Deuterostomes (from the Greek: second the mouth) are one of the two superphyla of animals in the taxonomic branch bilateria, the other being the protostomes. These groups reflect two main lines of evolution based on their pattern of early development.
In both protostomes and deuterostomes, the embryo consists of a little hollow ball of cells known as a blastula. In deuterostomes early divisions of cells are parallel or perpendicular to the polar axis, and so the cells are located above and below one another; this is called radial cleavage. A groups of cells move inward to form an opening called the blastophore. In deuterostomes the blastophore develops to be the anus.
Deuterostomes have indeterminate cleavage: The cells' fate is not determined early on, and so 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.
Last but not least, Deuterostomes are enterocoely, which means the mesoderm forms as "out-pocketings" of the developed gut that will pinch off and form the coelom.
The Deuterostomia comprise one of the major groups within the animal kingdom, and it is probably the most familiar to you, since you are a deuterostome!
In the Protostomia, which is the other major group of the Bilateria, the mouth develops from the edge of this pocket, where the inner and outer layer of cells meet; the anal opening develops later.
In the Deuterostomia, the reverse is true; the pocket edge develops into the anus, and the mouth is formed later.
The name Deuterostomia ("second mouth") refers to the development of the embryo blastopore into the anus, with the mouth developing as a secondary opening, in contrast to protostomes ("first mouth") with the process reversed.
Within the Deuterostomia, chordates and hemichordates were united by possession of ciliated pharyngeal gill pores/slits and a stiffened notochord-like structure (the notochord in chordates, the stomochord in hemichordates).
Deuterostomia is much lower down, probably the second branch of Bilateria to split off after the Acoelomorpha.
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