Phoronids (scientific name Phoronida, sometimes called horseshoe worms) are a small phylum of marine animals that filter-feed with a lophophore (a "crown" of tentacles), and build upright tubes of chitin to support and protect their soft bodies. They live in most of the oceans and seas, including the Arctic Ocean but excluding the

Antarctic Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...

, and between the

intertidal The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species ...

zone and about 400 meters down. Most adult phoronids are 2 cm long and about 1.5 mm wide, although the largest are 50 cm long. The name of the group comes from its

type genus In biological taxonomy, the type genus is the genus which defines a biological family and the root of the family name. Zoological nomenclature According to the International Code of Zoological Nomenclature, "The name-bearing type of a nominal ...

: '' Phoronis''.

Overview

The bottom end of the body is an ampulla (a flask-like swelling), which anchors the animal in the tube and enables it to retract its body very quickly when threatened. When the lophophore is extended at the top of the body, cilia (little hairs) on the sides of the tentacles draw food particles to the mouth, which is inside and slightly to one side of the base of the lophophore. Unwanted material can be excluded by closing a lid above the mouth or be rejected by the tentacles, whose cilia can switch into reverse. The food then moves down to the stomach, which is in the ampulla. Solid wastes are moved up the intestine and out through the anus, which is outside and slightly below the lophophore. A blood vessel leads up the middle of the body from the stomach to a circular vessel at the base of the lophophore, and from there a single blind vessel runs up each tentacle. A pair of blood vessels near the body wall lead downward from the lophophore ring to the stomach and also to blind branches throughout the body. There is no heart, but the major vessels can contract in waves to move the blood. Phoronids do not ventilate their trunks with oxygenated water, but rely on

respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellul ...

through the lophophore. The blood contains hemoglobin, which is unusual in such small animals and seems to be an adaptation to anoxic and hypoxic environments. The blood of ''Phoronis architecta'' carries twice as much

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

as a human of the same weight. Two

metanephridia The nephridium (plural ''nephridia'') is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove metabolic wastes from an animal's body. Ne ...

filter the body fluid, returning any useful products and dumping the remaining soluble wastes through a pair of pores beside the anus. One species builds colonies by budding or by splitting into top and bottom sections, and all phoronids reproduce sexually from spring to autumn. The eggs of most species form free-swimming actinotroch larvae, which feed on plankton. An actinotroch settles to the seabed after about 20 days and then undergoes a radical change in 30 minutes: the larval tentacles are replaced by the adult lophophore; the anus moves from the bottom to just outside the lophophore; and this changes the gut from upright to a U-bend, with the stomach at the bottom of the body. One species forms a "slug-like" larva, and the larvae of a few species are not known. Phoronids live for about one year. Some species live separately, in vertical tubes embedded in soft

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

, while others form tangled masses buried in or encrusting rocks and shells. In some habitats populations of phoronids reach tens of thousand of individuals per square meter. The actinotroch larvae are familiar among plankton, and sometimes account for a significant proportion of the zooplankton biomass. Predators include fish, gastropods (snails), and nematodes (tiny roundworms). One phoronid species is unpalatable to many

epibenthic Benthos (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone.parasites infest phoronids' body cavities, digestive tract and tentacles. It is unknown whether phoronids have any significance for humans. The International Union for Conservation of Nature (IUCN) has not listed any phoronid species as endangered. As of 2010 there are no indisputable body fossils of phoronids. There is good evidence that phoronids created

trace fossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...

s found in the Silurian, Devonian, Permian, Jurassic and Cretaceous periods, and possibly in the

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

and

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

. Phoronids,

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

s and bryozoans (ectoprocts) have collectively been called lophophorates, because all use lophophores to feed. From about the 1940s to the 1990s,

family trees A family tree, also called a genealogy or a pedigree chart, is a chart representing family relationships in a conventional tree structure. More detailed family trees, used in medicine and social work, are known as genograms. Representations of ...

based on

embryological Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''-logia'') is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and ...

and morphological features placed lophophorates among or as a sister group to the deuterostomes, a super-phylum which includes chordates and echinoderms. While a minority adhere to this view, most researchers now regard phoronids as members of the protostome super-phylum Lophotrochozoa. Although analysts using

molecular phylogeny Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

are confident that members of Lophotrochozoa are more closely related to each other than of non-members, the relationships between members are mostly unclear. Some analyses regard phoronids and brachiopods as sister-groups, while others place phoronids as a sub-group within brachiopoda.

