The flatworms, or Platyhelminthes, are bilaterally symmetrical metazoans with three tissue layers but no body cavity enclosing the viscera. There is cephalization with brain and sense organs concentrated at the anterior end.
Flatworms are complex animals with elaborate reproductive systems.
The three classes are the free-living flatworms, flukes, and tapeworms. The free-living flatworms belong to the class Turbellaria. They are carnivores and vary in size from less than 1 mm to several centimeters. Many are roughly the size of ciliate protozoans, with which the small ones are easily confused. The body is cylindrical in the small species, dorsoventrally flattened in the large ones.