THE INTEGUMENT 29 



ness and flexibility, and is usually very dense, impermeable, and 

 light, compared with the crust of the Crustacea, where the cuticle 

 becomes heavy and solid by the deposition of the carbonate and 

 phosphate of lime. This is due to the presence of a substance called 

 by Odier chitin'. 1 The cuticle is thin, delicate, and flexible between 

 the joints; it is likewise so in such diaphanous aquatic larvae as 

 that of Corethra, and in the gills of aquatic insects, also in the walls 

 of the tracheae and of the salivary ducts. The cuticle thus forms 

 a more or less solid crust which is broken into joints and pieces 

 (sclerites), forming supports for the attachments of the muscles and 

 serving to protect the soft parts within. 



Chitin. If we allow an insect to soak for a long time in acids, or 

 boil it in liquid potassa or caustic potash, the integument is not 

 affected. The muscles and the other soft parts are dissolved, leav- 

 ing the cuticle clear and transparent. This insolubility of the cuti- 

 cle is due to the presence of chitin, the insoluble residue left after 

 such treatment. It also resists boiling in acids, in any alkalies, 

 alcohol or ether. The chemical formula is C^H^N-jON). 2 



"Chitin forms less than one-half by weight of the integument, but it is so 

 coherent and uniformly distributed that when isolated by chemical reagents, and 

 even when cautiously calcined, it retains its original organized form. The color 

 which it frequently exhibits is not due to any essential ingredient ; it may be 

 diminished or even destroyed by various bleaching processes." (Miall and 

 Denny.) 



" The chemical stability of chitin is so remarkable that we might expect it to 

 accumulate like the inorganic constituents of animal skeletons, and form perma- 

 nent 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 info 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 setae. The shell of 

 Lingula and the pen of cuttle-fish contain true chitin (Krukenberg) . The integu- 

 ment of Limulus, of trilobites, and of Arachnida, as well as Myriopoda, appears 

 to consist of chitin. 3 



The chitin is rapidly deposited at the end of embryonic life, also 

 during the larval and pupal stages. As is well known, insects after 



1 Lassaigne gave it the name of entomoline. 



2 Miall and Denny ex Krukenberg ; Kolbe gives the formula as C 9 H 15 NO 6 or 

 CjsHjsNO^. As the result of his recent researches, Krawkow (Zeits. Biol., xxix, 

 1892, p. 177) states that the chemical composition of chitin may prove to be some- 

 what variable. 



3 On allowing portions of a locust, a piece of the integument of Limulus, a scorpion, 

 and a myriopod to soak for a month in white potash, neither were dissolved or 

 affected by the reagent. 



