Acoels: Acoela

Acoels (AY-seels) are tiny wormlike sea animals. They are the simplest animals with bilateral symmetry (bye-LAT-er-uhl SIH-muh-tree), meaning the right and left halves of the body match each other. Most acoels are no longer than about one-sixteenth of an inch (2 millimeters), although one species can reach a length of about five-eighths inch (15 millimeters). The bodies of acoels are flat ovals.

Acoels have either a simple mouth or none at all. The mouth is on the bottom of the animal. Acoels have no digestive tract, no system for eliminating waste or balancing salt content in their cells, and no reproductive organs. The nervous system is a loose net of fibers strung throughout the body. Most acoels have simple eyes. Almost all acoels have an organ for balance and for adjusting themselves to their surroundings.