ON DESIGN IN ANIMAL BODIES. 193 



priate and perfect, becoming more and more me- 

 chanical and complex in its construction, from the 

 lithophytes, testacea, Crustacea, reptiles, fishes, 

 mammalia, up to man. 



The texture of a sponge, its form and elasticity, 

 depend upon a membranous and horny substance, 

 to which both siliceous and calcareous spiculae 

 are added. 



Carbonate of lime is the hardening material of 

 shell, united to a membranous or cartilaginous 

 animal matter. Our author describes the slime 

 of a snail hardening into shell by the influence of 

 the atmosphere ; but this is a very imperfect, and 

 indeed erroneous view of the matter. The shell 

 of the oyster, and even the pearl, consist of con- 

 centric layers of membrane and carbonate of 

 lime ; and it is their lamellated arrangement 

 which causes the beautiful iridescence in the 

 polished surface of those shells.* In the rough 

 outer surface of an oyster-shell, we shall see the 

 marks of the successive layers. We have to un- 

 derstand, that that which now forms the centre 

 and utmost convexity of the shell was, at an earlier 

 stao^e, sufficient to cover the whole animal. But 

 as the oyster grows, it throws out from its surface 

 a new secretion, composed of animal matter and 

 carbonate of lime, which is attached to the shell 

 already formed, and projects further at its edges. 



*■ See the discoveries of 'Sir David Brewster on this subject. 

 Phil. Trans. 1814, p. 397. 



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