Animals with a true coelom can be divided into two groups: protostosomes and deuterostomes. 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 ball of cells known as a blastula. A groups of cells move inward to form an opening called the blastophore. However, protostomes differ from deuterostomes in that the blastophore develops to into the mouth, whereas in deuterostomes in develops to be the anus. Also, 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. Protostomes have their early cell divisions diagonal to the polar axis forming a spiral arrangement of cells; this is called spiral cleavage.
Protostomes have a determinate cleavage: the fate of how each embryonic cell will turn out to beor function is typically fixed very early; the first four cells are separate and each will develop into a fixed quarter of the larva. If a cell is removed from the blastula, a limb might not form, for the other cells don't compensate. In deuterostomes this is not the case, they 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. Protostomes on theother hand are schizocoely, where the mesoderm splits and the split widens into a cabity that becomes the coelom.
Phyla in deuterostomes: