An order of Echinothuriacea with solid and hollow primary radioles, diademoid ambulacral plates, noncrenulate tubercles, and the anus within the apical system. The extant members of the family Echinothuriidae are all deep-water forms. The Echinothuriidae have a large flexible test which collapses into a disk at atmospheric pressure, and the middle element of the diademoid plates is much larger than the other two elements. Some species carry venomous spines. Echinothuriids range from the Late Jurassic to present day.