Chitin is the main component of
the cell walls of fungi, the exoskeletons of
arthropods such as crustaceans (e.g., crabs,
lobsters and shrimps) and insects, the radulas of
mollusks, and the beaks and internal shells of
cephalopods, including squid and octopuses.
Combined with calcium carbonate, as in the
shells of Crustacean and mollusks, chitin
produces a much stronger composite.