260 SOLID PARTS OF ANIMALS. 



CHAPTER VI. 



OF CRUSTS. 



By crusts we understand those bony coverings of which the 

 whole external surface of crabs, lobsters, and other similar sea 

 animals are composed. Mr Hatchett found them composed of 

 three ingredients: 1. A cartilaginous substance, possessing the 

 properties of coagulated albumen ; 2. Carbonate of lime ; 

 3. Phosphate of lime. By the presence of this last substance 

 they are essentially distinguished from shells, and by the great 

 excess of carbonate of lime above the phosphate they are equally 

 distinguished from bones. Thus the crusts lie intermediate be- 

 tween bones and shells, partaking of the properties and consti- 

 tution of each. The shells of the eggs of fowls must be referred 

 likewise to the class of crusts, since they contain both phosphate 

 and carbonate of lime. The animal cement in them, however, 

 is much smaller in quantity. From the experiments of Berniard 

 and Hatchett, it is extremely probable that the shells of snails 

 are composed likewise of the same ingredients, phosphate of lime 

 having been detected in them by these chemists. 



Mr Hatchett examined the crusts of crabs, lobsters, prawns, 

 and cray fish. When immersed in diluted nitric acid these 

 crusts effervesced a little, and gradually assumed the form of a 

 yellowish-white soft elastic cartilage, retaining the form of the 

 crust. The solution yielded a precipitate to acetate of lead, and 

 ammonia threw down phosphate of lime. Carbonate of ammonia 

 threw down a much more copious precipitate of carbonate of lime. 

 On examining the crust which covers different species of echini, 

 Mr Hatchett found it to correspond with the other crusts in its 

 composition. Some species of star- fish yielded phosphate of 

 lime, others none ; hence the covering of that genus of animals 

 seems to be intermediate between shell and crust 



With these observations of Mr Hatchett the analysis of Me- 

 rat-Guillot corresponds. From lobster crust he obtained, 

 Carbonate of lime, . \ 60 

 Phosphate of lime, . 14 



Cartilage, . . 26 



100* 

 * Ann. de Chim. xxxiv. 71. 