Comparison of similar phyla

Description

Body structure

Most adult phoronids are 2 to 20 cm long and about 1.5 mm wide, although the largest are 50 cm long. Their skins have no cuticle but secrete rigid tubes of chitin, similar to the material used in

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

s' exoskeletons, and sometimes reinforced with

particles and other debris. Most species' tubes are erect, but those of ''Phoronis vancouverensis'' are horizontal and tangled. Phoronids can move within their tubes but never leave them. The bottom end of the body is an ampulla (a flask-like swelling in a tube-like structure), which anchors the animal in the tube and enables it to retract its body when threatened, reducing the body to 20 percent of its maximum length. Longitudinal muscles retract the body very quickly, while circular muscles slowly extend the body by compressing the internal fluid. For feeding and

each phoronid has at the top end a lophophore, a "crown" of tentacles with which the animal filter-feeds. In small species the "crown" is a simple circle, in medium-size species it is bent into the shape of a horseshoe with tentacles on the outer and inner sides, and in the largest species the ends of the horseshoe wind into complex spirals. These more elaborate shapes increase the area available for feeding and

. The tentacles are hollow, held upright by fluid pressure, and can be moved individually by muscles. The mouth is inside the base of the crown of tentacles but to one side. The gut runs from the mouth to one side of the

stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...

, in the bottom of the ampulla. The intestine runs from the stomach, up the other side of the body, and exits at the anus, outside and a little below the crown of tentacles. The gut and intestine are both supported by two

mesenteries In zoology, a mesentery is a membrane inside the body cavity of an animal. The term identifies different structures in different phyla: in vertebrates it is a double fold of the peritoneum enclosing the intestines; in other organisms it forms com ...

(partitions that run the length of the body) connected to the body wall, and another mesentery connects the gut to the intestine. The body is divided into coeloms, compartments lined with mesothelium. The main body cavity, under the crown of tentacles, is called the metacoelom, and the tentacles and their base share the mesocoelom. Above the mouth is the epistome, a hollow lid which can close the mouth. The cavity in the epistome is sometimes called the protocoelom, although other authors disagree that it is a coelom and Ruppert, Fox and Barnes think it is built by a different process. The tube comprises a three-layered organic inner cylinder, and an agglutinated external layer.

Feeding, circulation and excretion

When the lophophore is extended, cilia (little hairs) on the sides of the tentacles draw water down between the tentacles and out at the base of the lophophore. Shorter cilia on the inner sides of the tentacles flick food particles into a groove in a circle under and just inside the tentacles, and cilia in the groove push the particles into the mouth. Phoronids direct their lophophores into the water current, and quickly reorient to maximize the food-catching area when currents change. Their diet includes algae, diatoms, flagellates, peridinians, small invertebrate larvae, and detritus. Unwanted material can be excluded by closing the epistome (lid above the mouth) or be rejected by the tentacles, whose cilia can switch into reverse. The gut uses cilia and muscles to move food towards the stomach and secretes

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

s that digest some of the food, but the stomach digests the majority of the food. Phoronids also absorb amino acids (the building blocks of

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s) through their skins, mainly in summer. Solid wastes are moved up the intestine and out through the anus, which is outside and slightly below the lophophore. A blood vessel starts from the peritoneum (the

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. ...

that loosely encloses the stomach), with blind

capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...

