Holothurians (Holothuroidea) are a class of marine animals from the branch of soft, oblong echinoderms, with a circle of tentacles around the mouth. They are also called sea cucumbers. These animals, mostly benthic live, depending on the species, from the surface to the abyss. Sea cucumbers are usually 10 to 30 centimeters long, but some species like the Moorish cordon can exceed 3 meters.