Biologically, a skeleton is a hard or rigid framework that could provide protection and support in many types of organisms. Most of skeletons are mineralized, but some may not be mineralized (e.g., some annelid tubes being agglutinated). They could be subdivided into exoskeletons and endoskeletons, not including hydroskeletons. Exoskeletons are external hard parts. They may enclose the soft tissues and organs of the body, or may be external sclerites. Endoskeletons are internal hard parts (typical of many vertebrates). Most of endoskeletons are generally surrounded by skin and musculature. See entry “ Animal Skeletons, Advent” for further reading.