Making a Shell
The blood of a mollusk is rich in a liquid form of calcium. A soft,
outer organ called the mantle concentrates the calcium in areas where
it can separate out from the blood, forming calcium carbonate crystals.
The mantle deposits sheets of the crystal in varying thicknesses. The
individual crystals in each layer vary in shape and orientation. One
such layer is called mother-of-pearl, or nacre. It's often found in the
inner layer of shells of some gastropods, bivalves and cephalopods. The
layered construction increases the strength of the entire shell.
The mantle orchestrates the formation of the shell's external features,
such as ribs and spines. It also designs the colors of the shell.
Production of new shell material is influenced by several factors:
sexual hormones, intrinsic rhythms, diet, acidity of water and
temperature of water.
Colors in shells are derived from organic pigments found in food.
Glandular cells collect these pigments, mix them with fluid calcite,
and set this substance into the outer shell before it hardens. There
are four main pigments that produce the many colors seen in shells:
yellow carotenoids, black melanins, green porphyrins, and blue and red
indigoids. Some shell colors, such as blue-green iridescent sheens, are
caused not by color pigments, but by the refraction of light off
various layers of calcite in the shell.
Most color cells are located along the front edge of the mantle where
new shell material is added -- that's at the rim of the mouth for snail
shells or along the free edges of valves for clams. A straight color
line or ray is formed when the color cells remain in the same position
as the shell grows out. If color pigment production continually starts
and stops, a pattern of dots or dashes is drawn on the shell. If the
color cells actually migrate to one side, a slanting trail of color is
produced. Other kinds of behavior by color cells can produce circles,
triangles and other shapes.
Shells often have complex features such as spines, ribs, cords and
grooves. For some species, these features provide protection from
predators. Certain features also give the shells added strength and
assist in burrowing. Shells of creatures like the nautilus have complex
internal structures, such as multiple chambers containing water and gas
that allow the creature to adjust its buoyancy.
Most gastropods have coiled shells in the form of a logarithmic spiral,
the only form of a coil that can both increase in size and retain its
shape. Such an architecture is an efficient way to maintain strength
while retaining the compactness of the shell.
Some mollusks, called carrier shells, actually cement old discarded
shells and other seafloor debris to their own shells. The Pallid
Carrier shells of Japan and the Common West Indian Carrier shells in
fact appear like piles of dead shells. Scientists believe that the
mollusks use this technique as a means to camouflage themselves from
predators, for additional strength, and to prevent sinking into soft
sand or mud.
While many mollusks have shells for protection from predators and
environmental stresses, as well as support for their soft tissue,
shells also have their disadvantages. Shells are permanent structures
that mollusks must carry around for the rest of their lives, and its
weight could slow the creature down. Some mollusks, such as the squid
and octopus, have evolved to eliminate shells altogether.
Source:
http://www.all-about-seashells.com/article/index.php?id=13&q=Making_a_Shell