Carolynn Miller, age 12, of Libertyville, I11., for her question:
HOW ARE SHELLS FORMED?
Mother Nature designed shells to protect her small soft bodied animals from the hungry sea. At least 100,000 creatures build these protective homes for themselves without even trying. The main ingredient is a limy material called calcium carbonate. However, each shell building species uses a different design.
The clam builds a pair of rather humdrum shells that can be snapped shut or partly opened. The murex builds a single shell, handsomely coiled, spiked and lined with glossy pink. The snail also builds a single coiled shell and, as in most shells, those of the tropics are more gorgeously colored.
A collector of beauteous objects could spend a lifetime gathering seashells and never find all the different varieties. All these shell builders have a special flap of soft, moist tissue called the mantle. It wraps around the body and lies just below the shell. The amazing mantle contains special cells and glands for collecting the raw materials and building the shell.
It extracts limy calcium carbonate and perhaps other minerals from the water. Certain glands process these simple ingredients to form tacky liquid. Other glands add materials to make the liquid set hard and strong. More glands and coloring materials, sometimes in bright, eye catching designs.
As the soft bodied animal grows larger, extra layers are added around the edge of his shell to give him more living room. As a rule, the shell is built in three layers. Outside is the prismatic layer, made from molecules arranged in prism type patterns. This layer is usually rough and durable. It may be ridged or spiked, gorgeously tinted or colored to match the drab ocean floor.
The inside layer, next to the animal's delicate skin, is a glossy, smooth layer of nacre, also called mother of pearl. Sandwiched between the prismatic and nacre layers is the durable lamellar layer. The mantle arranges molecules in different patterns to form these different layers. The mixtures ooze out from tiny tubes, first one layer and then another around the edge of the shell.
Some of the loveliest shells are adorned with colorful stripes or spots. The colors are added by separate clumps of cells in the mantle. As new material is added, they may paint a bright stripe around the coils. In spotted shells, the color making cells work for a while, then take a rest. Meantime the mantle has added a strip of the background material.