Scientists say the first bilateral organisms, which means the first living things that were symmetrical from side to side, were worms that lived on the bottom of bodies of water. They say this worm had only one opening in its body instead of both a mouth to take food in and an anus to let waste out like an earthworm. A through-gut may already have developed with the ctenophora however. The scientists think 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.