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. Animal Classification Crabs and Lobsters Top of Page 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. Clams and Oysters Top of Page Most mollusks have a soft, skin-like organ covered with a hard outside shell.