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a nitrogen-containing polysaccharide, related chemically to cellulose, that forms a semitransparent horny substance and is a principal constituent of the exoskeleton, or outer covering, of insects, crustaceans, and arachnids.

an inert carbohydrate, (C 6 H 10 O 5 ) n, the chief constituent of the cell walls of plants and of wood, cotton, hemp, paper, etc.