45 8 APPENDIX. 



of lAmulus 1. 01, and in that of Sepia 1.22 per cent. These results 

 are especially interesting, as showing that chitin is not a substance 

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

 have it occurring in mesoblastic structures.'' 



III. Chitin and othee Substances. 



Chitin. — This substance, which is frequently impregnated with 

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

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

 Invertehrata. It is in the Arthropoda 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 

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

 odontophore) ; and in worms {e.g., the setse of the Arvnelida). It 

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

 in encysted forms of the Protozoa, &c." 



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 residue 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 : — 

 20,,H,,N,0,„ + 2H,0 = 3C,Hp, + 4CeH,3N0,. 



ConchioUn (CjjH^jNjOjj) is the skeletin or basis of the shells of 

 the Gasteropoda. 



Cornein (Cj^H^^NjOu) is the skeletin of Gorgonia and other 

 corals. 



Spongin is the skeletin of the Porifera. Its composition is 

 unknown. 



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

 posed. 



