Many primitive animals, like flatworms, have no coelom at all, just a rudimentary internal pouch, like sticking your finger deep into a ball of clay. We call such animals acoelomate , because they lack a coelom. Another group of animals, including nematode worms and rotifers, have a large body cavity that looks like a coelom, and functions like a coelom, but is actually formed in a different fashion. This pseudocoelom is a remnant of the hollow space inside the blastula, the blastocoel. We call these animals pseudocoelomates .
All higher animals have a true coelom, a body cavity formed from within the mesoderm tissue layer.
This cavity is lined by mesodermal membranes, and the internal organs grow and develop within this fluid filled cavity. Such animals are called eucoelomate . If we look at the overall pattern of animal evolution, we see that all of the coelomate animals are split into two distinct groups, one called protostomes, the other called deuterostomes. This is a very ancient split within the animal kingdom, going back at least 570 million years to the early Cambrian. These groups are separated by what happens to the blastopore, the small hole that opens in the gastrula, connecting the embryonic gut to the outside.