Fluke :

Fluke (invertebrate), common name given to members of a class of parasitic flatworm . They vary in length from 0.2 to 165 mm (up to 6y in); most species have flat, elongated bodies, although some blood flukes are nearly cylindrical. The possession of a digestive tract, specialized sensory organs, and, in most species, free-living stages places them closer in their evolutionary history to the free-living flatworms than to the parasitic tapeworms. The mouth of the fluke is situated on the underside and, in most species, near the front. Muscular sucking discs at the rear of the body serve to attach the fluke to the host; in the species that are external parasites (ectoparasites), these suckers are often equipped with hooks. Most species are hermaphroditic—that is, male and female organs are present in the same individual. Flukes that are parasitic on the surface of other organisms have a simple development; species that are internal parasites (endoparasites) frequently undergo a complex development requiring two or more hosts to complete their life cycles.
One endoparasitic species, commonly called the sheep-liver fluke, produces a disease in sheep, goats, and cattle called liver rot. This disease has a high mortality rate and is frequently epidemic in Europe and Australia. The sheep-liver fluke is about 2.5 cm (1 in) in length and has a dark red pigment, much like the liver in which it lives. The eggs leave the body of the animal in its faeces and, if they are discharged in a body of water, hatch to liberate ciliated larvae, called miracidia. Each miracidium swims in the water until it finds a snail in which it can develop. The miracidium burrows its way to the liver tissue of the snail and changes into a spore form, or sporocyst. Within the sporocyst, bodies called rediae develop by budding. These rediae produce more rediae, which then produce new larval forms called cercariae. The cercaria escapes from the snail and usually attaches itself to aquatic vegetation, where it encysts. It remains encysted until a sheep or other mammal swallows it. The cyst wall then breaks down, and the larva migrates to the liver of the host, where it develops into an adult fluke. The life cycle of this fluke is typical of the developmental history of many members of the class.
Flukes occur in most parts of the world where the hosts can thrive. They are parasitic in their adult form in many species of vertebrate animals. Each species of fluke is host-specific (able to parasitize only a few related vertebrates). Flukes commonly known as blood flukes infest the blood vessels in humans, causing the widespread, serious disease schistosomiasis, also known as bilharzia or bilharziasis.
Scientific classification: Flukes make up the class Trematoda. The sheep-liver fluke is classified as Fasciola hepatica. The flukes commonly known as blood flukes are classified in the genus Schistosoma.