What is Phoronida?
The '''Phoronida''', or '''horseshoe worms''', are a small [[phylum]] of [[Ocean|marine]] animals. There are twenty [[species]], in two [[genera]]. They are part of the [[Brachiozoa]], which also includes the [[brachiopod]]s. 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 [[chitin]]ous 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 [[Rock (geology)|rocks]], they may live in [[Colony (biology)|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]] [[pier]]s; they then live in these holes which they line with their secreted tubes. They feed using a [[lophophore]], a [[cilia]]ted structure that surrounds the mouth. Together with the [[Bryozoa]] and [[Brachiopoda]], the phoronids belong to the lophophorates, sometimes treated as a single phylum. == Anatomy == [[File:Phoronida.gif|thumb|right|230px|Structure of a phoronid.]] 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 [[intestine]]s which end in the [[anus]]. Phoronids have a simple [[blood]] system of one descending [[artery]] and ascending [[vein]] linked by a network of fine [[Capillary|capillaries]]. There are also blood vessels into each of the [[tentacle]]s. 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]]