A class of Echinozoa characterized by a cylindrical body and smooth leathery skin, and known as sea cucumbers. There are no arms, but a ring of five or more tentacles may surround the mouth, which is usually at one end of the body. There are no pedicellariae. Tube feet may be present or lacking. There are no ambulacral grooves, although they are represented by internal epineural canals overlying the radial nerves. E. Deichmann (1957) regards them as the most aberrant group of extant echinoderms.