Phoronids (

taxonomic 280px, Generalized scheme of taxonomy Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme of classes (a taxonomy) and the allocation ...

name Phoronida, sometimes called horseshoe worms) are a small

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

of marine animals that filter-feed with a

lophophore The lophophore () is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Hyolitha, and Phoronida, which collectively constitute the protostome group Lophophorata.chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

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 60th parallel south, 60° S latitude and encircling Antarctica. With a size of , it is the seco ...

, and between the

intertidal The intertidal zone or foreshore is the area above water level at low tide and underwater at high tide; in other words, it is the part of the littoral zone within the tidal range. This area can include several types of habitats with various sp ...

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 (''genus typica'') 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-bearin ...

: ''

Phoronis ''Phoronis'' is one of the two genus, genera of the horseshoe worm family (Phoronidae), in the phylum (biology), phylum Phoronida. The body has two sections, each with its own coelom. There is a specialist feeding structure, the lophophore, whic ...

''.

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 The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

(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 In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

, 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 Hemoglobin (haemoglobin, Hb or Hgb) is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. 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 a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

as a human of the same weight. Two

metanephridia The nephridium (: 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. Nephridia co ...

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 Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is kno ...

or by splitting into top and bottom sections, and all phoronids

reproduce sexually Sexual reproduction is a type of reproduction that involves a complex Biological life cycle, life cycle in which a gamete (haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to p ...

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 solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

, while others form tangled masses buried in or encrusting rocks and shells. Species able to bore into materials like limestone and dead corals do so by chemical secretions. In some habitats populations of phoronids reach tens of thousands of individuals per square meter. The actinotroch larvae are familiar among

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

, and sometimes account for a significant proportion of the zooplankton biomass. Predators include fish,

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

s (snails), and

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

s (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.parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

s 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 called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of part ...

s found in the

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

and

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

periods, and possibly in the

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

and

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

. Phoronids,

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of 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 e ...

s and

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...

ns (ectoprocts) have collectively been called

lophophorate The Lophophorata (also called Tentaculata; not to be confused with Tentaculata Eschscholtz 1825, a class within the Ctenophora) are a Lophotrochozoan clade consisting of the Brachiozoa and the Bryozoa. They have a lophophore. Molecular phyloge ...

s, 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 o ...

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 an ...

and morphological features placed lophophorates among or as a

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

to the

deuterostome Deuterostomes (from Greek: ) are bilaterian animals of the superphylum Deuterostomia (), typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia comprises three phyla: Chordata, Echinodermata, ...

s, a super-phylum which includes

chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...

s and

echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...

s. While a minority adhere to this view, most researchers now regard phoronids as members of the

protostome Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's memb ...

super-phylum

Lophotrochozoa Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, and brach ...

. 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 A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

but

secrete Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical mec ...

rigid tubes of

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

, similar to the material used in

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

exoskeleton An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...

s, 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 The lophophore () is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Hyolitha, and Phoronida, which collectively constitute the protostome group Lophophorata.filter-feeds. In small species the "crown" is a simple circle, in medium-size species it is bent into the shape of a

horseshoe A horseshoe is a product designed to protect a horse hoof from wear. Shoes are attached on the palmar surface (ground side) of the hooves, usually nailed through the insensitive hoof wall that is anatomically akin to the human toenail, altho ...

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 upper gastrointestinal tract of Human, humans and many other animals, including several invertebrates. The Ancient Greek name for the stomach is ''gaster'' which is used as ''gastric'' in medical t ...

, 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 human anatomy, the mesentery is an organ that attaches the intestines to the posterior abdominal wall, consisting of a double fold of the peritoneum. It helps (among other functions) in storing fat and allowing blood vessels, lymphatics, a ...

(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

coelom The coelom (or celom) is the main body cavity in many animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, i ...

s, compartments lined with

mesothelium The mesothelium is a membrane composed of simple squamous epithelium, simple squamous epithelial cells of mesodermal origin, which forms the lining of several body cavities: the pleura (pleural cavity around the lungs), peritoneum (abdominopelvic ...

. 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,

(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 Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

diatom A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and the ...

s, 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 An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s that digest some of the food, but the stomach digests the majority of the food. Phoronids also absorb

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s (the building blocks of

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s) through their skins, mainly in summer. Solid wastes are moved up the intestine and out through the

, which is outside and slightly below the lophophore. A blood vessel starts from the

peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mesotheli ...