supplying the stomach. The blood vessel leads up the middle of the body to a circular vessel at the base of the lophophore, and from there a single blind vessel runs up each tentacle. A pair of blood vessels near the body wall lead downward from the lophophore ring, and in most species these are combined into one a little below the lophophore ring. The downward vessel(s) leads back to the peritoneum, and also to blind branches throughout the body. There is no heart, but muscles in the major vessels contract in waves to move the blood. Unlike many animals that live in tubes, phoronids do not ventilate their trunks with oxygenated water, but rely on

by the lophophore, which extends above hypoxic sediments. The blood has hemocytes containing hemoglobin, which unusual in such small animals and seems to be an adaptation to anoxic and hypoxic environments. The blood of ''Phoronis architecta'' carries as much oxygen per cm³ as that of most

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

s; the blood's volume in cm³ per gm of body weight is twice that of a human.

Podocyte Podocytes are cells in Bowman's capsule in the kidneys that wrap around capillaries of the glomerulus. Podocytes make up the epithelial lining of Bowman's capsule, the third layer through which filtration of blood takes place. Bowman's capsule ...

s on the walls of the blood vessels perform first-stage filtration of soluble wastes into the main coelom's fluid. Two

, each with a funnel-like intake, filter the fluid a second time, returning any useful products to the coelom and dumping the remaining wastes through a pair of

nephridiopore A nephridiopore is part of the nephridium, an excretory organ found in many organisms, such as flatworms and annelids. Polychaetes typically release their gametes into the water column using nephridiopores. Nephridia are homologous to nephrons o ...

s beside the anus.

Nervous system and movement

There is a nervous center between the mouth and anus, and a nerve ring at the base of the lophophore. The ring supplies nerves to the tentacles and, just under the skin, to the body-wall muscles. '' Phoronis ovalis'' has two nerve trunks under the skin, whereas other species have one. The trunk(s) have giant axons (nerves that transmit signals very fast) which co-ordinate the retraction of the body when danger threatens. Except for retracting the body into the tube, phoronids have limited and slow movement: partial emerging from the tube; bending the body when extended; and the lophophore's flicking of food into the mouth.

Reproduction and lifecycle

Only '' Phoronis ovalis'' naturally builds colonies by budding or by splitting into top and bottom sections which then grow into full bodies. In experiments, other species have split successfully, but only when both parts have enough gonadal (reproductive) tissue. All phoronids breed sexually from spring to autumn. Some species are

hermaphroditic In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have s ...

(have both male and female reproductive organs) but cross-fertilize (fertilize the eggs of other members), while others are dioecious (have separate sexes). The gametes ( sperms and

ova , abbreviated as OVA and sometimes as OAV (original animation video), are Japanese animated films and series made specially for release in home video formats without prior showings on television or in theaters, though the first part of an OVA s ...

) are produced in the swollen gonads, around the stomach. The gametes swim through the metacoelom to the metanephridia. Sperm exit by the nephridiopores and some are captured by the lophophores of individuals of the same species. Species that lay small fertilized eggs release them into the water as plankton, while species with larger eggs brood them either in the body's tube or stuck in the center of the lophophore by adhesive. The brooded eggs are released to feed on plankton when they develop into larvae. Development of the eggs is a mixture of deuterostome and protostome characteristics. Early divisions of the egg are holoblastic (the cells divide completely) and radial (they gradually form a stack of circles). The process is regulative (the fate of each cell depends on interaction with other cells, not on a rigid program in each cell), and experiments that divided early embryos produced complete larvae. Mesoderm is formed from mesenchyme originating from the

archenteron The primary gut that forms during gastrulation in a developing embryo is known as the archenteron, the gastrocoel or the primitive digestive tube. It develops into the endoderm and mesoderm of an animal. Formation in sea urchins As primary mesen ...

