This marine animal's name comes from the fact that it has a cucumberlike shape with short tentacles at one end. It's also known as sea slug. Though it is seldom found fresh in the United States, it's sold dried (usually marketed as trepang, iriko or bêche-de-mer) in Asian markets. It must be soaked in water for at least 24 hours, during which time it doubles in size and takes on a gelatinous quality. Its texture is rather rubbery and it's therefore most often used in soups.