Systematics of lower Metazoa

Molecular, morphological, and paleontological evidence suggests the most primitive phyla are found within two para-phyletic groups, Parazoa and Radiata. The Parazoa includes two phyla, Porifera and Placozoa, which display the most primitive grade of organization and are at the base of the evolutionary tree. The Radiata contains the radiate phyla, Cnidaria and Ctenophora, the first true metazoans prior to the evolution of the bilateral body plan in the Precambrian. Some enigmatic groups with questionable affinities to either Radiata or Bilateria include the Monoblastozoa, Orthonectida, and Rhombozoa, often included under the name Mesozoa. The largest assemblage of animals is found within the Bilateria, a monophylum consisting of two large clades, Protostomia and Deuterostomia. Branching order within these clades is unresolved, though molecular evidence indicates the Protostomia may be subdivided into Ecdysozoa (molting animals) and Lophotrochozoa (ciliated animals). Lophotrochozoa contains several phyla, including the Lophophorata, a superphylum of Brachiopoda, Ectoprocta and Phoronida, still considered by many authors to be deuterostomes. Additionally, subordinate clades within the Protostomia define certain groups of lower Metazoa. These groups go by a variety of names, including Aschelminthes, Nemathelminthes, Gnathifera, Cycloneuralia, Introverta, and Cephalorhyncha. Nemathelminthes and As-chelminthes are older terms proposed in 1859 and 1910, respectively, to consume a variety of acoelomate and pesudo-coelomate taxa; neither group is considered monophyletic.

The monophylum Gnathifera was erected in 1995 to include three phyla, Gnathostomulida, Rotifera, and Acantho-cephala. The Micrognathozoa, discovered in 2002, is now included in this clade, and some biologists also place the Chaetognatha within Gnathifera. Cycloneuralia includes six acoelomate and pseudocoelomate taxa: Gastrotricha, Nema-toda, Nematomorpha, Kinorhyncha, Loricifera, and Priapul-ida. The Introverta makes up a subset of Cycloneuralia, and Cephalorhyncha is yet a smaller subset of Introverta. Phyla of uncertain taxonomic affinity include the Entoprocta and

Cycliophora. Some biologists consider entoprocts closely related to ectoprocts or a sister taxon of Cycliophora. Within Deuterostomia, subordinate clades include Lophophorata (also considered protostomes), Neorenalia, and Cyrotreta. The animal, Xenoturbella bocki, is perhaps the most enigmatic bilaterian to date, being variously regarded as a species of protobranch bivalve (Mollusca), member of the Acoela (Platy-helminthes), relative of acorn worms (Enteropneusta) and Holothuroidea (Echinodermata), or as the most basal bilaterian; Xenoturbella,s affinities remain undetermined.

The Parazoa (Greek: para = beside, zoa = animals, Meta-zoa) includes animals with the most primitive grade of organization: they lack definable nervous systems, true tissues, or organ systems, and epithelia do not rest on a basement membrane. Sponges (phylum Porifera) are the most basal para-zoans. According to some researchers, sponges are little more than colonial choanoflagellates, but arguments can be made to classify sponges as more advanced than colonial, het-erotrophic eurkaryotes such as Proterospongia. Sponges contain many more specialized cell types (including sex cells similar to other metazoans), have at least a rudimentary ability to form tissue-like epithelia via septate junctions (between choanocytes), and undergo an embryonic process of tissue formation (though probably not homologous to gastrulation of eumetazoa). The Porifera is a well-defined monophylum characterized by two synapomorphies, cellular totipotency and acquiferous systems of choanocyte chambers. Nearly all sponges are benthic, sessile suspension-feeders with the exception of Asbestopluma, a carnivorous demosponge. More than 5,000 living species of sponge are known globally, and many more fossil species exist, some dating as far back as 600 mya. Four sponge-like fossil groups are well known to have produced massive reef-like structures back in the early Paleozoic (550-480 mya): archaeocyathans, stromatoporids, sphinctozoans, and chaetetids.

