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The Platyhelminthes (Greek platy: flat; helminth: worm) are a phylum of relatively simple animals, called flatworms. The body is ribbon-shaped, soft-bodied and is flattened dorsoventrally, from top to bottom. They are bilaterally symmetrical and are triploblastic, meaning they have three germ layers; the outer ectoderm,an inner endoderm and a mesoderm between the two. However, they possess no respiratory or circulatory systems, and there is no true body cavity except the gut and hence are Acoelomates. Usually the digestive tract has one opening, but in particularly long worms or those with highly branched guts, there may be one or more anuses. In acoelomate flatworms, now thought to be unrelated to the Platyhelminthes, the gut is absent or non-permanent. Since most flatworms have only one opening to their digestive tract (their gastrovascular cavity), they can't feed, digest, and eliminate undigested particles of food simultaneously, as most animals with tubular guts can. Muscular contraction in the upper end of the gut cause a strong sucking force allowing flatworms to ingest their food and tear it into small bits. The gut is branched and extends throughout the body, functioning in both digestion and transport of food.
Flatworms were formerly considered to be basal among the protostomes. Molecular evidence suggests that this is only true of the orders Acoela and Nemertodermatida, which are thus given their own phylum Acoelomorpha. The true flatworms form a monophyletic group that developed from more complex ancestors, and is generally considered to belong among the Lophotrochozoa. The traditional classifications of flatworms recognizes three monophyletic classes:
The remaining flatworms are grouped together for convenience as the class Turbellaria, now comprising the following orders:
- Catenulida
- Macrostomida
- Lecithoepitheliata
- Rhabdocoela
- Prolecithophora
- Proseriata
- Tricladida
- Polycladida
Most of these groups include free-living forms. The flukes and tapeworms, though, are parasitic, and a few cause extreme damage to people and other animals.
References
- Crawley, John L., and Kent M. Van De Graff.(editors);A Photographic Atlas for the Zoology Laboratory; Colorado, Morton Publishing Company; BooksEnthsiast.com (fourth edition,2002)
Animals
Flatworms
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