Deuterostomes (from the Greek: second the mouth) are a major groups of animals. Together with the protostomes, they include most of the Bilateria, animals that are typically bilaterally symmetric and have three germ layers. They 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 main phyla of deuterostomes:
The phylum Chaetognatha (arrow worms) may also belong here.
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 .