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There are around 29,000 described species in the phylum Platyhelminthes (Zhang 2011). Platyhelminths, or flatworms, include both free-living and parasitic species. The phylum Platyhelminthes is divided into two monophyletic clades, the Catenulida (with just around 100 known species; Larsson and Jondelius 2008) and the Rhabditophora, which includes nearly all known platyhelminths (Timothy et al. 2004; Willems et al. 2006; Larsson and Jondelius 2008).

The majority of flatworms are obligately parasitic, either flukes (members of the classes Trematoda and Monogenea, such as the Schistosoma trematodes that cause schistosomiasis) or tapeworms (members of the class Cestoda, such as the Beef Tapeworm) (Brusca and Brusca 2003). These obligately parasitic groups together form a monophyletic clade known as Neodermata. Many of these parasitic flatworms have complex life cycles involving multiple hosts. The non-parasitic flatworms, which until recently were usually placed together in a group known as Turbellaria, include mainly free-living forms in marine and freshwater benthic (bottom) habitats; a few are terrestrial and some are symbiotic in or on other invertebrates (Brusca and Brusca 2003). Until relatively recently, Turbellaria was considered to include the Acoela ("acoel flatworms") and Nemertodermatida, which together comprise a group known as Acoelomorpha. Based on molecular phylogenetic studies, however, the acoelomorphs are no longer thought to fall within the Platyhelminthes at all, but rather to be the sister group to all non-acoelomorph bilaterians (a group that includes most animal phyla) (Larsson and Jondelius 2008 and references therein). Even with the acoelomorphs removed, however, Turbellaria is no longer generally recognized as a valid grouping since Neodermata is nested within it, making it paraphyletic.

Most flatworms are conspicuously flattened dorsoventrally (i.e., from "back" to "belly"). The free-living tapeworms range from less than 1 mm to around 30 cm long, but some tapeworms may reach several meters in length (Brusca and Brusca 2003). 

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