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A CHORDATE is an animal belonging to the phylum CHORDATA; chordates possess a notochord , a hollow dorsal nerve cord , pharyngeal slits , an endostyle , and a post-anal tail , for at least some period of their life cycle. Chordates are deuterostomes , as during the embryo development stage the anus forms before the mouth. They are also bilaterally symmetric coelomates with metameric segmentation and a circulatory system . In the case of vertebrate chordates, the notochord is usually replaced by a vertebral column during development.
Taxonomically, the phylum includes the following subphyla: the
Vertebrata , which includes fish , amphibians , reptiles , birds , and
mammals ; the
Tunicata , which includes salps and sea squirts ; and
Of the more than 65,000 living species of chordates, about half are
bony fish of the superclass
Osteichthyes . The world's largest and
fastest animals, the blue whale and peregrine falcon respectively, are
chordates, as are humans .
Hemichordata , which includes the acorn worms , has been presented as
a fourth chordate subphylum, but it now is usually treated as a
separate phylum. The Hemichordata, along with the Echinodermata (which
includes starfish , sea urchins , sea cucumbers , and crinoids ), form
* 1 Overview of affinities
* 1.1 Origin of name
* 2 Definition
* 3 Subdivisions
* 4 Origins
* 5 Classification
* 5.1 Taxonomy * 5.2 Phylogeny
* 6 Closest nonchordate relatives
* 6.1 Hemichordates * 6.2 Echinoderms
* 7 See also * 8 References * 9 External links
OVERVIEW OF AFFINITIES
Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. The current consensus is that chordates are monophyletic , meaning that the Chordata include all and only the descendants of a single common ancestor, which is itself a chordate, and that craniates ' nearest relatives are tunicates.
All of the earliest chordate fossils have been found in the Early
It has also proved difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "family trees " shows that many of the traditional classes are paraphyletic .
While this has been well known since the 19th century, an insistence on only monophyletic taxa has resulted in vertebrate classification being in a state of flux.
ORIGIN OF NAME
Although the name Chordata is attributed to William Bateson (1885), it was already in prevalent use by 1880. Ernst Haeckel described a taxon comprising tunicates, cephalochordates, and vertebrates in 1866. Though he used the German vernacular form, it is allowed under the ICZN code because of its subsequent latinization. 1 = bulge in spinal cord ("brain") 2 = NOTOCHORD 3 = DORSAL NERVE CORD 4 = POST-ANAL TAIL 5 = anus 6 = digestive canal 7 = circulatory system 8 = atriopore 9 = space above pharynx 10 = PHARYNGEAL SLIT (gill ) 11 = pharynx 12 = vestibule 13 = oral cirri 14 = mouth opening 15 = gonads (ovary / testicle ) 16 = light sensor 17 = nerves 18 = metapleural fold 19 = hepatic caecum (liver -like sack) Anatomy of the cephalochordate Amphioxus . Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.
Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following:
* A notochord, a fairly stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine , and in wholly aquatic species this helps the animal to swim by flexing its tail. * A dorsal neural tube . In fish and other vertebrates , this develops into the spinal cord , the main communications trunk of the nervous system . * Pharyngeal slits . The pharynx is the part of the throat immediately behind the mouth. In fish , the slits are modified to form gills , but in some other chordates they are part of a filter-feeding system that extracts particles of food from the water in which the animals live. * Post-anal tail. A muscular tail that extends backwards behind the anus . * An endostyle . This is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus . It also stores iodine , and may be a precursor of the vertebrate thyroid gland.
There are soft constraints that separate chordates from certain other biological lineages, but have not yet been made part of the formal definition:
* All chordates are deuterostomes . This means that, during the embryo development stage, the anus forms before the mouth. * All chordates are based on a bilateral body plan . * All chordates are coelomates , and have a fluid filled body cavity called a coelom with a complete lining called peritoneum derived from mesoderm (see Brusca and Brusca) .
