EXTERNAL STRUCTURE. - The skin of the Crustacea is more or less completely hardened by a horny deposit called " chitine," with or without the addition of lime,1 thus forming a defensive covering to the softer tissues of the animal. This hardened envelope serves also as an external skeleton, giving rigidity and support to the Animal matter 28.6 Phosphate of lime 6.0 Carbonate of lime 62.8 Phosphate of magnesia l '0 soda salts, &c. 1-6 internal organs ; and to its inflections, projections, and rugosities the muscles and membranes of the body and appendages are attached.