Deuterostomia is a superphylum of animals. They are the 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 the first opening is becomes to mouth. Deuterostomes are also known as enterocoelomates, because their coelom develops through enterocoely.
Superphylum deuterostomia was redefined in 1995 based on molecular sequence analyses when the lophophorates were removed from it. And combined with other protostome animals to form superphylum Lophotrochozoa. The phylum chaetognatha(arrow worms) may also belong here. Extinct groups may include the phylum vetulicolia, Echinodermata, hemichordata, and Xenoturbellida from the Clade Ambulacraria.
In both deuterostomes and protostomes, a zygote initially develops into a hollow ball of cells, called a blastula. In deuterostomia, 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. Most deuterostomia display indeterminate cleavage, in which the developmental fate of the cells in the developing embryo are not determined by the identity of the parent cell. 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.
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 including tunicates(in the larval stage). Some hemichordates also have a tubular nerve-cord. In the 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 deuterostomia evolved from a common ancestor that had pharyngeal gill slits, a hollow nerve cord circular and longitudinal muscles and a segmented body.
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