Author:
Jan Scholten
Book:
Qjurious
Type:
Info
Chapter:
4-310.00.00
Platyhelminthes
English: flatworms.
Greec: Platyhelminthes, platy means flat and helminth means worm.
Clades: Platyzoa; Protostomia; Animals.
Zoology: simple bilaterian, unsegmented, soft-bodied invertebrate animals; no body cavity, no specialized circulatory and respiratory organs, restricted to flattened shapes.
Taxonomy
Platyhelminthes were regarded as a primitive stage in the evolution of bilaterians (animals with bilateral symmetry and hence with distinct front and rear ends). However, analyses since the mid-1980s have separated out one sub-group, the Acoelomorpha, as basal bilaterians, in other words closer to the original bilaterians than to any other modern groups. The remaining Platyhelminthes form a monophyletic group, in other words 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 the Lophotrochozoa, one of the three main groups of more complex bilaterians. These analyses have also concluded that the redefined Platyhelminthes, excluding Acoelomorpha, consists of two monophyletic sub-groups, Catenulida and Rhabditophora, and that Cestoda, Trematoda and Monegenea form a monophyletic sub-group within one branch of the Rhabditophora. Hence the traditional platyhelminth sub-group "Turbellaria" is now regarded as paraphyletic since it excludes the wholly-parasitic groups although these are descended from one group of "turbellarians".
Over half of all known flatworm species are parasitic, and some do enormous harm to humans and their livestock. Schistosomiasis, caused by one genus of trematodes, is the second most devastating of all human diseases caused by parasites, surpassed only by malaria. Neurocysticercosis, which arises when larvae of the pork tapeworm Taenia solium penetrate the central nervous system, is the major cause of acquired epilepsy worldwide. The threat of platyhelminth parasites to humans in developed countries is rising because of organic 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 and irrigation projects increase the dangers presented by poor sanitation and unhygienic farming.
Two planarian species have been used successfully in the Philippines, Indonesia, Hawaii, New Guinea and Guam to control populations of the imported giant African snail Achatina fulica, which was displacing native snails. However, there is now concern that these planarians may themselves become a serious threat to native snails. In North-west Europe there are concerns about the spread of the New Zealand planarian Arthurdendyus triangulatus, which preys on earthworms.
Classes
Monogenea
Trematoda
Cestoda
Turbellaria