Which animals live inside empty seashells?

Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to enter empty scavenged mollusk shells in order to preserve their vulnerable exoskeletons. There are approximately 800 species of hermit crabs, the majority of which have an asymmetric abdomen hidden by a tight-fitting shell. Due to the flimsy abdominal exoskeleton of hermit crabs, they must inhabit shelter generated by other species or risk becoming vulnerable. Hermit crabs, unlike other crabs, have soft exoskeletons (outer coverings).

Their fragile bodies require shelter from the harsh conditions of underwater existence, as well as a safe haven from predators. They burrow into abandoned seashells to live. Because of its flexible body, the crab may twist and transform into a curled shell, leaving only its claws exposed. As it crawls down the ocean floor, a hermit crab will carry the shell on its back. When it outgrows its current shell, it goes on to a larger one.