Endoskeleton

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Endoskeleton of a swordfish

An endoskeleton is an internal support structure of an animal. In three phyla and one subclass of animals, endoskeletons of various complexity are found: Chordata, Echinodermata, Porifera and Coleoidea. 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. The Coleoidae do not have a true endoskeleton in the evolutionary sense; here, a mollusc exoskeleton evolved into several sorts of internal structure, the "cuttlebone" of cuttlefish being the best-known version.

In arthropods, the endoskeleton is derived from the epidermis, and is continuous with and an extension of the exoskeleton, although internalized. It is composed 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 proper, during ecdysis (moulting). However, the endoskeleton of spiders is partly derived from mesodermal tissue and therefore comparable to vertebrate cartilage^[1]