ParaHoxozoa
The first bilateral organisms (the first living things that were symmetrical from side to side) were worms which lived on the bottom of bodies of water. This type of worm probably had only one opening in its body instead of both a mouth and an anus, as bilateral organisms do today. A through-gut may already have developed with the ctenophora. The through-gut, the tubelike digestive system with two ends, may have evolved when the organism's one hole closed in the middle. For example, Acoela look like the planula larvae of some cnidarians.