Shell fish include crustacians (crabs and lobsters) and mullusks (clams and oysters). Mullusks were among the first inhabitants of the Earth. Fossils of mollusks have been found in rocks and date back over 500 million years. There fossils are usually well preserved because of the mollusks hard shell.
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|Crustaceans live mostly in the ocean or other
waters. Most commonly known crustaceans are the crab, lobster and barnacle.
Crustaceans have a hard, external shell which protects their body. Crustaceans have a head and abdomen. The head has antennae which are part of their sensory system. The abdomen includes the heart, digestive system and reproductive system.
The abdomen also has appendages, such as legs, for crawling and swimming. Many crustaceans also have claws that help with crawling and eating.
|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.