There is still much ongoing differential (DNA sequence based) comparison research that is trying to separate out the simplest forms of chordates. As some lineages of the 90% of species that lack a backbone or notochord might have lost these structures over time, this complicates the classification of chordates. Some chordate lineages may only be found by DNA analysis, when there is no physical trace of any chordate-like structures.
See also: List of chordate orders
Craniates , one of the three subdivisions of chordates, all have distinct skulls . They include the hagfish which have no vertebrae . Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or possibly all deuterostomes , are by their tails".
Most are vertebrates , in which the notochord is replaced by the vertebral column . These consist of a series of bony or cartilaginous cylindrical vertebrae, generally with neural arches that protect the spinal cord , and with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates, but as members of the craniates, the group from which vertebrates are thought to have evolved . However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may be degenerate vertebrates who have lost their vertebral columns.
The position of lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish . However, molecular phylogenetics , which uses biochemical features to classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish. If lampreys are more closely related to the hagfish than the other vertebrates, this would suggest that they form a clade , which has been named the Cyclostomata .
TUNICATA (TUNICATES, OR UROCHORDATES)
Main article: Tunicate Comparison of three invertebrate chordates A. Lancelet, B. Larval tunicate, C. Adult tunicate --------------------------------------------------------
1. Notochord , 2. Nerve chord, 3. Buccal cirri , 4. Pharynx , 5. Gill slit , 6. Gonad , 7. Gut, 8. V-shaped muscles, 9. Anus, 10. Inhalant syphon , 11. Exhalant syphon, 12. Heart, 13. Stomach, 14. Esophagus , 15. Intestines, 16. Tail, 17. Atrium, 18. Tunic Tunicates: sea squirts
Most tunicates appear as adults in two major forms, both of which are soft-bodied filter-feeders that lack the standard features of chordates: "sea squirts" are sessile and consist mainly of water pumps and filter-feeding apparatus; salps float in mid-water, feeding on plankton , and have a two-generation cycle in which one generation is solitary and the next forms chain-like colonies . However, all tunicate larvae have the standard chordate features, including long, tadpole -like tails; they also have rudimentary brains, light sensors and tilt sensors. The third main group of tunicates, Appendicularia (also known as Larvacea) retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of sea squirts or salps. The etymology of the term Urochorda(ta) (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail. The term TUNICATA (Lamarck 1816) is recognised as having precedence and is now more commonly used.
Cephalochordates are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs. These burrowing filter-feeders compose the earliest-branching chordate sub-phylum.
The majority of animals more complex than jellyfish and other
Fossils of one major deuterostome group, the echinoderms (whose
modern members include starfish , sea urchins and crinoids ), are
quite common from the start of the Cambrian, 542 million years ago.
Tunicates (Urochordates )
A consensus family tree of the chordates
The evolutionary relationships between the chordate groups and
between chordates as a whole and their closest deuterostome relatives
have been debated since 1890. Studies based on anatomical,
embryological , and paleontological data have produced different
"family trees". Some closely linked chordates and hemichordates, but
that idea is now rejected. Combining such analyses with data from a
small set of ribosome
Since early chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by molecular phylogenetics techniques—by analyzing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before 900 million years ago and the earliest chordates around 896 million years ago. However, molecular estimates of dates often disagree with each other and with the fossil record, and their assumption that the molecular clock runs at a known constant rate has been challenged.
A skeleton of the blue whale , the world's largest animal, outside the Long Marine Laboratory at the University of California, Santa Cruz A peregrine falcon , the world's fastest animal
The following schema is from the third edition of Vertebrate Palaeontology . The invertebrate chordate classes are from Fishes of the World . While it is structured so as to reflect evolutionary relationships (similar to a cladogram ), it also retains the traditional ranks used in Linnaean taxonomy .
