malacostracan
noun
mal·a·cos·tra·can ˌma-lə-ˈkä-stri-kən 
: any of a large subclass (Malacostraca) of crustaceans having a thorax consisting of eight segments usually covered by a carapace and including the decapods and isopods
malacostracan adjective
Word History
Etymology
ultimately from Greek malakostrakos soft-shelled, from malakos soft + ostrakon shell — more at mollify, oyster
First Known Use
1835, in the meaning defined above