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Deuterostomes (taxonomic term: Deuterostomia; from the Greek: "second mouth") are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes. Deuterostomes are distinguished by their embryonic development; in deuterostomes, the first opening (the blastopore) becomes the anus, while in protostomes it becomes the mouth.
There are four living phyla of deuterostomes:
The phylum Chaetognatha (arrow worms) may also belong here. Extinct groups may include the phylum Vetulicolia. The term Ambulacraria is sometimes used to unite the phyla Echinodermata, Hemichordata, and Xenoturbellida.
In both deuterostomes and protostomes, a zygote first develops into a hollow ball of cells, called a blastula. In deuterostomes, the early divisions occur parallel or perpendicular to the polar axis. This is called radial cleavage, and also occurs in certain protostomes, such as the lophophorates. Cleavage is indeterminate - the cells' fates are not determined early on. Thus if the first four cells are separated, each cell is capable of forming a complete small larva, and if a cell is removed from the blastula the other cells will compensate.
In deuterostomes the mesoderm forms as evaginations of the developed gut that pinch off, forming the coelom. This is called enterocoely.
Both the Hemichordata and Chordata have gill slits, and primitive fossil echinoderms also show signs of gill slits. A hollow nerve cord is found in all chordates, even tunicates (even if it disappears in the adults). Some hemichordates also have a tubular nerve cord. In early embryonic stage it looks like the hollow nerve cord of chordates. Because of the degenerated nervous system of echinoderms it is not possible to discern much about their ancestors in this matter, but based on different facts it is quite possible that all the present deuterostomes evolved from a common ancestor which had gill slits, a hollow nerve cord and a segmented body. It could have resembled the small group of Cambrian deuterostomes named Vetulicolia.
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