Trematodes, or flukes, are another class of helminths that have parasitic species. Adult flukes are typically flat, oval-shaped worms that have a layer of muscles just below the tegument, or skin, that allow the worm to expand and contract its shape and, thus, move its body. Flukes usually have an oral sucker on their anterior end, sometimes ringed with hooks, that is used to attach themselves to the host's tissues.
The life cycle of a typical trematode begins when eggs, that are passed out of a previous host's digestive tract, find themselves in fresh water. The ciliated larval form, called miracidia, emerge from the eggs and swim until they find the appropriate species of their intermediate host: usually a snail. The miracidia penetrate the snail and change into another form, called sporocysts. The sporocysts undergo further changes resulting in yet another form of the parasite called cercariae, which burrow out of the snail and pass into the water again. A cercaria has a flagella-like tail that helps it swim through the water in search of its final host, typically a mammal or avian species. The cercariae make contact with the skin of a host and burrow in. Host animals may also become infected with flukes by ingesting meat, usually fish or crustaceans, that are harboring the cercariae in an encysted form, called metacercaria, within their tissues. In either form, once inside the final host the parasite moves through tissues or the blood to the desired organ, often the intestine or liver, where it matures into a reproducing adult, starting the cycle again.
Clonorchis sinensis, a fluke common in the Far East, is a trematode that uses fish as one of its intermediate hosts and fish-eating mammals, including humans, as a final host. The adult C. sinensis flukes eventually make their way through the bile ducts to the liver of the host.
Another fluke that uses both a snail and a second intermediate host is Paragonimus westermani. Freshwater crabs and crayfish can harbor P. westermani metacercaria that, in turn, may be consumed. The adult of this parasite makes its way into the lungs of its host.
Some of the most infamous flukes are species of the genus Schistosoma that cause the often fatal schistosomiasis. The cercariae of these flukes infect human hosts directly by burrowing into the skin of a person wading or swimming in infected water. One species, S. mansoni, enters the blood stream as an immature worm and can be carried through various organs, including the lungs and heart, before maturing in the liver.