458 APPENDIX. 



of Limultis i.oi, and in that of Sejiia 1.22 per cent. These results 

 are especially intei-esting, as showing that chitin is not a substance 

 which is exclusively epiblastic in origin, but here, at least, we 

 have it occiurring in mesoblastic structures." 



III. Chitin and other Substances. 



Chitin. — This sul)stance, which is frequently impregnated with 

 salts (calcareous salts in the Crustacea, silica in the lingual ribbon 

 of certain Molhisca), " has a very wide distribution among the 

 Ioive7~tebrata. It is in the Arthrojjoda that it is found to the 

 greatest extent ; it forms the membrane of the ovum, the cuticle 

 of the adult, with its appendages, the supporting substance in the 

 trachefe of insects, &c. It is also found in the Mollusca (jaws and 

 odontophore) ; and in worms {e.g., the seta? of the Annelida). It 

 foi-ms the membrane of the ova in other groups, and the cyst-wall 

 in encysted forms of the Protozoa, etc." 



Chitin is readily prepared by treating the shells of crabs and 

 lobsters with HCl, so as to dissolve out the calcareous salts. It 

 is also obtained by digesting the wings of beetles and other insects 

 in a solution of NaHO. In both cases the chitin remains undis- 

 solved. The residvie is then dissolved in strong HCl, and re- 

 precipitated from this solution by the addition of water. This 

 operation is repeated two or three times, when the chitin is 

 obtained in a state of purity. 



Chitin is a colourless substance, devoid of crystalline structure, 

 and is only soluble in strong mineral acids. When heated with 

 strong acids, it is decomposed into acetic acid and glucosamine : — 

 2C,,H,,N,0,„ + 2H,0 = 3G,HA + 4CeH,3NO,. 



Conchiolin (Cj^H^gNj^Oj,) is the skeletin or basis of the shells of 

 the Gasterojjoda. 



Cornein (i^^^^^,f>^^ is the skeletin of Gorgonia and other 

 corals. 



S2Jongin is the skeletin of the Porifera. Its composition is 

 unknown. 



Fihrorin is the substance of which the webs of spiders are com- 

 posed. 



