The Collaborative International Dictionary of English v.0.48:

Exoskeleton \Ex`o*skel"e*ton\, n. [Exo- + skeleton] (Anat.)
   The hardened parts of the external integument of an animal,
   including hair, feathers, nails, horns, scales, etc.,as well
   as the armor of armadillos and many reptiles, and the shells
   or hardened integument of numerous invertebrates; external
   skeleton; dermoskeleton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

exoskeleton
    n 1: the exterior protective or supporting structure or shell of
         many animals (especially invertebrates) including bony or
         horny parts such as nails or scales or hoofs