Cellulose contains a hydroxy group, and chitin contains acetamide. Usually, polymers are man-made. Crabs, beetles, worms and mushrooms contain large amount of chitin. Chitin is a very adaptive material for a creature. Insects and animals with chitin coats usually shed these coats, or molt, at least once a year.
Chitin is a very firm material, and it help protect an insect against harm and pressure.
Depending on its thickness, chitin can be rigid or yielding. Often, insect coats contain thick, stiff layers of chitin. The areas around legs and face contain very thin, pliable layers. In addition to being a naturally-occurring molecule, chitin is also notable for the process in which it is broken down. Because many sea creatures shed shells of this material every year, one would expect the ocean to be full of chitin.