Lynn Charleston, age 12, of Gadsden, Ala., for her question:

HOW MANY KINDS OF MOLLUSKS ARE THERE?

A mollusk is a soft bodied animal without bones. Mollusks include octopuses, clams, oysters and snails. They make up the largest group of water animals. There are about 100,000 known kinds of living mollusks. Fossils of another 100,000 species have been found by scientists.

Each year, scientists discover about 1,000 new species of mollusks.

There are five classes of mollusks, each with a common name and a scientific name. Here are the common names of the classes: univalves, bivalves, tooth shells, octopuses and squids and chitons.

A sixth class is called Monoplacophora, but this group is very rare. Scientists have found shells only as fossils in the deepest waters of the Pacific Ocean.l

Univalve means "one shell." It is the largest class of mollusks and includes slugs, snails and limpets. Many in this class have two pairs of tentacles or feelers on their heads.

Second largest class is the bivalve form. This group includes clams, oysters, muscles and scallops. All bivalves have two shells that are held together by hinges that look like small teeth. The shells are usually open but strong muscles pull them shut when the animal i s frightened.

Octopuses and squids are the most active mollusks. All of them live in the oceans and they swim about freely. Octopuses have eight tentacles and squids have 10.

Tooth shell mollusks have slender, curving shells that resemble tusks. All tooth shells live in the oceans where they burrow in the mud or sand. Tooth shells have no head or eyes and they feed on one celled plants and animals that are swept into the mouth by tentacles.

Chitons have flat, oval bodies covered by eight shell plates. the plates are held together by a tough girdle. Chitons usually cling firmly to rocks and roll up into balls when forced to let go.

Chitons have small heads and mouths, but no eyes or tentacles. Their long radula is crisscrossed with teeth, which they use to scrape seaweed from rocks for food. A radula is a ribbon of teeth that works like a rough file and tears apart the animal's food.

All mollusks have a skin like organ called a mantle, which produces the substance that makes the shell. The edges of the mantle squeeze out liquid shell materials and add them to the shell as the mollusk grows.

In mollusks with no outside shell, the mantle forms a tough wrapper around the body organs.

Mollusks live in most parts of the world. Some are found in the deepest parts of oceans while others live in wooded slopes of high mountains. Some can even be found in hot, dry deserts.

Wherever mollusks live, they must keep their bodies moist to stay alive. Most land mollusks live in damp places such as under leaves or in soil.