* PHYLUM CHORDATA
* Subphylum TUNICATA (Urochordata) – (tunicates; 3,000 species)
* Class ASCIDIACEA (sea squirts) * Class THALIACEA (salps) * Class APPENDICULARIA (larvaceans) * Class SORBERACEA
* Subphylum CEPHALOCHORDATA (Acraniata) – (lancelets; 30 species)
* Class LEPTOCARDII (lancelets)
* Subphylum VERTEBRATA ( Craniata ) (vertebrates – animals with backbones; 57,674 species)
* Infraphylum incertae sedis CYCLOSTOMATA
* Superclass 'AGNATHA \' paraphyletic (jawless vertebrates; 100+ species)
Myxini (hagfish; 65 species)
* Infraphylum GNATHOSTOMATA (jawed vertebrates)
* Class †PLACODERMI (Paleozoic armoured forms; paraphyletic in relation to all other gnathostomes) * Class CHONDRICHTHYES (cartilaginous fish; 900+ species) * Class †ACANTHODII (Paleozoic "spiny sharks"; paraphyletic in relation to Chondrichthyes)
* Superclass OSTEICHTHYES (bony fish; 30,000+ species)
* Class ACTINOPTERYGII (ray-finned fish; about 30,000 species) * Class SARCOPTERYGII (lobe-finned fish: 8 species)
* Superclass TETRAPODA (four-limbed vertebrates; 28,000+ species) (The classification below follows Benton 2004, and uses a synthesis of rank-based Linnaean taxonomy and also reflects evolutionary relationships. Benton included the Superclass Tetrapoda in the Subclass Sarcopterygii in order to reflect the direct descent of tetrapods from lobe-finned fish, despite the former being assigned a higher taxonomic rank.)
* Class AMPHIBIA (amphibians; 7,000+ species) * Class SAUROPSIDA (reptiles (including birds ); 9,000+ species) * Class SYNAPSIDA (mammals ; 5,700+ species)
Cladogram of the
Myllokunmingia fengjiaoa †
Zhongjianichthys rostratus †
Placodermi † (paraphyletic in relation to all other Gnathostomata)
Lepidosauromorpha (lizards , snakes , tuatara , and their extinct relatives )
CLOSEST NONCHORDATE RELATIVES
Acorn worms or Enteropneusts are example of hemichordates.
Main article: Hemichordate
Hemichordates ("half (½) chordates") have some features similar to those of chordates: branchial openings that open into the pharynx and look rather like gill slits; stomochords, similar in composition to notochords , but running in a circle round the "collar", which is ahead of the mouth; and a dorsal nerve cord—but also a smaller ventral nerve cord.
There are two living groups of hemichordates. The solitary enteropneusts , commonly known as "acorn worms", have long proboscises and worm-like bodies with up to 200 branchial slits, are up to 2.5 metres (8.2 ft) long, and burrow though seafloor sediments . Pterobranchs are colonial animals, often less than 1 millimetre (0.039 in) long individually, whose dwellings are interconnected. Each filter feeds by means of a pair of branched tentacles, and has a short, shield-shaped proboscis. The extinct graptolites , colonial animals whose fossils look like tiny hacksaw blades, lived in tubes similar to those of pterobranchs.
Echinoderms differ from chordates and their other relatives in three conspicuous ways: they possess bilateral symmetry only as larvae - in adulthood they have radial symmetry , meaning that their body pattern is shaped like a wheel; they have tube feet ; and their bodies are supported by skeletons made of calcite , a material not used by chordates. Their hard, calcified shells keep their bodies well protected from the environment, and these skeletons enclose their bodies, but are also covered by thin skins. The feet are powered by another unique feature of echinoderms, a water vascular system of canals that also functions as a "lung" and surrounded by muscles that act as pumps. Crinoids look rather like flowers, and use their feather-like arms to filter food particles out of the water; most live anchored to rocks, but a few can move very slowly. Other echinoderms are mobile and take a variety of body shapes, for example starfish , sea urchins and sea cucumbers .
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