An endoskeleton is a structure that holds an animal from inside.
One can find endoskeletons in three phyla and one subclass of animals: in Chordata, Echinodermata, Porifera and Coleoidea. An endoskeleton allows the body to move and gives the body structure and shape.
A true endoskeleton develops from mesodermal tissue. Such a skeleton is present in echinoderms and chordates. The Coleoidae do not have a true endoskeleton in the evolutionary sense; here, a mollusc exoskeleton developed into several sorts of internal structure, the "cuttlebone" of cuttlefish being the best-known version. An important advantage of an endoskeleton over an exoskeleton is that the endoskeleton provides more structural support.