Sometimes known as horseshoe worms, these are sedentary worms inhabiting secreted chitinous tubes that are buried in soft sediments or attached to harder substrata. There are only 20 known species, and they range in size up to 50 cm long. There can be as many as 50 000 tentacles in the lophophore, see Phoronis architecta right. The tentacles can be completely withdrawn inside the tube if the animal is disturbed. The lophophore is formed in two ridges curved into a horseshoe shape, hence the common name. They are most closely related to the Brachiopoda.