Unlike Porifera, the phylum Placozoa has no known fossil record. Discovered in 1883, Trichoplax adhaerens, the only known member of Placozoa, is best characterized as a minute (0.07-0.11 in [2-3 mm]), flattened bag of epithelial cells with dorso-ventral polarity and no anterior-posterior polarity or symmetry. The outer epithelium is composed of monociliate cells (one cilium per cell) interconnected with belt desmo-somes and perhaps septate junctions. The narrow space between epithelial layers is filled with a gel-like mesenchyme. Asexual and sexual reproduction is known, but details of sexual reproduction are incomplete. The existence of striated ciliary rootlets in the monociliate cells suggests an evolutionary link to eumetazoans, and the lack of autapomorphies defining the placozoan body plan suggests a placozoan-like ancestor may have given rise to the Eumetazoa.

Eumetazoa (Greek: eu = good) refers to the remaining animal groups, approximately 30 phyla. Eumetazoans share several synapomorphies, including a synaptic nervous system, gap junctions between cells, a basal lamina, striated myofib-rils, organized gonads, definable germ layers (ectoderm, en-doderm), and body symmetry. The Cnidaria is the most basal eumetazoan phylum and contains approximately 10,000 extant species and several thousand fossil species. The fossil record extends back to the Precambrian (540 mya), though many fossils, especially those of Ediacara and Burgess Shale, have questionable affinities to the Cnidaria. The phylum is defined by at least three synapomorphies: cnidae (nemato-cysts), planula larva, and coelenteron with mouth surrounded by tentacles. Other synapomorphies might characterize the group, depending on one's view of the interrelationships. The phylum is traditionally divided into four classes: Hy-drozoa, Scyphozoa, Cubozoa, and Anthozoa. Body morphology is highly variable within the classes but is generally a variation on one or two basic body plans: polyp and medusa. Polyps are generally sessile, benthic forms with a ring of tentacles around the mouth, although some polyps have become colonial and taken up a pelagic existence (e.g., Velella velella). The medusa, or jellyfish, is often a flattened disc-shaped animal that lives in the water column, though some species have adopted a polypoid form and become benthic (Haliclystus). The polyp may be a sexual form that produces gametes (e.g., Anthozoa) or an asexual form that produces medusae (e.g., Cubozoa, Schyphozoa, Hydrozoa). Only polyps within Anthozoa form large reefs in mostly tropical seas. Interrelationships within the phylum are contentious.

Two prominent theories on the origin of Cnidaria have become established: the medusa theory and the polyp theory. The medusa theory postulates that the cnidarian ancestor was a planula-like animal that first gave rise to the medusoid form, subsequently followed by evolution of the polyp stage. Such a scenario places the trachyline hydrozoans as basal cnidarians, and therefore the medusoid body form would be a synapomorphy of the phylum. The polyp theory states that the polypoid form is ancestral, and therefore the Anthozoa is the most basal group within the phylum. In this scenario, Anthozoa is a paraphyletic taxon and the medusoid form is a synapomorphy of more derived clades within the Cnidaria. These theories have played important roles in the understanding of metazoan evolution, particularly regarding the origin of flatworms, the origin of the coelom, and the origin of mesodermal muscles (Rieger, 1986).

The triploblasts, or Triploblastica (Greek: triplo = three, blast = bud), refer to phyla that possess three embryonic germ layers (ectoderm, mesoderm, endoderm) formed during gas-trulation, though mesoderm is probably not homologous between protostomes and deuterostomes. Prior to 1985, the phylum Ctenophora was generally considered a diploblastic phylum because true mesoderm (entomesoderm) was not found; however, subsequent studies have revealed the presence of mesoderm during embryogeny. Ctenophora is considered the most primitive of the triploblastic phyla because it is hypothesized to have originated prior to the evolution of the bilateral body plan. Ctenophora is a small phylum of approximately 80 gelatinous animals known as comb jellies and sea gooseberries. Their paleontological history extends back to the Cambrian. The phylum is well defined by the presence of adhesive colloblasts, eight rows of ciliary plates, a unique apical sense organ, and a cydippid larval stage. Interrelationships among the seven inclusive orders are not well resolved, and there is disagreement around which form of ctenophore is most primitive, the tentaculate or atentactulate form. The ctenophore body plan has played a historical role in various theories on metazoan evolution (e.g., ctenophore-polyclad theory, ctenophore-trochophore theory).

