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LETTER XIV. 



ON THEIR SECRETIONS. 



This letter you may properly consider as a continuation 

 of the preceding ; for the blood is the material out of which 

 not only the growth and repairs of animal bodies, but like- 

 wise all the secretions, are directly derived, — the latter 

 obtaining their peculiar properties from differences in the 

 structure and action of the various glands or vessels through 

 which they percolate. 



1. Shell. — The most important and general secretion 

 of molluscous animals is shell, but as the structure and 

 formation of this must occupy a separate letter, I will 

 at present confine myself to some account of its chemical 

 composition, with such other remarks as may not find a 

 more appropriate place in our correspondence. 



However varied in external character, shells differ very 

 little in their chemical composition. They all consist of 

 carbonate of lime united to a soft albuminous matter, and 

 any variation that occurs in different shells is merely in the 

 relative proportions of these constituent parts. Mr. Hat- 

 chett, to whom we are indebted for all our correct informa- 

 tion on this subject, has divided shells into two classes, 

 according to the proportion and state of their animal matter. 

 The first class he names Porcellaneous Shells, since they 

 resemble porcelain, are usually of a compact texture, and 

 have an enamelled surface, which is often finely variegated. 

 The convolute shells afford good examples of this class. 

 They consist of carbonate of lime, cemented together by 

 so small a portion of albumen, that, when immersed in a 

 dilute acid, the shell is completely dissolved, and not a 

 sensible trace of it left behind. " The shells belonging 

 to the second class are usually covered with a strong epider- 

 mis, below which lies the shell in layers, and composed 

 entirely of the substance well known by the name of 

 mother-of-pearl. They have been distinguished by the 

 name of Mother-of-pearl Shells." The fresh-water mussels, 

 the oysters, the Haliotis, and pearly Turbines are examples 

 of this class. " When immersed in acids they effervesce, 

 at first strongly, but gradually more and more feebly, till 



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