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John Wood Jr., age 13, of Tacoma, Wash., for his question:


Snails are divided into three groups: those that live on land, those that live in fresh water and sea snails, or those that live in salt water. All together, there are about 80,000 different kinds of snails on earth.

Salt water, marine or sea snails form the largest group of snails. There are about 55,000 of this kind. Some of the snails live along the seashore. Others live on the floors of the world's oceans.

The snail's shells are made of three layers: an outer, middle and 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 minerals in these layers provide a hard covering for the shell.

Food eaten by the sea snail provides the minerals that form the shell and give it color. The bloodstream 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 makes the shell in the form of a liquid.

Other glands in the mantle add a hardening material so that the liquid quickly becomes strong and firm. Still other glands give the color.

The pattern on a sea snail's shell depends on whether color is added continuously and the number of places in the mantle from which color is added. If color is added continuously from only one place in the mantle, the shell will have one stripe. Color might be added from four places, in which case the shell will have four stripes. Spots or bars form if the flow of color is interrupted from time to time.

Most sea snails continue to add material to their shells throughout their lives. As long as the animal grows, its shell also grows. The shells begin to grow before the snails hatch. After they leave the egg, their bodies rapidly increase in size.

Many kinds of water animals have shells. These include clams, lobsters, oysters, shrimps and snails. Largest of all shells belongs to the giant clam of the South Pacific Ocean. This creature's shell measures up to four feet long and weighs as much as 500 pounds.

Smallest of all shells belongs to the sea snail called a Vitrinellid. It has a shell that is no larger than a grain of sand.

A sea snail that is only one tenth of an inch long when it hatches may grow five to six inches in six months. Most sea snails continue to grow for about six years.

Most sea snails have gills.

Collecting shells is a very popular hobby in many parts of the world. People who collect them especially prize the colorful shells of the various kinds of sea snails.


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