Phoronida

The Phoronida, or horseshoe worms, are a small phylum of marine animals. There are twenty species, in two genera. They are part of the Brachiozoa, which also includes the brachiopods.

Phoronids are worm-shaped, but with a gut that loops and exits the body near the mouth. That explains the name 'horseshoe worms'. They are found in all oceans and seas (except the polar seas), and all species have wide geographical ranges.

They occur at depths up to 400 metres, but mainly between 0 to 70 metres. The life span is thought to be about a year. The adults are tube worms, and secrete chitinous tubes in which they live. These tubes can be buried in the mud or sand or rest on the surface of a rocky substrate. If on rocks, they may live in colonies with their tubes become twisted around each other for support. Some species can dissolve away holes in rocks such as limestone, calcareous seashells or even cement piers; they then live in these holes which they line with their secreted tubes.

They feed using a lophophore, a ciliated structure that surrounds the mouth. Together with the Bryozoa and Brachiopoda, the phoronids belong to the lophophorates, sometimes treated as a single phylum.