. The coelom is formed by schizocoely, and the blastopore (a dent in the embryo) becomes the mouth. Phoronid ASlotwinski

The slug-like larva of ''Phoronis ovalis'' swims for about 4 days, creeps on the seabed for 3 to 4 days, then bores into a carbonate floor. Nothing is known about three species. The remaining species develop free-swimming actinotroch larvae, which feed on plankton. The actinotroch is an upright cylinder with the anus at the bottom and fringed with cilia. At the top is a lobe or hood, under which are: a ganglion, connected to a patch of cilia outside the apex of the hood; a pair of

protonephridia The nephridium (plural ''nephridia'') is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove metabolic wastes from an animal's body. Neph ...

(smaller and simpler than the metanephridia in the adult); the mouth; and feeding tentacles that encircle the mouth. After swimming for about 20 days, the actinotroch settles on the seabed and undergoes a catastrophic metamorphosis (radical change) in 30 minutes: the hood and larval tentacles are absorbed and the adult lophophore is created around the mouth, and both now point upward; the gut develops a U-bend so that the anus is just under and outside the lophophore. Finally the adult phoronid builds a tube. Phoronids live for about one year.

Ecology

Phoronids live in all the oceans and seas including the Arctic and excepting the

, and appear between the

zone and about 400 meters down. Some occur separately, in vertical tubes embedded in soft sediment such as sand, mud, or fine gravel. Others form tangled masses of many individuals buried in or encrusting rocks and shells. In some habitats populations of phoronids reach tens of thousand of individuals per square meter. The actinotroch larvae are familiar among plankton, and sometimes account for a significant proportion of the zooplankton biomass. '' Phoronis australis'' bores into the wall of the tube of a cerianthid

anemone ''Anemone'' () is a genus of flowering plants in the buttercup family Ranunculaceae. Plants of the genus are commonly called windflowers. They are native to the temperate and subtropical regions of all continents except Australia, New Zealand an ...

, ''Ceriantheomorphe brasiliensis'', and uses this as a foundation for building its own tube. One cerianthid can house up to 100 phoronids. In this unequal relationship, the anemone experiences no significant benefits nor harm, while the phoronid benefits from: a foundation for its tube; food (both animals are filter-feeders); and protection, as the cerianthid withdraws into its tube when danger threatens, and this alerts the phoronid to retract into its own tube. Although predators of phoronids are not well known, they include fish, gastropods (snails), and nematodes (tiny roundworms). ''Phoronopsis viridis'', which reaches densities of 26,500 per square meter on tidal flats in

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

(USA), is unpalatable to many

metacercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...

and cysts of trematodes in phoronids' coelomic cavities; unidentified gregarines in phoronids' digestive tract; and an ancistrocomid

ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a differen ...

parasite, '' Heterocineta'', in the tentacles. It is unknown whether phoronids have any significance for humans. The International Union for Conservation of Nature (IUCN) has not listed any phoronid species as endangered.

Evolutionary history

Fossil record

As of 2016 there are no indisputable body fossils of phoronids. Researching the Lower Cambrian Chengjiang fossils, in 1997 Chen and Zhou interpreted '' Iotuba chengjiangensis'' as a phoronid since it had tentacles and a U-shaped gut, – cited by Emig (Mar 2010) and Taylor, Vinn and Wilson(2010) and in 2004 Chen interpreted ''

Eophoronis ''Iotuba chengjiangensis'' (sometimes misspelt ''Lotuba'') is a Cambrian fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bon ...

'' as a phoronid. However, in 2006 Conway Morris regarded ''Iotuba'' and ''Eophoronis'' as synonyms for the same genus, which in his opinion looked like the

priapulid Priapulida (priapulid worms, from Gr. πριάπος, ''priāpos'' 'Priapus' + Lat. ''-ul-'', diminutive), sometimes referred to as penis worms, is a phylum of unsegmented marine worms. The name of the phylum relates to the Greek god of fertility ...

