The cnidarians, the only radial animals, have many such sections which all revolve around one point. An animal is considered bilateral or radial by the way they develop, e.g. the echinoderms, which appear as an adult to be radial, are, because they develop bilaterally, a bilateral organisms. The next division in Kingdom Animalia is the develpment of the body cavity. Flatworms have only one body cavity, and it does not go through their entire body. When they eat and excreat they use the same opening.
Roundworms have a sudocoelem which suspends their digestive tracts in fluid.
The roundworm's digestive tract goes all the way through it, so what it eats is excreated through an anus instead of being excreted through its mouth. A earthworm has a true coelem, which means that its digestive tract is held in place by tissue, instead of floating freely. All animals that are more complex than a round worm have a coelem. The remaining animals are split into deuterostomes and protozomes. When deuterostomes develop their cells line up in such a way that in the eight cell stage they can be devided in half with no cells being destroyed.