Why some worms regenerate and others do not

Why are so few species able to regenerate damaged or missing body parts, even though regeneration might seem an obvious survival advantage? Researchers at the Max Planck Institute (MPI) for Multidisciplinary Sciences in Göttingen, ...

How plankton hold secrets to preventing pandemics

Whether it's plankton exposed to parasites or people exposed to pathogens, a host's initial immune response plays an integral role in determining whether infection occurs and to what degree it spreads within a population, ...

Decapitated flatworms still sense light

A team of researchers from the Institute for Stem Cell Science and Regenerative Medicine, the Technology & Research Academy University and the University of Hyderabad, all in India, has found that flatworms are able to sense ...

Finding the Achilles' heel of a killer parasite

Two studies led by UT Southwestern researchers shed light on the biology and potential vulnerabilities of schistosomes—parasitic flatworms that cause the little-known tropical disease schistosomiasis. The findings, published ...

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Flatworm

The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes (from the Greek πλατύ, platy, meaning "flat" and ἕλμινς (root: ἑλμινθ-), helminth-, meaning worm) are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals. Unlike other bilaterians, they have no body cavity, and no specialized circulatory and respiratory organs, which restricts them to flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion.

In traditional zoology texts Platyhelminthes are divided into Turbellaria, which are mostly non-parasitic animals such as planarians, and three entirely parasitic groups: Cestoda, Trematoda and Monogenea. Turbellarians are mostly predators, and live in water or in shaded, humid terrestrial environments such as leaf litter. Cestodes (tapeworms) and trematodes (flukes) have complex life-cycles, with mature stages that live as parasites in the digestive systems of fish or land vertebrates, and intermediate stages that infest secondary hosts. The eggs of trematodes are excreted from their main hosts, whereas adult cestodes generate vast numbers of hermaphroditic, segment-like proglottids which 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 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 Monogenea 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.

This text uses material from Wikipedia, licensed under CC BY-SA