The Phoronida, also named horseshoe worms, is an exclusively marine group of lophophorate animals: sedentary infaunal, benthic suspension-feeders, with a vermiform body enclosed in a slender, chitinous tube in which it moves freely and it is anchored by the ampulla, the end-bulb of the body. The tubes are vertically embedded in soft sediments (sand, mud or fine gravel) or form tangled masses of many individuals, buried in, or encrusting, limestone rocks and shells of dead mollusks. One species, Phoronis australis, is embedded into the tube of cerianthid anemones. Phoronida are considered as phylum or class among the Lophophorata and are without intermediate hierarchical level until the genus level. Two genera, Phoronis and Phoronopsis, are recognized with respectively seven and three well-defined species. Phoronids 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 from the intertidal zone to about 400 m depth, but mainly between 0 to 70 m. The body is more or less elongate, ranging in length from some millimeters to more than 45 cm, and bears a terminal, bilaterally symmetrical, crown of tentacles named lophophore. The lophophore may form an oval to a horseshoe shape more or less complex with two spiral to helicoidal coils; the complexity of the form is proportional to the increase of the general body size. Phoronida have a characteristic ciliated and free-swimming pelagic larva, named Actinotrocha or actinotroch. Separate names for larval and adult forms are still used in taxonomy. Despite the priority of the larval name Actinotrocha, the International Commission of Zoological Nomenclature accepted also as valid the name Phoronis. Consequently, the actinotroch keeps a separate "generic" name considered as a technical term under Actinotrocha followed by a species name, which is sometimes still different from the adult species name.