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“The chemical stability of chitin is so remarkable that we might expect it to accumulate like the inorganic constituents of animal skeletons, and form permanent deposits. Schlossberger (Ann. d. chem. u. pharm., bd. 98) has, however, shown that it changes slowly under the action of water. Chitin kept for a year under water partially dissolved, turned into a slimy mass, and gave off a peculiar smell. This looks as if it were liable to putrefaction. The minute proportion of nitrogen in its composition may explain the complete disappearance of chitin in nature.” (Miall and Denny, The Cockroach, p. 29.)

Chitin, or a substance closely similar to it, occurs in worms and in their tubes, especially in the pharyngeal teeth of annelids and in their setæ. The shell of Lingula and the pen of cuttle-fish contain true chitin (Krukenberg). The integument of Limulus, of trilobites, and of Arachnida, as well as Myriopoda, appears to consist of chitin.^[11]

The chitin is rapidly deposited at the end of embryonic life, also during the larval and pupal stages. As is well known, insects after moulting are white, but in a few hours turn dark, and those which live in total darkness are white, showing that light has a direct effect in causing the dark color of the integument.