CART.
FORMATION OF THE SHELL
It is the mantle which secretes the shell, with each
part of the mantle specializing in secreting a different
layer. At its edges the mantle forms a roll or lobe, the
outer part of which produces the periostracum or outer
layer of the shell. That outer layer is made of a horney
organic substance called conchioline which is laid down
in overlapping layers --- similar, if you will, to the layered
feathers on a bird. The external wall of the mantle lobe
secretes the rainbow-colored middle layer of the shell,
consisting of calcite crystals in a conchioline matrix. The
total external surface of the mantle within the lobe
secretes crystals of calcium carbonate called aragonite,
which is deposited in layers on a delicate web of protein
matter (conchioline) and forms the inner layer of the shell.
The inner layer is what we call mother-of-pearl, the edges
of which have the coloration (greenish-black with purple
highlights) characteristic of the Pinctada Margaritifera
and also of the so-called "black" pearls of Polynesia.
Since the periphery of the mantle secretes a tinted
mother-of-pearl and the center produces a white nacre, it
is clear that if we want to produce black pearls it is the
edge of the mantle that must be grafted. The central
portion would be more likely to produce white pearls. The
external face of the mantle (the part touching the shell) is
lined with an epithelial layer made up of cells comparable
to those that form the human skin and glands, such as
the sebaceous and sweat glands, that are indentations of
the skin. In the oyster these epithelial cells are
responsible for secreting the organic matter and calcium
carbonate that make up the shell. It is useful to know how
the shell is produced, i.e. by a process quite similar to the
growth of our fingernails, because it is this same process
that produces pearls.
PINCTADA MARGARITIFERA
HOLD POINTER OVER IMAGE FOR DESCRIPTION
CLICK ON IMAGE FOR LARGER VIEWING OF DIFFERENT
STAGES IN POLISHING THE OUTSIDE OF THE
PINCTADA MARGARITIFERA SHELL
