Fossils of mollusks have been found in rocks and date back over 500 million years. Mollusk fossils are usually well preserved because of their hard shell. Most mollusks have a soft, skin-like organ covered with a hard outside shell. Some mollusks live on land, such as the snail and slug. Other mollusks live in water, such as the oyster, mussel, clam, squid and octopus.
Land living mollusks, like the snail, move slowly on a flat sole called a foot.
Ocean living mollusks move or swim by jet propulsion. They propel themselves by ejecting water from their body. For example, the squid ejects water from a cavity within its body, and the scallop ejects water to move by clamping its shell closed. Other ocean living mollusks, like the oyster, attach themselves to rocks or other surfaces, and can't move. They feed by filtering small food particles from water that flows through them.