A general term applied to the hard covering of many invertebrates. It may be a shell growing at the edges only (by accretion) or a series of plates. The term is most commonly applied to the horny skeleton enclosing the body of all Arthropoda and secreted by the underlying cellular layer. The exoskeletal material (cuticle) is composed of a complex glycoprotein, is relatively impermeable to water, and has a high strength-to-weight ratio. It provides insertion sites for muscles (apodemes) and is divided into separate plates that facilitate movements. The plates are connected by thin, untanned cuticle. In trilobites (Trilobita) and crustaceans (Crustacea) the cuticle is impregnated with mineral salts (calcium carbonate and calcium phosphate) which give increased strength. The exoskeleton is moulted periodically (ecdysis) to permit body growth.