Crabs are crustaceans that can also survive out of water. Their flat bodies are covered by a hard shell that provides protection. They have long, thin legs that allow them to walk under water, swim and dig. The claws, used to attack and to catch prey, are their first pair of legs. The antennae have several functions, including as sensory organs, allowing them to find food. The protruding eyes can retreat inside the shell for added security. The crab is a nocturnal animal, which usually spends the day hidden between rocks and coral.