Deuterostome

Deuterostomes (taxonomic term: Deuterostomia; from the Greek: "other mouth") are a major group of animals. They are a subgroup of the Bilateria, 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 three phyla of deuterostomes:

• Phylum Chordata (vertebrates and their kin)
• Phylum Echinodermata (starfishes, sea urchins, sea cucumbers, etc.)
• Phylum Hemichordata (acorn worms)

The phylum Chaetognatha (arrow worms) and the isolated genus Xenoturbella may also belong here. Extinct groups may include the phylum Vetulicolia.

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.