The ancestors of the Apodida, Elasipodida and the lineage leading to the remaining orders diverged in the middle to late Paleozoic Era between about 350 to 250 million years ago. The Aspidochirotida, Molpadiida, Dendrochirotida and Dactylochirotida began diverging somewhat later in the Triassic and Jurassic of the early Mesozoic Era about 200 million years ago. Assignment to different orders is largely based on the form of the calcareous ring and tentacles, as well as the presence of certain organs, such as the respiratory trees or the muscles that retract the oral region. Contains about 269 species in 32 genera and three families. Tentacles are digitate, pinnate or, in some small species, simple.
Respiratory trees are absent.
Tube feet are completely absent. The calcareous ring is without posterior projections. The body wall is very thin and often transparent. Found in both shallow and deep water. Contains about 141 species in 24 genera and five families.