urichordates develope
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A chordate is an animal belonging to the phylum Chordata; they 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. In the case of vertebrate chordates, the notochord is usually replaced by a vertebral column during development, and they may have body plans organized by segmentation.
Taxonomically, the phylum includes the subphyla Vertebrata, which includes fish, amphibians, reptiles, birds, and mammals; Tunicata, which includes salps and sea squirts; and Cephalochordata, comprising the lancelets. There are also additional extinct taxa. The Vertebrata are sometimes considered as a subgroup of the clade Craniata, consisting of chordates with a skull; the Craniata and Tunicata compose the clade Olfactores.
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. Fossil chordates are known from at least as early as the Cambrian explosion.
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 the Ambulacraria, the sister taxon of the Chordates. The Chordata and Ambulacraria form the superphylum Deuterostomia, composed of the deuterostomes.
Herdmania (a tunicate) is the most appropriate one of the best examples of retrogressive metamorphosis in which the highly developed tadpole larva undergoes retrogressive changes to become the most degenerated and sedentary adult.