Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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