Echinoderm is the usual name given to any member of the Phylum Echinodermata of marine animals. They live mostly in the benthic layer of the ocean, but they can generally be found on all the layers. They are very large predators of the molluscs, cnidarians, and crustaceans. Almost any animal belonging to this phylum is characterized by a hard, spiny covering or skin. The uniting feature of echinoderms is a water-vascular system. This is a system of canals branching throughout the body that branch into many sections called tube feet. There are at least 2,000 tube feet, which can penetrate the body wall and skeleton in places called ambulacral grooves, in most echinoderms. These tube feet, and in many echinoderms arms and even organs, can be regenerated. The 6,000 species that make up this phylum do not show body segmentation, and are radially symmetrical when fully grown for the most part.