Deuterostomes (from the greek: second the mouth) are one of the two divisions of the taxonomic group 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.

Phyla in deuterostomes: