The parasitic flatworms of class Trematoda, also called flukes, have oral suckers, sometimes supplemented by hooks, with which they attach to their vertebrate hosts. Trematodes have retained the same body form and digestive cavity as the turbellarians. However, practically the entire interior is occupied by the reproductive system; the organism is capable of producing huge numbers of offspring. Trematodes of the order Digenea have complex life cycles involving two or more hosts. The larval worms occupy small animals, typically snails and fish, and the adult worms are internal parasites of vertebrates. Many species, such as the liver fluke Clonorchis sinensis and the blood fluke ( Schistosoma ), cause serious diseases in humans.