'' Louisella''. In 2009 Balthasar and Butterfield found in western Canada two specimens from about 505 million years ago of a new fossil, ''Lingulosacculus nuda'', which had two shells like those of

s but not mineralized. In the authors' opinion, the U-shaped gut extended beyond the hinge line and outside the smaller shell. This would have precluded the attachment of muscles to close and open the shells, and the 50% of the animal's length beyond the hinge line would have needed longitudinal muscles and also a cuticle for protection. Hence they suggest that ''Lingulosacculus'' may have been a member of a phoronid

stem group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

within the

linguliform Linguliformea is a subphylum of inarticulate brachiopods. These were the earliest of brachiopods, ranging from the Cambrian into the Holocene. They rapidly diversified during the Cambrian into the Ordovician, but most families became extinct by ...

brachiopods. Another alternative is that '' Eccentrotheca'' lies somewhere in the phoronid stem lineage. There is good evidence that species of ''Phoronis'' created the

s of the

ichnogenus An ichnotaxon (plural ichnotaxa) is "a taxon based on the fossilized work of an organism", i.e. the non-human equivalent of an artifact. ''Ichnotaxa'' comes from the Greek ίχνος, ''ichnos'' meaning ''track'' and ταξις, ''taxis'' meaning ...

''Talpina'', which have been found in the Devonian, Jurassic and Cretaceous periods. The Talpina animal bored into calcareous algae, corals,

echinoid Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...

tests (shells), mollusc shells and the

rostra The rostra ( it, Rostri, links=no) was a large platform built in the city of Rome that stood during the republican and imperial periods. Speakers would stand on the rostra and face the north side of the comitium towards the senate house and de ...

of belemnites. Hederellids or Hederelloids are fossilized tubes, usually curved and between 0.1 and 1.8 mm wide, found from the Silurian to the Permian, and possibly in the

and

. Their branching colonies may have been made by phoronids.

Family tree

Phoronids,

s and bryozoans (ectoprocts) are collectively called lophophorates, because all feed using lophophores. From about the 1940s to the 1990s,

based on

and morphological features placed lophophorates among or as a sister group to the deuterostomes, a super-phylum that includes chordates and echinoderms. In the early development of their embryos, deuterostomes form the anus before the mouth, while protostomes form the mouth first. Nielsen (2002) views the phoronids and brachiopods as affiliated with the deuterostome pterobranchs, which also filter-feed by tentacles, because the current-driving cells of the lophophores of all three have one cilium per cell, while lophophores of bryozoans, which he regards as protostomes, have multiple cilia per cell. Helmkampf, Bruchhaus and Hausdorf (2008) summarise several authors' embryological and morphological analyses which doubt or disagree that phoronids and brachiopods are deuterostomes: * While deuterostomes have three coelomic cavities, lophophorates such as phoronids and brachiopods have only two. * Pterobranchs may be a sub-group of enteropneusts ("acorn worms"). This suggests that the ancestral deuterostome looks more like a mobile worm-like enteropneust than a sessile colonial pterobranch. The fact that lophophorates and pterobranchs both use tentacles for feeding is probably not a

synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to hav ...

of lophophorates and deuterostomes, but evolved independently as convergent adaptations to a sessile lifestyle. * The mesoderm does not form by

enterocoely Enterocoely (adjective forms: enterocoelic and enterocoelous) describes both the process by which some animal embryos develop and the origin of the cells involved. In enterocoely, a mesoderm (middle layer) is formed in a developing embryo, in whic ...

in phoronids and bryozoans, but does in deuterostomes, while there are disagreements about whether brachiopods form the mesoderm by enterocoely.

