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. The body wall may be hardened from enlarged plate-like ossicles. Live either attached to hard bottoms or burrow in soft sediment.
Most species live in shallow water.
Contains about 35 species in seven genera and three families. Tentacles are simple or with a few small digits. Respiratory trees are present. The calcareous ring is without posterior projections. Possess muscles for retracting the oral introvert.