Chordate

Chordates (phylum Chordata) are a group of animals that includes the vertebrates, together with several closely related invertebrates. They are united by having, at some time in their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail. Some scientists argue that the true qualifier should be pharyngeal pouches rather than slits.

The phylum Chordata is broken down into three subphyla: Urochordata, Cephalochordata, and Vertebrata. Urochordate larvae have a notochord and a nerve cord but these are lost in adulthood. Cephalochordates have a notochord and a nerve cord but no vertebrae. In all vertebrates except for Hagfish, the dorsal hollow nerve cord has been surrounded with cartilaginous or bony vertebrae and the notochord generally reduced.

The chordates and two sister phyla, the hemichordates and the echinoderms, make up the deuterostomes, a superphylum. It is the largest grouping of deuterostomes.

The extant groups of chordates are related as shown in the phylogenetic tree below. Many of the taxa listed do not match traditional classes because several of those classes are paraphyletic. Different attempts to organize the profusion of chordate clades into a small number of groups, some with and some without paraphyletic taxa, have thrown vertebrate classification into a state of flux. Also, the relationships of some chordate groups are not very well understood.

Taxonomy

 * Phylum Chordata
 * Subphylum Tunicata - (tunicates, formerly Urochordata; 3,000 species)
 * Subphylum Cephalochordata - (lancelets, 30 species)
 * Subphylum Vertebrata (vertebrates - animals with backbones; 57,739 species)
 * Infraphylum Agnatha (jawless vertebrates; 100+ species)
 * Class Myxini or Hyperotreti (hagfish; 65 species)
 * Class Conodonta (Conodonts)
 * Class Hyperoartia (Lampreys)
 * Class Cephalaspidomorphi (Paleozoic jawless fish)
 * Class Pteraspidomorphi (Paleozoic jawless fish)
 * Infraphylum Gnathostomata (jawed vertebrates)
 * Class Placodermi (Paleozoic armoured forms)
 * Class Chondrichthyes (cartilaginous fish; 300+ species)
 * Class Acanthodii (Paleozoic "spiny sharks")
 * Superclass Osteichthyes (bony fishes; 30,000+ species)
 * Class Actinopterygii (ray-finned fish; about 30,000 species)
 * Class Sarcopterygii (lobe-finned fish)
 * Superclass Tetrapoda (four-legged vertebrates; 18,000+ species)
 * Class Amphibia (amphibians; 6,000 species)
 * Series Amniota (with amniotic egg)
 * Class Sauropsida - (reptiles; 8,225+ species)
 * Class Aves (birds; 8,800-10,000 species)
 * Class Synapsida (mammal-like "reptiles"; 4,500+ species)
 * Class Mammalia (mammals; 5,800 species)

Phylogeny
Note: Lines show probable evolutionary relationships, including extinct taxa which are denoted with a dagger, †.

Origins
The origin of chordates is currently unknown. The first clearly-identifiable chordates are reduced fish- or lancelet-like specimens from the Cambrian. Most speculations about their origin fit into one or more of these categories:


 * A sediment-dwelling worm-like animal that evolved a flatter body and/or fins for swimming.
 * A sessile tubular filter-feeder that evolved into a free-swimming animal via usage of fins. (Tunicates, considered a chordate, are sessile filter feeders that have a tadpole-like larvae.)
 * A drifting or swimming larva of some other kind of animal that eventually retained its swimming features into adulthood.

The notochord's stiffness in many chordates may have evolved to facilitate the effectiveness of alternating muscle contractions for swimming (in S-shaped movements). In other words, in order to bend the body, a muscle needs a rigid structure to pull against. Lack of a stiff body part would merely result in the shorting of the animal during muscle contractions instead of the bending motions needed for swimming.