chitin

noun1

1. •
   A complex polysaccharide, a polymer of N-acetylglucosamine, found in the exoskeletons of arthropods and in the cell walls of fungi; thought to be responsible for some forms of asthma in humans.

   • Chitin usually occurs throughout Invertebrates in the form of an investment to the outermost cellular layer or ectoderm. — 1880