What is a Mollusk and types of mollusks?

What is a Mollusk?

Mollusk is an invertebrate animal with a soft body and no bones. 
Snails, mussels and oysters are mollusks. 
Each of these has a hard shell on the outside that protects its soft body. Some mollusks, such as cuttlefish and squid, have hard shells that grow inside their bodies.
Octopuses and slugs are also mollusks. Octopuses have no shell at all.

Where do mollusks live?

Some mollusks live in water, and others live on the land. All mollusks need to keep their bodies moist to stay alive.
So those that live on the land find their way to damp places. They bury themselves in sand or soil, or crawl under leaves.

Snails

There are different types of snails.
Many snails that live in fresh water eat plants and dead animals.
Ocean snails feed on seaweed and other sea creatures.
Land snails often eat rotting plants.
Although most snails are small, some grow to be 2 feet (61 centimeters) long.
Many snails are an important food for fish and birds.
But some snails are poisonous, and others carry certain diseases.

A snail of most species has a soft body covered with a tube-like shell.
This shell coils around itself as it grows.
A snail's shell starts growing before the snail hatches, and stops growing only when the

What are Bivalves?

Mussels, oysters, and clams belong to a group of mollusks called bivalves. Bivalves have a hard outer shell made up of two halves that are held together by a hinge.
Bivalves live in water, and most of them live in the sea. 
Bivalves usually have their shells open so they can feed on tiny pieces of food from the water that they pump through their bodies.
When a bivalve is frightened, or taken out of the water, it closes its shell tightly.

Octopuses

Octopuses have eight long arms, called tentacles, whose undersides are covered with suckers.
They eat crabs, shrimps, and other small creatures caught with their tentacles.