crus·ta·cean 
(krŭ-stāshən)
(krŭ-stāshən)n.
Any of various predominantly aquatic arthropods of the subphylum (or class) Crustacea, including lobsters, crabs, shrimps, and barnacles, characteristically having a segmented body, a chitinous exoskeleton, paired jointed limbs, and two pairs of antennae.
[From New Latin Crūstācea, class name, neuter pl. of crūstāceus, hard-shelled, from Latin crusta, shell; see kreus- in the Appendix of Indo-European roots.]
crus·tacean adj.