Mollusks, sometimes spelled molluscs, are invertebrates (animals without backbones) that belong to the phylum Mollusca.
They live in the oceans, in freshwater and on land.
Clams and oysters, octopuses and squids, slugs and snails are all mollusks.
A mollusk's body is divided into three parts: the head, the visceral mass and the foot.
The head contains the mollusk's brain and sense organs.
Within the visceral mass lie the internal organs.
The foot, which is found at the bottom of the mollusk's body, is muscular. In some mollusks, the foot is used for locomotion. In others, it acts as a sucker and attaches the mollusk to a hard surface.
All mollusks have a mantle, which is a fold of the outer wall of the body. The mantle covers the visceral mass.
The mantle surrounds a mantle cavity that contains organs for breathing, excreting and reproducing.
In a large number of mollusks, the mantle secretes substances that form a shell. This shell is made mostly of calcium carbonate.
Mollusks that live in the water and have shells are often known as shellfish.
Many mollusks have an organ called a radula that is made up of chitin, the same material that makes up the exoskeletons of arthropods. The radula acts like a tongue. It has a rough surface and the mollusk uses it to scrape away food.
The oldest known fossils of mollusks are about 500 million years old.