Chris Beeson, age 11, of High Point, No.Carolina, for her question:
How are seashells made?
Our planet Earth is jam packed with exquisite creations, most of them hidden where human eyes are not likely to see them. For example, thousands and thousands of assorted shells are in the sea, each one a beauteous work of art. The boneless little bodies that build them live and die out of sight. We get a chance to admire them only when divers find them below or when the empty shells wash up on the beaches.
One could spend a lifetime collecting lovely seashells and still fail to find 11 the different varieties. They are created by fleshy little animal called mollusks. And though each species builds the shell of his own special design, all of them build in more or less the same basic way.
The shell, of course, serves as a sturdy home to protect the bone¬less little body. The soft body is a package of internal organs, plus a cloak of fleshy skin called the mantle. This all important mantle wraps loosely around the main body, under the shell. It contains the highly specialized cells that do the shell building.
The main building material is calcium carbonate, the same mineral that the earth uses to form marble and limestone. The shellfish absorbs this material with his food and special cells in the mantle process it into a limy liquid. Also in the mantle are glands that create a harden¬ing material. The shell is built layer by layer as these basic ingredients ooze from the mantle. As the limy liquid is secreted, hardening material is added to make it set hard and solid.
In most cases, the shell is a sandwich of three different layers. The outside layer is tough and often rough to cope with exposure to the salty sea. The inside layer is assembled from wafers of tiny prisms to provide a smooth, non scratchy surface. Often these inside walls are tinted with pale moonbeam colors to form lovely material called nacre, or mother of pearl.
The inside of an oyster shell is lined with smooth and flowing mother of pearl. But the outside of the shell is rough and drab to blend with the silty sea bed. But many shellfish, especially those of tropical seas, build elegant designs with very colorful exteriors. The colors are manufactured by cells within the mantle and added as the basic shell building materials are secreted.
For example, if the mantle contains two clusters of color cells, a couple of stripes are added as the cell grows. Spots are added by color clusters that stop working from time to time. In any case, the magic mantle continues to add layers to the shell as long as the animal lives, which usually is about six years.
The miracle, of course, is how each species knows how to build a perfect copy of the shells its ancestors built. The answer is DNA, the miraculous biochemical in the nucleus of every living cell. Every shell¬fish inherits DNA from its ancestors. This miraculous material contains detailed instructions to build the proper shell and also to carry on all the other duties of the living cells, just as its ancestors did.