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Overview

Brief Summary

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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There are a total of around 20,000 species of flatworms, some of which live in the sea, others in freshwater and some of which are parasitic. As the name suggests, flatworms are flat and relatively broad. They are primitive animals, but have a definite head and tail and a developed nervous system. The body and reproduction deviate so much from other worms that they have been categorized separately. The larger species glide through the water just like snails. Despite the fact that they are primitive, these animals are capable of learning. For example, they reacted to a lamp in experiments and can also learn to react to it always in the same way.
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Members of the Phylum Platyhelminthes are typically called flatworms because of their flattened bodies. These animals lack appendages, and their bodies can take many forms, such as slender, leaflike, or ribbon-like. This diverse group of bilateral invertebrates ranges widely in size from 1mm or less to tapeworms that grow to many meters in length. There are both free-living and parasitic flatworms, and species are found in both freshwater and marine environments.

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Ecology

Associations

Known predators

Platyhelminthes (fluke) is prey of:
Anatidae
Phalacrocoracidae

Based on studies in:
USA: New York, Long Island (Marine)

This list may not be complete but is based on published studies.
  • G. M. Woodwell, Toxic substances and ecological cycles, Sci. Am. 216(3):24-31, from pp. 26-27 (March 1967).
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Known prey organisms

Platyhelminthes (fluke) preys on:
Plantae

Based on studies in:
USA: New York, Long Island (Marine)

This list may not be complete but is based on published studies.
  • G. M. Woodwell, Toxic substances and ecological cycles, Sci. Am. 216(3):24-31, from pp. 26-27 (March 1967).
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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
Specimen Records: 10833
Specimens with Sequences: 7846
Specimens with Barcodes: 3909
Species: 669
Species With Barcodes: 532
Public Records: 4073
Public Species: 398
Public BINs: 453
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Barcode data

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