The Shell-builders. 



What of the material out of which the animal con- 

 structs its shell, and the apparatus used in the work ? 

 Carbonate of lime is the essential building material, and 

 is obtained from the seaweeds and other vegetable matter 

 which the animal eats ; while, should it be a carnivorous 

 Mollusc, by devouring others that are plant eaters it 

 obtains its supply in a second-hand manner. In the 

 work of constructing the shell the animal has no elaborate 

 apparatus at its command, the work being performed by 

 the external membranous layer that invests the body 

 the mantle, as it is called, albeit a delicate and remark- 

 able organ. The thickened margin of the mantle is 

 glandular, and contains in its substance patches of 

 various colours. It is this thickened margin which 

 produces the material for the increase in size of the 

 shell, while the colour patches correspond both in their 

 tint and relative position with the colours that decorate 

 the exterior of the shell. When the little shell-builder 

 is engaged in increasing the dimensions of its portable 

 dwelling, the margin of its mantle is protruded, and 

 firmly adheres to the margin of the shell. In this posi- 

 tion the mantle secretes the calcareous matter, deposit- 

 ing it in a soft condition upon the extreme edge of the 

 shell, where this secretion soon hardens into a solid 

 shelly layer. This process is repeated again and again 

 at regular intervals, each succeeding layer further enlar- 

 ging the diameter of the shell. 



The quantity of this calcareous material secreted by 

 the edge of the mantle appears to be much more copious 

 at certain periods than at others, and it is at such times 

 of abundant supply of material tl>at ridges, broad plates, 

 or spines of different length are formed by the mantle 



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