Chitin, the substance which forms most of the hard parts of jointed-footed animals (arthropods), such as crustaceans, insects, and spiders. It was discovered by Odier in 1823, but regarded erroneously as free from nitrogen; rediscovered by Lassaigne in 1843; and since then recognised in all the four chief classes of arthropods and in some other types. Huxley has given an account of its formation in the crayfish. Chitin has been demonstrated in many arthropods, also in the pen of cuttle-fishes (Mollusca), and in the stalk and shell of Lingula anatina, a brachiopod. Its presence is at least probable in many other cases. In arthropods it is not confined to forming the firm and often very hard exoskeleton, but occurs internally in supporting plates, &c. among the tissues. In the crayfish it seems even to form the sheath of the strong ventral nerves. In arthropods the greater portion of the gut is formed as an intucking of the outer skin (ectoderm) from in front and behind, and the resulting portions known as fore- and hind-gut are also lined by this chitin, which frequently exhibit special internal thickenings for food-grinding purposes. The chitinous coating or cuticle is formed from underlying skin cells. In some cases the epidermic cells probably sweat it off after the manner of other secretions; but Huxley has shown in regard to the crayfish that the superficial portion of the cells undergoes a chitinous modification, being literally turned into chitin. The outer coat of crustacea is hardened by the addition of calcium carbonate or phosphate, especially the former; such calcareous hardening is very rare in insects, but copper has been demonstrated in some beetle cuticles.
Chitin is an amorphous white substance. It contains nitrogen, but is free from sulphur. Its resistance to acids and alkalies is very great; it is unaffected by digestive ferments, by water, hot or cold, by alcohol or ether. It may be dissolved by strong mineral acids (hydrochloric or sulphuric), and prepared from the cleaned exoskeleton of a lobster, or better still from the pen of a squid. Chemically it is regarded as a derivative of carbohydrates, and may be split up into sugar and glycosamin. According to Ledderhose, its formula is ; according to Sundwik, . See Krukenberg, Vergleichend-physiologische Vorträge (1886).