The Bilateria (Greek: bi = two, latus = side) is a well-defined monophylum characterized by a bilaterally symmetric body plan, a pronounced antero-posterior axis, and cephal-ization (brain). The phylum Platyhelminthes has figured prominently in nearly all discussions of the origin and relationships of the Bilateria. Originally classified within the taxon Vermes by Linnaeus in his 1735 book, Systema Naturae, the Platyhelminthes has since had a contentious history. In 1851 Vogt was first to isolate the phylum (with the nemerteans) into a single taxon called Platyelmia, later changed to Platyelminthes (now Platyhelminthes) by Gregenbaur in 1859. Platyhelminthes was subsequently erected to phylum-level status containing four classes: Turbellaria, Nemertea, Trematoda, and Cestoda. Minot (1876) later dropped Nemertea from this taxon. As of recent times, the Platyhelminthes contains close to 20,000 species of free-living and parasitic flatworms, with very little fossil record. The phylum is divided into three monophyletic clades: Acoelomorpha, Catenulida, and Rhabditophora. The terms Acoelomorpha and Rhabditophora are recent systematic additions to the platyhelminth literature (Ehlers, 1985). Acoelomorphs include the subclades (orders) Acoela and Nemertodermatida, and Rhabditophora includes all remaining groups (free living and parasitic), with the exception of Catenulida.

Historically, the Catenulida, both acoelomorph taxa, and all free-living groups of Rhabditophora (approximately nine orders) made up class Turbellaria; however, since recently, the Turbellaria is considered paraphyletic and there is little agreement on its precise composition because of the lack of well-defined synapomorphies. Turbellarian worms are best characterized as lower platyhelminths with a ciliated epidermis and most use cilia for locomotion. The remaining parasitic classes within the Rhabditophora are often referred to together as Neodermata (Latin: neo = new, derm = skin), in reference to their tegument. Class Trematoda historically contained all parasitic flukes (endo- and ectoparasites), but has subsequently been changed to include only the digenetic and aspidogastrean flukes. Class Monogenea contains only mono-genetic flukes, and class Cestoda contains the tapeworms.

Relationships among all major clades and classes are unknown and highly speculative. Morphological and molecular data argue for a major restructuring of the Platyhelminthes, particularly in the removal of the taxon name Turbellaria, recognition of its (potentially) evolutionary independent clades, and a search for synapomorphies to unite the major taxa. Arguments based on molecular data, mostly sequence data in the form of 18S rDNA, assert a polyphyletic Platy-helminthes, with complete removal of the Acoelomorpha and its transference to a position at the base of the Bilateria. Mor-phologists recognize the uniqueness of the Acoelomorpha, but contend removal from the phylum is not yet warranted. They emphasize the search for shared, derived characters to unite the three main clades, and assume a cladistic stance that accentuates the similarities that unite taxa (synapomorphies), not the differences that separate them. Potential synapomor-phies exist at different levels within the phylum. According to

Rieger and Ladurner (2001), the mode of epidermal replacement during growth or general maintenance appears to be unique to the Platyhelminthes. New epidermal cells arise from neoblasts below the body wall, instead of from dividing epidermal cells as occurs in all other metazoans. Tyler (2001) mentions the unregionalized sac-like gut as a potential au-tapomorphy of the phylum, and the structure of the hermaphroditic reproductive system and presence of biflagellate spermatozoa may be synapomorphies of Acoelomorpha and Rhabditophora. Within the Rhabditophora, the exclusively parasitic classes, Trematoda, Monogenea, and Cestoda, are related by the shared presence of a syncytial tegument that arises through the developmental replacement of the larval epidermis. Relationships among the smaller clades of free-living and symbiotic "turbellarians" are still a work in progress. The phylogenetic position of the Platyhelminthes remains unknown and will likely prevail as the main topic of discussion among biologists interested in the origins and early radiation of the Metazoa. Enigmatic animals such as Xenoturbella and the turbellariomorph annelid, Lobatocerebrum, along with en-teropneust models proposed by Tyler (2001), may also figure prominently in understanding platyhelminth evolution.

The phylum Nemertea is a small group of approximately 900 predatory worms occupying marine, freshwater, and terrestrial habitats. There is no reliable fossil record. Ne-merteans have been known since 1758, but it was not until 1876 that they were recognized as a group distinct from the turbellarian flatworms. The monophyletic nature of Ne-mertea is confirmed by the presence of their unique proboscis apparatus, but other synapomorphies may exist depending on their accepted theory of origin. Two classes exist, Anopla and Enopla, based on the structure of the proboscis and the position of the proboscis pore. The Anopla is generally considered primitive to the Enopla and may be a paraphyletic taxon since the defining features are symplesiomorphies. Relationships to other metazoans are less well known. Spiral cleavage and the presence of a true coelom place the nemerteans within the Protostomia, but questions have been raised as to the ho-mology of the nemertean coelom (rhynchocoel, bloodvessel system) with other protostomes. The traditional placement of Nemertea close to the Platyhelminthes is often favored because both groups are functionally acoelomate, though no synapomorphies are evident.

