Theflatworms, known in scientific literature asPlatyhelminthesorPlathelminthes(from theGreekπλατύ,platy, meaning "flat" and ἕλμινς (root:ἑλμινθ-),helminth-, meaning worm)
In traditional zoology texts, Platyhelminthes are divided intoTurbellaria, which are mostly nonparasiticanimals such asplanarians, and three entirely parasitic groups:Cestoda,TrematodaandMonogenea; however, since the turbellarians have since been proven not to bemonophyletic, this classification is now deprecated.
Free-living flatworms are mostly predators, and live in water or in shaded, humid terrestrial environments such asleaf litter. Cestodes (tapeworms) and trematodes (flukes) have complex life-cycles, with mature stages that live as parasites in the digestive systems of fish or landvertebrates, and intermediate stages that infest secondary hosts.
The eggs of trematodes are excreted from their main hosts, whereas adult cestodes generate vast numbers ofhermaphroditic, segment-likeproglottidswhich detach when mature, are excreted, and then release eggs. Unlike the other parasitic groups, the monogeneans are external parasites infesting aquatic animals, and their larvae metamorphose into the adult form after attaching to a suitable host.
Because they do not have internal body cavities, for over a century, Platyhelminthes were regarded as a primitive stage in theevolutionof bilaterians (animals with bilateral symmetry and hence with distinct front and rear ends). However, analyses since the mid-1980s have separated out one subgroup, theAcoelomorpha, asbasalbilaterians - closer to the original bilaterians than to any other modern groups. The remaining Platyhelminthes form amonophyleticgroup - one that contains all and only descendants of a common ancestor that is itself a member of the group. The redefined Platyhelminthes is part of theLophotrochozoa, one of the three main groups of more complex bilaterians. These analyses have also concluded the redefined Platyhelminthes, excluding Acoelomorpha, consists of two monophyletic subgroups,CatenulidaandRhabditophora, and Cestoda, Trematoda and Monogenea form a monophyletic subgroup within one branch of the Rhabditophora. Hence, the traditional platyhelminth subgroup "Turbellaria" is now regarded asparaphyletic, since it excludes the wholly parasitic groups, although these are descended from one group of "turbellarians".
Over half of all known flatworm species areparasitic, and some do enormous harm to humans and their livestock.Schistosomiasis, caused by onegenusof trematodes, is the second-most devastating of all human diseases caused by parasites, surpassed only bymalaria.Neurocysticercosis, which arises when larvae of the pork tapewormTaenia soliumpenetrate thecentral nervous system, is the major cause of acquiredepilepsyworldwide. The threat of platyhelminth parasites to humans in developed countries is rising because oforganic farming, the popularity of raw or lightly cooked foods, and imports of food from high-risk areas. In less developed countries, people often cannot afford the fuel required to cook food thoroughly, and poorly designed water-supply andirrigationprojects increase the dangers presented by poor sanitation and unhygienic farming.
Twoplanarianspecies have been used successfully in the Philippines, Indonesia, Hawaii, New Guinea, and Guam tocontrolpopulations of theimportedgiant AfricansnailAchatina fulica, which was displacing native snails. However, there is now concern that these planarians may themselves become a serious threat to native snails. In northwest Europe, there are concerns about the spread of the New ZealandplanarianArthurdendyus triangulatus, which preys onearthworms.
