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All trematodes are parasitic flatworms. Previous classification systems included the Monogenea amongst the trematoda, alongside the Digenea and Aspidogastrea, on the basis that they were all vermiform parasites. The taxonomy of the Platyhelminthes is being subjected to extensive revision thanks to modern phylogenetic studies, and modern sources place the Monogenea in a separate class within the phylum.
There are no known cases of human infection with Aspidogastreans, therefore the use of the term "fluke" in relation to human infection refers solely to digenean infections.
These can be classified into two areas, on the basis of the system which they infect. Tissue flukes, are species which infect the bile ducts, lungs, or other biological tissues. This group includes the lung fluke, Paragonimus westermani , and the liver flukes, Clonorhis sinensis and Fasciola hepatica. The other group are known as blood flukes, and inhabit the blood in some stages of their life cycle. Blood flukes include various species of the genus Schistosoma.
Trematodes have a complex life cycle, often involving several hosts. The eggs pass from the host with the feces. When the eggs reach water, they hatch into free-swimming forms called miracidia . The miracidia penetrate a snail or other molluscan host to become sporocysts . The cells inside the sporocysts typically divide by mitosis to form rediae. Rediae, in turn, give rise to free-swimming cercariae , which escape from the mollusk into water. Using enzymes to burrow through exposed skin, cercariae penetrate another host (often an arthropod) and then encyst as metacercariae . When this host is eaten by the definitive host, the metacercariae excyst and develop and the life cycle repeats. For more information on life cycles, see the respective pages on digenea and aspidogastrea.
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