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A polymer of repeating sugar molecules (a slightly modified glucose, poly-N-acetyl-D-glucosamine). See image. Chitin is the material which makes up the exoskeletons of and other . In arthropods, chitin occurs in a crosslinked form, α-chitin, and is strengthened with the additions of structural proteins and calcium carbonate salts. Significantly, it is also found in the radular "teeth" and operculums of , the setae (bristles) and jaws of worms, and the cell walls of . So, this is exceedingly ancient stuff, predating the split between animals, fungi and protists and possibly even predating the split between bacteria and metazoans. [What may be of sociological interest is that the 1,5 aldose linkage was missing from the middle glucose in this diagram and no one spotted it for over a year ....]
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