They are adapted to recognize and penetrate the second intermediate host, and contain behavioral and physiological adaptations not present in earlier life stages. The metacercariae are an adapted cystic form dormant in the secondary intermediate host. The adult is the fully developed form which infects the definitive host.The first stage is the miracidium that is triangular in shape and covered by a ciliated ectoderm which is the outermost layer of the three germ layers. The epidermis and epidemic tissues of the parasite are going to evolve from the miracidium. They also have an anterior spin which helps them drill into the snail. The miracidium develops into the sporocyst, which is a sac-like structure, and in this sac the larvae begin to develop. The cells multiply. The rediae and cercariae develop from the larvae which are then released and encyst as metacercariae on aquatic plants. Humans as well as larger sea creatures then get infected when they eat these plants. When they infect humans, it takes 3–4 months for the metacercariae to mature into adult flukes and lay eggs. Life cycle of liver flukes
Liver flukes, one of the different species, are responsible for causing liver fluke disease which is also known as fasciolosis. They are hermaphroditic internal parasites. They are caused by the migration of a large number of immature flukes through the liver passageway or by adult flukes that migrate to the bile ducts. Liver flukes infect all grazing animals and are passed from human to human when they eat raw or undercooked fish.
Blood flukes include species of the genus Schistosoma.Lung flukes: there are eight species of lung flukes that infect humans: Paragonimus westermani, P. heterotremus, P. skryabini, P. miyazakii, P. africanus, P. uterobilateralis, P. kellicotti, and P. mexicanus. Paragonimus westermani needs three different hosts in order to survive, reproduce, and complete its life cycle. The completion of a lung flukes life cycle follows the cycle of first, second, and final hosts. The first intermediate host of the lung fluke is a snail, the second intermediate host is a crab or crayfish, and the final host for lung flukes is an animal or human host.Liver flukes: include Clonorchis, Opisthorchis, and Fasciola. Clonorchis and Opisthorchis are considered the most important human pathogens because they are classified as class 1 carcinogens. Liver flukes are commonly found within bile ducts, liver, and gallbladder in mammalian and avian species. There are high numbers of human infection with all three of these liver flukes.Blood flukes: include five species of Schistosoma: Schistosoma japonicum, S. mekongi, S. mansoni, S. haematobium, and S. intercalatum. Humans get infected by the cercariae, which are the larval forms of trematodes. Any contact with water can cause potential human infection. Adult blood flukes can live indefinitely in human or animal reservoir hosts. Anatomy
Trematodes are flattened oval or worm-like animals, usually no more than a few centimeters in length, although species as small as 1 millimetre (0.039 in) are known.