(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. Bi ...

that loosely encloses the stomach), with blind

capillaries A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the in ...

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

, 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 () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

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 ...

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 the smallest species of horseshoe worms, '' Phoronis ovalis'', naturally builds colonies by

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

gonad A gonad, sex gland, or reproductive gland is a Heterocrine gland, mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gon ...

al (reproductive) tissue. All phoronids breed sexually from spring to autumn. Some species are

hermaphroditic A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...

(have both male and female reproductive organs) but cross-fertilize (fertilize the eggs of other members), while others are

dioecious Dioecy ( ; ; adj. dioecious, ) is a characteristic of certain species that have distinct unisexual individuals, each producing either male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproduction is ...

(have separate sexes). The

gamete A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...

s (

sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...

s and

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

) 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

and

characteristics. Early

divisions Division may refer to: Mathematics *Division (mathematics), the inverse of multiplication * Division algorithm, a method for computing the result of mathematical division Military *Division (military), a formation typically consisting of 10,000 t ...

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 The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical ...

is formed from

mesenchyme Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood, or bone. The interactions between mesenchyme and epithelium help to form nearly ever ...

originating from the

archenteron The archenteron, also called the gastrocoel, is the internal cavity formed in the gastrulation stage in early embryonic development that becomes the cavity of the primitive gut. Formation in sea urchins As primary mesenchyme cells detach fro ...

. The coelom is formed by

schizocoely Schizocoely (adjective forms: schizocoelous or schizocoelic) is a process by which some animal embryos develop. The schizocoely mechanism occurs when secondary body cavities (coeloms) are formed by splitting a solid mass of mesodermal embryonic ti ...

, and the

blastopore Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals, the blastocyst, is reorganized into a two-layered or three-layered embryo known as ...

(a dent in the embryo) becomes the mouth. Phoronid ASlotwinski

The slug-like larva of ''Phoronis ovalis'', the only known species with a lecithotrophic (non-feeding) larvae, lack tentacles and 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 (: 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. Nephridia co ...

(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 juvenile body forms from the larva's metasomal sack. The adult lophophore is created around the mouth, and by growing a ventral side that is extremely long compared to the dorsal side, 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 The Arctic (; . ) is the polar regions of Earth, polar region of Earth that surrounds the North Pole, lying within the Arctic Circle. The Arctic region, from the IERS Reference Meridian travelling east, consists of parts of northern Norway ( ...

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

, and sometimes account for a significant proportion of the zooplankton biomass. '' Phoronis australis'' bores into the wall of the tube of a cerianthid anemone, '' 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,

s (snails), and

s (tiny roundworms). ''Phoronopsis viridis'', which reaches densities of 26,500 per square meter on tidal flats in

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

(USA), is unpalatable to many

s infest phoronids: progenetic

metacercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two Sucker (zoology), suckers: one ventral and the other Mouth, oral. Trematodes are covered by a Tegument (helminth), tegument, that protects the o ...

and

cysts A cyst is a closed Wikt:sac, sac, having a distinct Cell envelope, envelope and cell division, division compared with the nearby Biological tissue, tissue. Hence, it is a cluster of Cell (biology), cells that have grouped together to form a sac ...

trematode Trematoda is a Class (biology), class of flatworms known as trematodes, and commonly as flukes. They are obligate parasite, obligate Endoparasites, internal parasites with a complex biological life cycle, life cycle requiring at least two Host ( ...

s in phoronids'

ic cavities; unidentified

gregarine The gregarines are a group of Apicomplexan alveolates, classified as the Gregarinasina or Gregarinia. The large (roughly half a millimeter) parasites inhabit the intestines of many invertebrates. They are not found in any vertebrates. Gregarines ...

s 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 flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...

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 The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

Chengjiang fossils, in 1997 Chen and Zhou interpreted ''

Iotuba ''Iotuba chengjiangensis'' (sometimes mis-spelt ''Lotuba'') is a 515 million year old Cambrian worm known from the Chengjiang biota. Originally interpreted as a phoronid, the organism is now recognized as an annelid cage worm affiliated with the ...

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'' 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 ''Louisella'' is a genus of worm known from the Middle Cambrian Burgess Shale. It was originally described by Charles Walcott in 1911 as a holothurian echinoderm, and represents a senior synonym of ''Miskoia'', which was originally described as ...

''. 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

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. ''Ichnotaxon'' comes from the Ancient Greek (''íchnos'') meaning "track" and English , itself derived from ...

