A seashell, also known as a sea shell, or simply as a shell, is the common name for a hard, protective outer layer, a shell, or in some cases a test, that was created by a sea creature, a marine organism. The shell is part of the body of a marine animal. In most cases a shell is an exoskeleton, usually that of an animal without a backbone, an invertebrate. Seashells are most often found on beaches.