What's the "een" in Halloween?
endoskeleton en·do·skel·e·ton (ěn'dō-skěl'ĭ-tn)
An internal supporting skeleton, derived from the mesoderm, that is characteristic of vertebrates and certain invertebrates.
The internal supporting framework of humans and other vertebrates, usually made of bone. Certain invertebrates, such as sponges and echinoderms, also have endoskeletons. Compare exoskeleton.