Flatworms

Commonly known as flukes, there are over 6,000 species of flatworm in this class. Descended from the parasitic flatworm, flukes grow slightly larger, to about 0.8-1.2 in (2-3 cm) long. A fluke must live in two or more hosts during its lifetime because its developmental needs are different than its adult needs. The first host is usually a mollusk and the final host-which houses the fluke during its mature, sexual stage-is invariably a vertebrate, sometimes a human. In general, flukes lay tens of thousands of eggs to increase their offspring's chances of survival.

Three families in this class contain blood flukes, those that live in the bloodstream of their hosts. Blood flukes, called schistosomes, are of particular importance to humans, since an estimated 200 million people are affected by them. Second only to malaria among human parasites, they usually do not kill their victims immediately; rather, they make their hosts uncomfortable for years until a secondary illness kills them.

As larvae, some species inhabit snails but, upon destroying their hosts' livers, leave and swim freely for several days. They are then absorbed through the skin of a second host, such as a human, and live in veins near the stomach. There they mature and can live for 20 years or more. Unlike other species in the phylum, blood flukes have clearly defined genders.