Chitin is a:

Chitin is a long-chain polymer of a N-acetylglucosamine/ structural polysacchride, a derivative of glucose, and is found in many places throughout the natural world.It is the main component of the cell walls of fungi, the exoskeletons of ahropods such as crustaceans (e.g. crabs, lobsters and shrimps) and insects, the radulas of mollusks and the beaks of cephalopods, including squid and octopi. Chitin may be compared to the polysaccharide cellulose and to the protein keratin.