There are about 95 species in 11 genera and four families. Tentacles are simple. Respiratory trees are present. The calcareous ring is without posterior projections. The body wall is generally soft and pliant.
Most forms live in shallow water, though one family is restricted to the deep sea.
Contains about 550 species in 90 genera and seven families. Tentacles are highly branched and extended to filter material from the water column. Respiratory trees are present. Some members with a calcareous ring composed of numerous small pieces or having long posterior extensions. Possess muscles for retracting the oral introvert.