Publisher Summary Chitin that has a widespread distribution amongst invertebrates and the lower forms of plant life, is the most abundant of those polysaccharides which contain amino sugars, and it consists predominantly, if not entirely, of unbranched chains of β-(1 → 4)-linked 2-acetamido-2-deoxy-D-glucose (N-acetyl-D-glucosamine) residues. It may thus be regarded as a derivative of cellulose, in which the C2 hydroxyl groups have been replaced by acetamido residues, and, indeed, chitin resembles cellulose in many of its properties. Both polysaccharides serve as structural and defensive materials in Nature, chitin being most prominent in the fungi and in the arthropods, where it is a principal component in the exoskeletons of crustaceans and insects. Chitin usually forms a part of very complex systems and is not found alone; thus, insect exoskeletons are largely composed of chitin-protein complexes, whereas crustacean shells usually contain large proportions of calcium carbonate in addition to protein. A purified chitin can be obtained by immediate precipitation of the crude material from its solution in concentrated hydrochloric acid, but x-ray diffraction studies present that a considerable shortening in the chain-length of the molecule results.