Phoronids, common name horseshoe worms, are a little-known phylum of marine worms that look like small delicate flowers on the ocean floor. Phoronids are lophophorates, like brachiopods (an ancient group superficially similar to clams) and bryozoans ("moss animals"), to which they are related. Lophophorates, including phoronids, feed using a lophophore, a ciliated organ located outside the mouth. Phoronids are different enough from other animals that they are considered a distinct phylum, one that includes just 30 species.
Like other lophophorates, phoronids are stationary animals. They burrow tubes into the ocean floor in which they conceal most of their worm-shaped bodies, while their fanned lophophore sticks above the surface. The adults surround themselves with a chitinous tube, secreted using specialized cells. Rare among animals, the phoronids' anus and mouth are at the same end of the body, both near the top. This is necessary, otherwise the phoronid would have nowhere to expel waste.
Like many other filter feeders, the phoronid is careful to orient its lophophore against the prevailing water currents to maximize its chances of picking up organic detritus. Its lophophores are covered with a sticky mucus to help it pick up particles. When a particle, either a bit of detritus or a small living organism, is captured, it is ferried along the lophophores towards the mouth, where it is consumed.
Phoronids are unusual for having one of the largest length-width ratios of any animal. Most phoronids are long, up to 50 cm (30 in) but typically just 3 mm (1/8 in) wide. That is a length to width ratio of 240:1. Most contrast, most animals have a length to width ratio of 1:1 or 2:1.
Phoronids live in dense colonies and have a lifespan of about a year. Like many other marine animals, their favored depth is within 70 m (230 ft) of the surface, though they can occur at depths of up to 400 m (1300 ft).
Phoronids, being soft-bodied, have a poor fossil record, though the characteristic burrows left by phoronids are found in strata as old as the Devonian, 360 million years ago. There are even some Lower Cambrian fossils reminiscent of phoronids. If these are true phoronids, that would make the phylum as much as 535 million years old.
