Chitin (C8H13O5N)n (/ˈkaɪtɪn/ KY-tin) is a long-chain polymer of N-acetylglucosamine, an amide derivative of glucose. This polysaccharide is a primary component of cell walls in fungi, the exoskeletons of arthropods, such as crustaceans and insects, the radulae of molluscs, cephalopod beaks, and the scales of fish and skin of lissamphibians, making it the second most abundant polysaccharide in nature, behind only cellulose. The structure of chitin is comparable to cellulose, forming crystalline nanofibrils or whiskers. It is functionally comparable to the protein keratin. Chitin has proved useful for several medicinal, industrial and biotechnological purposes.