Ahmed Matiur Rahman
If youve ever walked along a beach, youve probably seen sea-shells lying on the sand where it has been washed in by the waves. The shell will nearly always be empty, for it is the home of some sea animal that had died.
By the way, shells are also found in wood lands, rivers and ponds as well as the sea. When people speak of shells, they usually mean those of soft bodied animals known as mollusks.
Most mollusks have shells outside their soft bodies. The shell is a mollusks skeleton. It is part of the animal and the mollusk is attached to it by muscles. The soft animal inside can never leave its shell and return to it.
The shell is made of a form of limestone and is built by the mollusk itself. Certain glands in the mollusk are able to take lime stone from the water and deposit it in tiny particles at the edge of, and along the inside of, the shell. As a mollusk grows in size, its shell increases in thickness and size. You can see lines of growth that are marked by ridges that sun parallel to the outer edge. Youve probably noticed these growth lines in the shells of oysters. The other ridges are caused by ridges in the mantle of mollusk, or by muscles in its body.
The shell of a mollusk consists of three layers. The outside is covered with a thin layer of hornlike material that contains no line. Under this is a layer of carbonate of lime. The inside layer is the mother of pearl, or nacre. It is made up very think alternate layers of carbonate of lime and a horny substance.
The coloring of the shell comes from some glands of the mollusk that contain coloring matter. So a shell may be spotted, all one color, or marked with lines. Some shells are so tiny they can only be seen with a magnifying glass, while the giant clam has a shell that can be four feet long.