Phoronid

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Phoronis
Phoronopsis

Phoronids ('Phoronida'), commonly known as horseshoe worms, are a relatively small animal phylum: twenty species are known, in two genera, Phoronis and Phoronopsis. Phoronids are worm-shaped, but with a gut that loops and exits the body near the mouth, instead of running the length of the animal, as in annelids (and many vertebrates). They are found in all oceans and seas (except the polar seas) and all species have wide geographical ranges and most are cosmopolitan. They occur at depths ranging to about 400 metres, but mainly between 0 to 70 metres. The life span is thought to be about one year. The adults are tube worms, and secrete chitinous tubes in which to live. These tubes can be buried in the mud or sand that makes up the sea bed or can be resting on the surface of a rocky substrate, in this case they tend to live in colonies and their tubes become twisted around each other for support to form a large impenetrable mass. 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. There are 10 representative species, like Phoronopsis californica, a large (30cm / 1 ft long) orange-coloured species found along the west coast of North America; Phoronis psammophila a smaller (12cm / 4 in long) cosmopolitan species occurring along the coasts of North America and Phoronis hippocrepia found around most European coasts.

Contents

Anatomy

Though they are normally long (up to 50cm / 30 in) Phoronids are normally very thin. Phoronopsis harmeri for example is 20cm long but only 3 mm (1/8 of an inch) in diameter. Many species are however much shorter than this, though they are all very thin, Phoronis hippocrepia is 3–5 cm long. The smallest species is Phoronis ovalis, which measures only 6 mm in length and lives in colonies on the shells of oysters, where there may be as many as 150 animals per sq. cm.

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