Contains about 141 species in 24 genera and five families. Tentacles are shield-shaped and used in shovelling sediment. Respiratory trees are present. The calcareous ring is without posterior projections. With the exception of Deimatidae, the body wall is soft to gelatinous.
All forms live in deep water.
There are about 340 species in 35 genera and three families. Tentacles are shield-shaped. Respiratory trees are present. The calcareous ring is without posterior projections. The body wall is generally soft and pliant.