The clade Gnathifera contains four phyla defined by the ultrastructure of jaw-like elements in the mouth, though the orientation and number of elements are phylum specific. Historically, Rotifera is the best-known group of gnathiferans, containing approximately 2,000 species of exclusively microscopic, pseudocoelomate animals with a ciliated wheel-like organ on their head (Latin: rota = wheel, fera = bearer). The fossil record is sparse and only dates back to the Eocene epoch (54 mya) where animals are preserved in Dominican amber. The phylum is monophyletic and characterized by the ciliated corona, a retrocerebral organ, and unique jaw structure. Some researchers prefer to include the phylum Acantho-cephala within the Rotifera based on the shared presence of an intracytoplasmic lamina and anterior flagellum on the sperm. The name Syndermata is often used to refer to both phyla together. Acanthocephala consists of approximately

Middle Silurian crinoids (Calliocrinus cronutus) based on fossils from the Racine Dolomite of the Chicago-Milwaukee area. Although they resemble flowers, crinoids are animals related to starfish. With a net of delicate, feathery arms splayed out in the current, crinoids were able to trap microscopic food particles. The food was transported down the arms into the mouth, centrally located on the bulb-like calyx. The long stem-like column served to lift the animal above the sea floor, exposing it to currents for feeding. (Photo by ┬ęChase Studios, Inc/Photo Researchers, Inc. Reproduced by permission.)

Middle Silurian crinoids (Calliocrinus cronutus) based on fossils from the Racine Dolomite of the Chicago-Milwaukee area. Although they resemble flowers, crinoids are animals related to starfish. With a net of delicate, feathery arms splayed out in the current, crinoids were able to trap microscopic food particles. The food was transported down the arms into the mouth, centrally located on the bulb-like calyx. The long stem-like column served to lift the animal above the sea floor, exposing it to currents for feeding. (Photo by ┬ęChase Studios, Inc/Photo Researchers, Inc. Reproduced by permission.)

1,100 species of obligate intestinal parasites of vertebrates. Traditionally, acanthocephalans were closely aligned with turbellarians or Aschelminthes, but recent studies support the sister-group relationship of Acanthocephala and Rotifera. If considered part of Gnathifera, acanthocephalans must be considered highly derived, having lost the main gnathiferan characteristic (jaws) in their evolution towards parasitism.

Autapomorphies of Acanthocephala include a hook-bearing proboscis with unique ligament system and the absence of a gut. The phylum is traditionally divided into three classes, though relationships among the classes are poorly known. Gnathostomulids were first described in 1956 by Peter Ax as turbellarians, but later raised to phylum status in 1969. The phylum is small, containing approximately 100 species divided into two monophyletic classes and characterized by the unique jaw structure. A fossil record is lacking. Micrognathozoa is the most recent group of gnathiferans to be discovered, described by Reinhardt Kristensen and Peter Funch in 2000. The group consists of microscopic, ciliated, jawed worms found in a cold spring at Disko Island, Greenland. Only a single species has been described, Limnognathia maerski. The authors chose to treat Micrognathozoa as a class within the phylum Gnathifera, relegating the phyla Rotifera, Acantho-cephala, and Gnathostomulida to classes within Gnathifera. Gnathostomulida is the sister group to the clade containing Micrognathozoa, Acanthocephala, and Rotifera.

Cycloneuralia is a morphologically defined clade containing Gastrotricha, Nematoda, Nematomorpha, Priapula, Ki-norhyncha, and Loricifera. Molecular evidence indicates the clade is paraphyletic, but several potential synapomorphies exist: terminal mouth, radial pharynx, and tripartite peripha-ryngeal brain. The phylum Gastrotricha is the most primitive group of cycloneuralians, and the one often considered having the weakest affiliation with the clade. Gastrotrichs are microscopic (0.003-0.11 in [0.1-3 mm]) ciliated worms that bear cuticular adhesive organs. There is no fossil record. Synapomorphies defining the small phylum of approximately 600 species include a multilayered cuticle that covers the cilia, tube-like dual-gland adhesive organs, the unique structure of their sensory cilia, and helicoidal muscles that surround the digestive tract. The two orders within the phylum are mono-phyletic. Relationships of gastrotrichs to specific clades within the Protostomia are uncertain; the possession of both a cuticle and external cilia places gastrotrichs in an intermediate position between Ecdysozoa and Lophotrochozoa.

