Jimmy Lombard, age 12, of Poughkeepsie, N.Y., for his question:
HOW ARE SEA SHELLS FORMED?
Sea shells, generally speaking, cover the bodies of mollusks, a group of animals that include clams, oysters, cowries, conches and snails. The shells are formed in layers.
There are actually about 100,000 kinds of mollusk shells. Each kind has its own special design and shape, but all are formed in much the same way. Some mollusks that grow the shells live in the ocean, some in fresh water and others on land.
Most shells consist of three layers: an outer, middle and inner layer. These layers are also called the prismatics layer (outer layer), the lamellar layer (middle layer) and the nacreous layer (inner layer).
Each layer contains a form of calcium carbonate, a kind of limestone also found in marble and other kinds of rocks. In most shells, the mineral in these layers provides a hard covering for the shell.
In the outer layer, the mineral may be in the form of small particles called prisms. In the inner layer, the mineral is often produced as a smooth, shiny substance called nacre or mother of pearl. The nacre of certain kinds of shells, including those of clams and scallops, has a dull appearance.
The food eaten by a mollusk provides the minerals that form the shell and give it color. The blood stream of the animal carries the minerals to the mantle, a fleshy skinlike tissue inside the shell. Special glands in the mantle produce the substance that make the shell in the form of a liquid. Other glands in the mantle add a hardening material so that the liquid quickly becomes firm and strong. Still other glands produce the color.
The pattern of the color depends on whether color is added continuously and the number of places in the mantle from which color is added.
As long as the animal grows, its shell also grows.
Most kinds of mollusks add material to their shells throughout their lives. Claims and snails begin to grow shells before they hatch. After they leave the egg, their bodies rapidly increase in size.
A sea snail that is only one tenth of an inch long when it hatches may grow five or six inches in six months. Most clams and sea snails grow for about six years.
If color is added continuously from only one place in the mantle, the shell will have one stripe. If color is added continuously from four places, the shell will have four stripes. If the color flow is interrupted from time to time, spots or bars will form on the shell.
Some kinds of shells have beautiful shapes and bright colors. Others are plain and colorless.
Among the smallest kinds are the shells of the Vitrinellid, a sea snail found in many parts of the world. Some Vitrinellid shells grow only about as big as a grain of sand. The largest shell is that of the giant clam of the South Pacific Ocean. Its shell may measure four feet long and weigh 500 pounds.