Endoskeleton
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An endoskeleton is an internal support structure of an animal. Three phyla of animals possess true endoskeletons of various complexity: Chordata, Echinodermata, and Porifera. An endoskeleton allows the body to move and gives the body structure and shape. A true endoskeleton is derived from mesodermal tissue. Such a skeleton is present in echinoderms and chordates. The poriferan "skeleton" consists of microscopic calcareous or siliceous spicules or a spongin network. In arthropods, the endoskeleton is derived from the epidermis, and is continuous with the exoskeleton, though internalized. It is comprised of various internal rigid support rods (e.g., the tentorium), the membranous tracheae, and the anterior and posterior portions of the digestive tract; all of these are shed along with the exoskeleton during ecdysis (moulting).
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