Once considered closely related to Gastrotricha, phylum Nematoda (also Nemata) is the largest phylum of cycloneu-ralians, containing greater than 20,000 described species. Together with its sister group Nematomorpha, these two phyla form a basal group within the Introverta, a subclade of Cy-cloneuralia defined by the loss of external cilia, a molted cuticle, and an introvert (an anterior part of the head than can be invaginated). The Nematoda is monophyletic and well defined by the presence of sensory organs called amphids and a unique form of excretory system (renette cells). The presence of an introvert is not often considered part of the ground pattern of the phylum (only known in Kinonchulus), so relationships to other Introverta are tenuous based on this character. The fossil record is sparse and dates back only to the Carboniferous Period (354-290 mya). Molecular evidence suggests the class of predominantly marine, free-living nematodes, Adenophorea, may be paraphyletic as it includes the ancestors of Secernentea, the class of terrestrial parasites. The closely related phylum, Nematomorpha, contains approximately 325 species of exclusively parasitic worms, often called horsehair worms. The phylum is defined by the presence of a unique larval stage that parasitizes arthropods, and an adult body devoid of a gut. The fossil record is sparse and extends only back to the Eocene (15-45 mya). Nematomorphs share several characteristics with nematodes that suggest a sister-group relationship: collagenous cuticle without microvilli, absence of circular muscles, and ectodermal longitudinal cords. According to some zoologists, nematomorphs may have evolved from the mermithoid nematodes that they resemble.

The Cephalorhyncha, a sub-clade of Introverta within the Cycloneuralia, contains the phyla Priapula, Kinorhyncha, and Loricifera. These phyla are united by the shared presence of chitin in their cuticles, of rings of scalids on their introverts, flosculi, and two rings of introvert retractor muscles attached through their brains. Phylum Priapula is the most primitive of the cephalorhynchs, contains fewer than 20 extant species, and has an extensive fossil record. Recent species range in size from minute to inches (millimeters to centimeters) and are exclusively benthic predators. Autapomorphies of Priapula include a large body cavity containing amoebocytes and erthry-ocytes, and perhaps the unique structure of their urogenital system. The fossil record for priapulans is unique within the Cycloneuralia because it extends back to the Cambrian: Ot-toia prolifica is the most abundant fossil species and closely resembles the extant Halicryptus spinulosus. The two remaining phyla, Kinorhyncha and Loricifera, are closely united by the structure of their introvert, which actually protrudes out rather than evaginates out (turns inside out by eversion of an inner surface) as in priapulans. The Kinorhyncha is a small phylum of approximately 150 species, all microscopic with a body composed of 13 segments, and with an internally segmented musculature and nervous system. Despite the tough cuticle, no fossilized kinorhynchs have been found. Reinhardt Kristensen, the co-discoverer of Micrognathozoa and Cy-cliophora, first described Loriciferans in 1983. The Loricifera is a small phylum of 100 species characterized by scalids (spines) with intrinsic muscles and a lifecycle with a Higgins larva. No fossils are known.

The phyla Entoprocta and Cycliophora comprise a mono-phylum, according to cladistic studies. Phylum Entoprocta consists of 150 extant species and few fossilized forms that extend back to the Upper Jurassic (200 mya). Species are either individual or colonies of globular polyps with ciliated tentacles on a short stalk. Entoprocts are functionally acoelomate but may have small pseudocoelic sinuses. Embryonic cleavage is clearly of the spiral pattern and results in a trochophore larva, placing entoprocts within the Protostomia. Traditionally, entoprocts were allied with ectoprocts in the phylum Bry-ozoa, but a lack of synapomorphies led to their eventual separation in 1921. Danish biologist Claus Nielsen (2001) still supports a close relationship between the taxa and suggests several possible synapomorphies: myoepithelial cells in the apical organ, similar metamorphosis, and the ultrastructure of larval eyes. Cladistic studies have pointed out a potential relationship to Cycliophora, a recently discovered phylum of symbiotic micrometazoans that live on the mouthparts of lobsters (Funch and Kristensen, 1995). The phylum is characterized by a highly complicated lifecycle involving asexual female stages, sexual female stages, sexual dwarf males, and a variety of larval forms (pandora larvae, chordoid larvae). The chordoid larva is thought to be homologous with the typical protostome trochophore larva, and may have developed neotenically from entoprocts, according to some researchers. A single synapomorphy unites the two phyla: mushroom-shaped extensions from the basal lamina into the epidermis.

