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SHELLS. 
Shells are envelopes, formed by a calcareous substance, of a foliated 
texture, and almost as heavy and hard as marble. They form coverings for a 
great number of animals of the class of Mollusca ; and every one knows that 
the variety of their forms, their more or less vivid colours, and the brilliancy 
of their mother of pearl, constitute some of the finest ornaments of the 
cabinets of virtuosi. We have sufficiently explained these forms, and we shall 
presently show their relation with the Orders and Genera of the animals 
which inhabit them : at present we have only to consider their texture, their 
growth, andthe manner in which they are united to the rest of the body) 
They are composed, like bones, of a calcareous matter ; intimate y con- 
nected with a gelatinous substance, and which may be, in like manner, 
separated by means of acids ; but this matter is not disposed in lamina, or in 
fibres ; it is uniformly extended throughout the whole body of the shell. 
It is only in some species that we find strata easily separated, and, as it 
were, agglutinated to each other like the leaves of paper in the formation of 
pasteboard. We know, from observation, that these strata do not all exist in 
younganimals; they haye only the most external, which are, at the same 
time, the smallest. In proportion as the animal increases in age, it forms a 
new stratum on the internal surface of the shell, which extends beyond the 
edges of all the preceding strata, so that each operation of this kind adds to 
the size of the shell, in length, breadth, and thickness. These are certain 
facts ; to prove them it is only necessary to compare some shells of the same 
species that have belonged to individuals of different ages; the fewest strata 
will always be found in the shells of the young. Muscles, which may be 
observed when very young, and eyen before they quit the matrix of their 
mother, have, at that period, shells consisting of one stratum only; but the 
shell is not therefore soft and gelatinous; it possesses the same firmness as the 
adult shell, and its greater fragility is merely owing to its thinness. 
But are the strata which thus SiICERSmneAR augment the dimensions of shells, 
produced by developement, or by a simple juxta-position? Do the nutritive 
vessels deposit the calcareous juice at different points, or does it only transude 
through the skin of the animal, and attach wself to the pre-existing strata ? 
These are questions with respect to which physiologists are not agreed. 
The body of the snail appears to adhere to its shell only where the muscles 
are attached ; but Reaumur having placed thin pellicles between the body and 
parts of the shell, which he purposely broke, these fractures were not 
repaired ; but when this, or any other obstacle, no longer prevented the 
juices flowing from the surface of the skin, the injured part was speedily 
regenerated. 
These facts fayour the idea of the simple juxta-position of a transuded 
matler : we observe, however, on the other hand, that the oyster and muscle 
adhere to the shell not only by their muscles, but by the whole border of 
their mantle; besides, the oyster has always between the two last strata of 
the conyex valve. a considerable yacuity, which is filled with a feetid acrid 
liquor, and which communicates with the interior of the body by a particular 
aperture. How is this vacuity produced? and, above all, how is it removed 
upon the formation of each new stratum, if the arterial and absorbent vessels 
do not penetrate into the centre of the strata, to regulate its position, and to 
remove, from time to time, the particles of the shell? 
Some observations seem to prove that there are testaceous animals, which, 
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