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 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.
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.
Phoronids have three parts in larval and adult forms. Each one contains its own coelom. As in the Brachiopoda the anterior or front section is highly reduced. It consists of the prosome (with the protocoelom) forming the epistome, a fold extending along the inner row of tentacles and overhanging the mouth. The mesosome (with the mesocoelom) bears mainly the lophophore. The lophophore consists of between 1 and many hundreds of tentacles depending on species. The metasome (or trunk, with the metacoelom) is slender and cylindrical and is used to anchor the body at the tube end.
The digestive tract of Phoronids consists of a short oesophagus which leads into a spherical stomach and then into the intestines which end in the anus. Phoronids have a simple blood system comprising a single descending artery and ascending vein linked by a network of fine capillaries. There are also blood vessels into each of the tentacles. The blood is colourless but contains corpuscles with a haemoglobin-like pigment that helps in carrying oxygen.
The nervous system is mainly composed by the nervous ganglion between mouth and anus, a ring nerve at the basis of the lophophore, one or two giant nerves fibres which issue from the ganglion and extend along the body wall. There are two tubular excretory organs, metanephridia open into the metacoelom by one or two funnels, and discharge to the exterior via a nephridiopore, located on the anal Papilla. The morphological characteristics of the nephridia are of prime taxonomic importance to identify a species.
Phoronids are hermaphroditic or dioecious, and reproduce asexually. Gametes are released through the nephridia. Fertilisation is most probably internal. Phoronids generally follow one of two main types of reproductive strategies. Some species such as Phoronis ovalis lay only a few (12 - 25) large eggs which have a lot of yolk. These eggs are brooded within the adults tube, they are released only when they have hatched. The second strategy is to lay a much larger number (up to 500) of smaller eggs. These eggs are released as soon as they are fertilised. They hatch a few days later into what is called an 'actinotrocha' larvae. The larvae undergo a planktotrophic development (for all species except one) during 2-3 weeks and settle after about 20 days. Metamorphosis is "catastrophic", occurring in less than 30 minutes and leading to a slender young phoronid.
Phoronids can regenerate the lophophore if it becomes damaged, in fact Phoronis ovalis voluntarily loses its lophophore in order to lay its eggs. Once the eggs are laid the animal grows a new lophophore.
Phoronids are suspension-feeders. They orient their lophophores into the prevailing water current. Food particles in the water current are trapped in a stream of mucous that travels along the tentacles until it reaches the oral ring from where it is drawn into the mouth and then on into the digestive tract. Direct uptake of amino acids through the epidermis displays seasonal variations.
There is poor fossil record of soft-bodied phoronids: burrows and borings (created by bioerosion) attributed to phoronids are known since the Devonian. Iotuba chengjiangensis, a form known from three specimens from the Lower Cambrian fauna, has been interpreted as a phoronid, having a U-shaped gut and is tentaculated. Phoronids may be related to the common but mysterious tubular fossils known as hederellids.