The Deuterostomia is a morphologically well-defined group with a long fossil history. Traditionally, the Deuteros-tomia is characterized by embryological criteria such as radial cleavage (a symplesiomorphy), blastopore becomes the anus, enterocoelous archenteric mesoderm, and a tripartite coelom. The deuterostome clade generally includes the Chaetognatha, Lophophorata (Brachiopoda, Ectoprocta, Phoronida), Hemi-chordata (Enteropneusta, Pterobranchia), Echinodermata, and Chordata. Several phyla, namely Chaetognatha and the lophophorate groups, share characteristics with both proto-stomes and deuterostomes, and their phylogenetic affinities are muddled. The advent of molecular phylogenetics in the 1990s also raised several questions regarding traditional deuterostome relationships. The hemichordates, echino-derms, and chordates make up a characteristic clade of derived deuterostomes defined by at least five synapomorphies, depending on views about their evolutionary origins: glomerular complex, epithelia that binds iodine and secretes iodothyrosine, separate gonoducts, pharyngeal gill slits, and a dorsally concentrated nervous system. The Hemichordata is a small phylum of a few hundred species, with a fossil record dating back to the Cambrian, and generally considered mono-phyletic by the presence of an anterior evagination of the gut called the stomochord. Brusca and Brusca (2002) consider the glomerulur complex, an excretory organ comprising a heart and blood vessel network in the prosome of the proboscis, an additional synapomorphy, but Nielsen (2001) regards it as a synapomorphy of Neorenalia (= Deuterostomia: Hemichordata + Echinodermata + Chordata). Nielsen also considers the Hemichordata to be polyphyletic with pterobranchs forming the sister group of Echinodermata. Alternatively, molecular studies suggest that enteropneusts and echinoderms are sister groups, or echinoderms are the closest sister group to the chordates.

Despite their extensive fossil record extending to the Cambrian, echinoderms and chordates have been notoriously difficult in terms of identifying a closest sister group. Echinoderms are defined primarily by the endoskeletal system of plates with a stereom structure, and external ciliary grooves for suspension feeding. Two extinct groups, the Car-poids and Helicoplacoids, are basal groups with the phylum. According to some authors, the Carpoids represent the ancestor to most fossil and modern echinoderms, but the lack of radial symmetry and evidence of a water vascular system make their presence within the phylum debatable. The He-licoplacoids, however, had triradial symmetry and water vascular systems with open ambulacral grooves, and are so considered basal echinoderms. The six remaining classes are united by the characteristic pentaradial symmetry, with mouth and anus on oral surface, and attachment to the substratum by the aboral surface.

According to the paleontologist R. P. S. Jefferies (1986), a group of extinct echinoderms called calcichordates gave rise to the chordate lineage. Jefferies claims to have identified chordate-like structures such as gill slits, a brain, notochord, and dorsal nerve cord in fossils of calcichordates. This calci-chordate theory is still hotly contested, although many paleontologists have rejected the theory because they do not accept Jefferies' identification of the chordate features in these extinct echinoderms. The traditional branching of the chordate line is generally well accepted with Urochordata arising first, followed by Cephalochordata and Vertebrata. The cephalochordate-vertebrate line is hypothesized to have evolved via paedomorphosis from a urochordate-like ancestor. Some zoologists even prefer Garstang's theory (1928) that vertebrates evolved directly from a larval urochordate by pae-domorphosis. However, lately, the model cephalochordate, Branchiostoma (Amphioxus), is considered to be the closest liv ing relative of Vertebrata. Synapomorphies uniting Cephalo-chordata and Vertebrata include segmental muscles (my-omeres), a ventral pulsating blood vessel (homologous with the vertebrate heart), an intestinal diverticulum (embryonic precursor of the vertebrate liver), and separate dorsal and ventral roots of the spinal cord.

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