The Collaborative International Dictionary of English v.0.48:

Malacostraca \Mal`a*cos"tra*ca\, prop. n. pl. [NL., from Gr.
   malako`s soft + ? shell of a testacean.] (Zool.)
   A subclass of Crustacea, including Arthrostraca and
   Thoracostraca, or all those higher than the Entomostraca.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Malacostraca
    n 1: largest subclass of Crustacea including most of the well-
         known marine, freshwater, and terrestrial crustaceans:
         crabs; lobsters; shrimps; sow bugs; beach flies [syn:
         Malacostraca, subclass Malacostraca]