Explain what an exoskeleton is and what it is made of.

The bodies of arthropods are supported, not by internal bones, but by a hardened exoskeleton made of chitin, a substance produced by many non-arthropods as well. In arthropods, the nonliving exoskeleton is like a form-fitting suit of armor. It is produced by the "skin" and then hardens into a protective outer-covering.