Mollusks were among the first inhabitants of the Earth. Fossils of mollusks have been found in rocks that date back over 500 million years. Mollusk fossils are usually well preserved because of their hard shells. Most mollusks have a soft, skin-like organ covered with a hard outside shell.
Some mollusks, such as snails and slugs, live on land. Land living mollusks, like the snail, move slowly on a flat sole called a foot.
Other mollusks, such as the oyster, mussel, clam, squid, and octopus, live in water. Ocean living mollusks move or swim by jet propulsion, propelling themselves by ejecting water from the body. For example, the squid ejects water from a cavity within its body, and the scallop ejects water 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 the water that flows through them.