An illustration of a Oneirophanta Mutabilis, a type of sea cucubmer. The sea cucumber (also known as trepang, bêche-de-mer, balate, or ambiguously, sea slug) is an echinoderm of the class Holothuroidea, with an elongated body and leathery skin, which is found on the sea floor worldwide. It is also named because of its cucumber-like shape. The body contains a single, branched gonad. Like all echinoderms, sea cucumbers have an endoskeleton just below the skin, calcified structures that are usually reduced to isolated microscopic ossicles (or sclerietes) joined by connective tissue.