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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, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a muscular tail extending past the anus. 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 they 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. The extant groups of chordates are related as shown in the phylogenetic tree, below. They do not match up very well with the traditional groups, and as a result vertebrate classification is in a state of flux, although their relationships are not very well understood.
Classes of Chordata In the subphylum Urochordata classes Ascidiacea, Thaliacea, Larvacea are found. Includes the sea squirts and tunicate worms. In subphylum Cephalochordata the lancelet worms are found. In the subphylum Vertebrata (all animals with vertebrae) classes Myxini (hagfish), Conodonta, Hyperoartia (lampreys), Cephalaspidomorphi, Pteraspidomorphi, Placodermi, Chondrichthyes (sharks, rays and skates), Acanthodii (spiny sharks), Actinopterygii (ray-finned fish), Sarcopterygii (lobe finned fish), Amphibia (amphibians), Sauropsida (reptiles), Synapsida, Aves (birds), and Mammalia (mammals) are found. Taxonomy & Phylogeny | |||||||||||||||||||||||||||||||||
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