268 The Commercial Products of the Sea. 



To understand the composition of shells, a little pre- 

 liminary scientific definition must be given. It has been 

 observed that shells may be regarded as epidermal in their 

 character, being formed upon the surface of a filmy cloak- 

 like organ, called a mantle, which answers to the true 

 skin of other animals. A slimy juice, consisting of a 

 membranaceous tissue, consolidated by an admixture of 

 carbonate of lime, exudes from the glands of this important 

 organ, and, thickening in successive layers, becomes hard- 

 ened and moulded on the body, at first simple and un- 

 adorned, but subsequently embellished according to the 

 taste or inclination of the occupant. Each shell is therefore 

 composed of animal and calcareous matter ; the first con- 

 stitutes a membranaceous basis, which is equally curious 

 and beautiful, being either formed of cells with hexagonal 

 walls, or else of laminae, more or less wrinkled, like morocco 

 leather. Shells which are always concealed by the mantle 

 are colourless ; and those which are covered by the mantle- 

 lobes, when the animal expands, acquire a glazed or en- 

 amelled surface, like the cowries ; when the shell is deeply 

 immersed in the foot of the animal it becomes partly 

 glazed, as in Cymba. In all other shells there is an outer 

 layer of gelatinous matter forming what is called the 

 epidermis, although it is sometimes very thin and trans- 

 parent. 



Woodward well remarks that the forms and colours of 

 shells (as of other natural objects) answer some particular 

 purpose, or obey some general law ; but besides this there 

 is much that seems specially intended for our study and 

 calculated to call forth enlightened admiration. Thus the 

 tints of many shells are concealed during life by a dull 

 external coat, and the pearly halls of the nautilus are seen 

 by no other eyes than ours. 



