group embraces several
   orders; as the Phyllopoda, Ostracoda, Copepoda, and
   Pectostraca. See Copepoda, Phyllopoda, and Cladocera.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Entomostraca
    n 1: in some older classifications includes the Branchiopoda and
         Copepoda and Ostracoda and Cirripedia; no longer in
         technical use [syn: Entomostraca, subclass
         Entomostraca]