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SHELLS. 



Shells are envelopes, formed by a calcareous substance, of a foliated 

 texture, and almost as heavy and bard 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, and the manner in which they are united to the rest of the body. 



They are composed, like bones, of a calcareous matter ; intimately 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, thatth&e strata do not all exist in 

 young animals; they have 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 fe^tends beyond the 

 edges of all the preceding strata, so that each operation of ttu s 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 C« same 

 species that have belonged to individuals of different ages ; the fewest &. tra , ta 

 will always be found in the shells of the young. Muscles, which may tZ°' 

 observed when very young, and even 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 successively 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 itself 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. * J 



These facts favour the idea of the simple juxta-position of a transuded 

 matter : 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 convex valve, a considerable vacuity, which is filled with a foetid 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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