Mollusks were among the first inhabitants of the Earth. 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.