exoskeleton (cuticle) The horny skeleton that encloses the body of all Arthropoda. It is secreted by an underlying cellular layer (the hypodermis). The structure is complex. Immediately above the epidermis is the Schmidt layer; above that an endocuticle of chitin; and above that an exocuticle of tanned chitin which is frequently dark in colour. There is a thin epicuticle (the outermost layer) covered with wax or grease. Internal to the epicuticle is the procuticle, comprising an outer exocuticle and an inner endocuticle. Both are composed of a complex glycoprotein formed from the binding of chitin with protein; the molecular structure of the exocuticle is further stabilized by phenolic tanning and additional cross-linkages. The precise nature of these layers–whether they are tanned, lamellar, etc.–varies from species to species, and also from place to place in the exoskeleton of an individual insect, according to its species. In Crustacea, the procuticle is also impregnated with mineral salts, calcium carbonate, and calcium phosphate. Cuticle is relatively impermeable to water and has a high strength-to-weight ratio, two characteristics important for the successful conquest of the land by the arthropods. Ingrowths of the cuticle, called ‘apodemes’, provide insertion sites for muscles. The division of the cuticle into separate plates facilitates movement: the plates are connected by the so-called arthrodial membrane, which comprises thin, flexible, untanned cuticle. Periodic moulting of the cuticle (ecdysis) permits body growth.

exoskeleton 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 phośphate) which give increased strength. The exoskeleton is moulted periodically (ecdysis) to permit body growth.

ex·o·skel·e·ton / ˌeksōˈskelitn/ • n. Zool. a rigid external covering for the body in some invertebrate animals, esp. arthropods, providing both support and protection. Compare with endoskeleton.DERIVATIVES: ex·o·skel·e·tal / ˌeksōˈskelətl/ adj.

exoskeleton A rigid external covering for the body in certain animals, such as the hard chitinous cuticle of arthropods. An exoskeleton protects and supports the body and provides points of attachment for muscles. The cuticle of arthropods must be shed at intervals to allow growth to occur (see ecdysis). Other examples of exoskeletons are the shells of molluscs and the bony plates of tortoises and armadillos. Compare endoskeleton.

exoskeleton Protective skeleton or hard supporting structure forming the outside of the soft bodies of certain animals, notably arthropods and molluscs. In arthropods, it consists of a thick horny covering attached to the outside of the body and may be jointed and flexible. The exoskeleton does not grow as the animal grows; instead it is shed periodically and the animal generates a new one.