The class Trematoda consists of flukes. Flukes are parasitic flatworms that require two hosts to complete their life cycle. They begin their lives in a lower life form like a snail, then move on to invade humans and other animals. Flukes, says Animal Planet, are generally found in the tropics and Far East, but can also be found in other parts of the world. They spread infection and disease in humans and animals. Common flukes include blood flukes, Chinese liver flukes and lancet liver flukes.
The digestive system of a trematoda, when one is present, is similar to that of a turbellarian. Like a turbellarian, a trematode has a two-branched gut that extends throughout the body, but a trematode lacks a turbellarian’s extensible pharynx.
Many trematodes have no digestive system at all. Since many of them live inside the digestive systems of their hosts, they can simply absorb pre-digested matter from their hosts. Since they don’t have to devote any energy or body volume to digestion, this leaves these animals all the more energy and body space to devote to reproduction.