Trematodes generally lack any specialized sense organs, although some ectoparasitic species do possess one or two pairs of simple ocelli. Reproductive system
Most trematodes are simultaneous hermaphrodites, having both male and female organs. Blood flukes are the only form of trematodes that are not hermaphrodites, meaning they have both a male and female sex. Blood flukes are unique in the way that they can reproduce sexually and asexually. In order for the reproduction of blood flukes to occur asexually, infecting a freshwater snail must happen to act as an intermediate host. Acting as an intermediate host, an organism that supports non-reproductive parasites, allows for the blood flukes to gather nourishment to reproduce. The asexual reproduction takes place in the hepatopancreas of the snail. Reproduction of blood flukes sexually involves finding a mammal host to act as an intermediate host to support their nutritional needs.The reproductive system of trematodes usually consist of having two testes, which are found posterior and dorsal to the ventral sucker. Having sperm ducts that join together on the underside of the front half of the animal. This final part of the male system varies considerably in structure between species, but may include sperm storage sacs and accessory glands, in addition to the copulatory organ, which is either eversible, and termed a cirrus, or non-eversible, and termed a penis.There is usually only a single ovary. Eggs pass from it into an oviduct. The distal part of the oviduct, called ootype, is dilated.
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.