The Collaborative International Dictionary of English v.0.48:

Endoskeleton \En`do*skel"e*ton\, n. [Endo- + skeleton.] (Anat.)
   The bony, cartilaginous, or other internal framework of an
   animal, as distinguished from the exoskeleton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

endoskeleton
    n 1: the internal skeleton; bony and cartilaginous structure
         (especially of vertebrates)