''Talpina'', which have been found in the

and

periods. The Talpina animal bored into

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines. In zoology ''Calcare ...

coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...

echinoid Sea urchins or urchins () are echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body covered by a spiny p ...

tests Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film) ...

(shells), mollusc shells and the

rostra The Rostra () 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 deliver orations to t ...

belemnite Belemnitida (or belemnites) is an extinct order (biology), order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous (And possibly the Eocene). Unlike squid, belemnites had an internal skeleton that made up the cone ...

s. Hederellids or Hederelloids are fossilized tubes, usually curved and between 0.1 and 1.8 mm wide, found from the

to the

, and possibly in the

and

. Their branching colonies may have been made by phoronids.

Family tree

Phoronids,

s and

ns (ectoprocts) are collectively called

s, because all feed using lophophores. From about the 1940s to the 1990s,

based on

and morphological features placed lophophorates among or as a

to the

s, a super-phylum that includes

s and

s. In the early development of their embryos, deuterostomes form the

before the mouth, while

s form the mouth first. Nielsen (2002) views the phoronids and brachiopods as affiliated with the deuterostome

pterobranchs Pterobranchia, members of which are often called pterobranchs, is a class of small worm-shaped animals. They belong to the Hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water wi ...

, which also filter-feed by tentacles, because the current-driving cells of the lophophores of all three have one

cilium The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike pr ...

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

ic 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 Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...

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

enterocoely Enterocoelom (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 whi ...

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 protostomes

From 1988 onwards analyses based on

, which compares

biochemical Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, ...

features such as similarities in

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

, have placed phoronids and brachiopods among the

, a protostome super-phylum that includes

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

annelid The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...

s and

flatworm Platyhelminthes (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a Phylum (biology), phylum of relatively simple bilaterian, Segmentation (biology), ...

s but excludes the other main protostome super-phylum

Ecdysozoa Ecdysozoa () is a group of protostome animals, including Arthropoda (insects, chelicerates (including arachnids), crustaceans, and myriapods), Nematoda, and several smaller phylum (biology), phyla. The grouping of these animal phyla into a single ...

, 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

are generally divided into:

Lophophorata The Lophophorata (also called Tentaculata; not to be confused with Tentaculata Eschscholtz 1825, a class within the Ctenophora) are a Lophotrochozoan clade consisting of the Brachiozoa and the Bryozoa Bryozoa (also known as the Polyzoa ...

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

Trochozoa The Trochozoa are a proposed Lophotrochozoa clade that is a sister clade of Bryozoa and Platyzoa. The clade would include animals in five phyla: the Nemertea, the Annelida, the Mollusca, and the two Brachiozoan phyla, Brachiopoda and Phoronida ...

(animals many of which have

trochophore A trochophore () 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 in order to capture it more easily. ...

larvae), including molluscs,

echiura The Echiura, or spoon worms, are a small group of ocean, marine animals. Once treated as a separate phylum, they are now considered to belong to Annelida. Annelids typically have their bodies divided into Segmentation (biology), segments, but e ...

ns,

sipuncula The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a class containing about 162 species of marine annelid worms, that have secondarily lost their segmentation. Sipuncula was once considered a phylum of unsegmented ...

ns and

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

ns; and some other phyla (such as

Platyhelminthes Platyhelminthes (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates commonly called f ...

Gastrotricha The gastrotrichs (phylum Gastrotricha), common name, commonly referred to as hairybellies or hairybacks, are a group of Microscopic scale, microscopic (0.06–3.0 mm), cylindrical, acoelomate animals, and are widely distributed and abundant ...

Gnathostomulida Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956. ...

Micrognathozoa ''Limnognathia'' is a genus of microscopic acoelomate freshwater animal that was discovered in Disko Island, Greenland, in 1994. Since then, it has also been found on the Crozet Islands of Antarctica as well as in the British Isles and the Sp ...

, and

Rotifera The rotifers (, from Latin 'wheel' and 'bearing'), sometimes 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 Harris ...

). Molecular phylogeny indicates that Phoronida are closely related to Brachiopoda, but

Bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...

(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 In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

. Recently the term "Lophophorata" has been applied only to the Phoronida and Brachiopoda, and Halanych (2004) 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 Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

. 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

s and the brachiopod+phoronid clade diverged between 900 Ma and 560 Ma, most probably about 685 Ma.

Taxonomy

The phylum has two

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

, with no

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

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