Relationships of Phoronida to other Bilateria:

From 1988 onwards analyses based on

, which compares biochemical features such as similarities in DNA, have placed phoronids and brachiopods among the Lophotrochozoa, a protostome super-phylum that includes molluscs, annelids and flatworms but excludes the other main protostome super-phylum Ecdysozoa, whose members include

s. Cohen wrote, "This inference, if true, undermines virtually all morphology–based reconstructions of phylogeny made during the past century or more." While analyses by molecular phylogeny are confident that members of Lophotrochozoa are more closely related to each other than of non-members, the relationships between members are mostly unclear. The Lophotrochozoa are generally divided into:

Lophophorata The Lophophorata are a Lophotrochozoan clade consisting of the Brachiozoa and the Bryozoa. They have a lophophore. Molecular phylogenetic analyses suggest that lophophorates are protostomes, but on morphological grounds they have been assessed ...

(animals that have lophophores), including Phoronida and Brachiopoda;

Trochozoa The Trochozoa are a proposed Lophotrochozoa clade that is a sister clade of Bryozoa. The trochozoans clade include animals in six phyla: the Nemertea, the Annelida, the Cycliophora, the Mollusca, and the two Brachiozoa Brachiozoa is a groupin ...

(animals many of which have

trochophore A trochophore (; also spelled trocophore) is a type of free-swimming planktonic marine larva with several bands of cilia. By moving their cilia rapidly, they make a water eddy, to control their movement, and to bring their food closer, to captur ...

larvae), including molluscs, annelids,

echiura The Echiura, or spoon worms, are a small group of marine animals. Once treated as a separate phylum, they are now considered to belong to Annelida. Annelids typically have their bodies divided into segments, but echiurans have secondarily los ...

ns,

sipuncula The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a class containing about 162 species of unsegmented marine annelid worms. The name ''Sipuncula'' is from the genus name ''Sipunculus'', and comes from the Latin ' ...

ns and

nemertea Nemertea is a phylum of animals also known as ribbon worms or proboscis worms, consisting of 1300 known species. Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many h ...

ns; and some other phyla (such as Platyhelminthes, Gastrotricha, Gnathostomulida, Micrognathozoa, and

Rotifera The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John H ...

). Molecular phylogeny indicates that Phoronida are closely related to Brachiopoda, but Bryozoa (Ectoprocta) are not closely related to this group, despite using a similar lophophore for feeding and respiration. This implies that the traditional definition "Lophophorata" is not monophyletic. Recently the term "Lophophorata" has been applied only to the Phoronida and Brachiopoda, and Halanych thinks this change will cause confusion. Some analyses regard Phoronida and Brachiopoda as sister-groups, while others place Phoronida as a sub-group within Brachiopoda, implying that Brachiopoda is paraphyletic. Cohen and Weydman's analysis (2005) concludes that phoronids are a sub-group of inarticulate brachiopods (those in which the hinge between the two valves have no teeth and sockets) and sister-group of the other inarticulate sub-groups. The authors also suggest that the ancestors of molluscs and the brachiopod+phoronid clade diverged between 900 Ma and 560 Ma, most probably about 685 Ma.

Taxonomy

The phylum has two genera, with no class or order names. Zoologists have given the larvae, usually called an actinotroch, a separate genus name from the adults. In 1999 Temereva and Malakhov described '' Phoronis svetlanae''. In 2000 Temereva described a new species, '' Phoronopsis malakhovi'', while Emig regards it as a synonym for '' Phoronopsis harmeri''. Santagata thinks '' Phoronis architecta'' is a different species from both '' Phoronis psammophila'' and '' Phoronis muelleri'', and that " he phoronids'species diversity is currently underestimated". In 2009 Temereva described what may be larvae of '' Phoronopsis albomaculata'' and '' Phoronopsis californica''. She wrote that, while there are 12 undisputed adult phoronid species, 25 morphological types of larvae have been identified.

Notes

References

External links

PHORONIDAPhoronida World database
ttps://web.archive.org/web/20080930064242/http://www.tafi.org.au/ Tasmanian Aquaculture & Fisheries Institutebr>Bioerosion website at The College of Wooster
{{Good article * Taxa named by Berthold Hatschek