[1] : The Collaborative International Dictionary of English v.0.48

malacostraca \mal`acos"traca\, 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

[2] : WordNet (r) 2.0

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
see also:
subclass malacostraca 

Malacostraca >> M Words
Malacostraca, definition of term: Malacostraca
malacostraca_pag1.html