What makes up the hard exoskeletons of an arthropod?

The exoskeleton of arthropoda is made of a tough protein called chitin, a long-chain polymer comparable to cellulose. It fills the same role as the protein keratin in other animals (found in hair, nails, hooves, claws, beaks, etc). Some arthropods, like crustaceans, further harden their exoskeleton by biomineralization with calcium carbonate. Because it is inflexible, the organism has to periodically shed their entire exoskeleton in order to grow.