How Animals Work. 



extending beyond its usual position at the circumfer- 

 ence f of the shell. In this way the external surface of 

 the shell is exclusively formed of layers deposited in 

 succession by the margin of the mantle. The coloured 

 spots in the tissue of the mantle are pigment glands, 

 capable of manufacturing the necessary colours for the 

 external ornamentation of the shell, and the pigment 

 furnished by these glands becomes mixed up with the 

 calcareous matter at the time of its deposition. Accord- 

 ing to the amount of pigment produced, and whether a 

 continuous supply is kept up during the whole period 

 that fresh material is being deposited in the process of 

 enlargement of the shell, or the supply of pigment is 

 only forthcoming at intervals, so the shape and size of 

 the bands and patches of colour on the surface of the 

 shell will vary. 



The margin of the mantle, therefore, is the sole 

 mechanism for enlarging the circumference of the shell 

 and its coloration ; but the growth in thickness of the 

 shell is attained by the secretion of a kind of calcareous 

 varnish produced by the external surface of the mantle 

 generally. This secretion is deposited layer by layer 

 over the whole of the previously existing shell, and in 

 this way the weight and solidity of the shell are pro- 

 gressively built up. Moreover, this thickening material 

 differs from that produced by the margin of the mantle, 

 inasmuch as it is not pigmented, but is of a beautiful, 

 lustrous white hue, and forms the well-known iridescent 

 material called nacre, or mother-of-pearl. 



The edge of the mantle is very often fringed with 

 small, delicate, and sensitive tentacles, and in some of 

 the Clams and Scallops (Pecteri) numerous brightly 



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