chitin, white, horny substance found in the outer skeleton of insects, crabs, and lobsters and in the internal structures of other invertebrates. It is a polysaccharide consisting of units of the amino sugar glucosamine. As a by-product of crustacean processing, chitin is used as a flocculating agent for waste water, a wound-healing agent, a thickener and stabilizer for foods and pharmaceuticals, an ion-exchange resin, a membrane for chromatography and electrodialysis, a binder for dyes, fabrics, and adhesives, and a sizing and strengthening agent for paper.