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.

Anatomy 

Though they are normally long, up to 50cm (30 in). Phoronids are normally very thin. 

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 of one 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 to carry 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 nerve fibres which issue from the ganglion and extend along the body wall. There are two tubular excretory organs, which discharge to the exterior via nephridiopores, on the anal papilla.

Reproduction
Phoronids may be hermaphrodites or single sexed, and may also reproduce asexually. Gametes are released through the nephridia. Fertilisation is probably internal. Phoronids follow one of two types of reproductive strategy. 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 adult's 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 planktonic development for 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.

Feeding 
Phoronids are suspension-feeders. They move 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. There it is drawn into the mouth and then on into the digestive tract. Direct uptake of amino acids through the epidermis also occurs.

Fossils 
The fossil record of phoronids is poor. There are borings dating back to the Devonian which have been attributed to phoronids. Iotuba chengjiangensis, a form known from only three specimens in the Lower Cambrian, has been interpreted as a phoronid because it appears to have had a U-shaped gut and was tentaculated. Phoronids may be related to the common but mysterious tubular fossils known as hederellids.

Other websites
Phoronidae - Guide to the Marine Zooplankton of south eastern Australia , Tasmanian Aquaculture & Fisheries Institute
Bioerosion website at The College of Wooster

References 

 Phoronida
 Phoronida World database
 Systematics of the phylum Phoronida
Emig C.C. & C. de Mittelwihr, 1999. What is a phoronid?
http://www.itis.usda.gov 

Minor phyla