Image from Teichlebewesen.

The Phylum Platyhelminthes is dominated by the parasitic flukes (Monogenea, 3000 spp. and Trematoda, 8000 spp.) and tapeworms (Cestoda, 5000 spp.). However, there are roughly 3000 species of free-living flatworms of the grade “Turbellaria.” Traditionally, turbellarians had been regarded as a Linnaean class of this phylum, but since they have been recognized as paraphyletic, the classification of free-living flatworms has become very complicated.

Of the free-living flatworms, ca. 1300 species from a dozen or so different orders are known from fresh waters, and more than 200 spp. are known from North America (Schockaert et al., 2008).

While the parasitic flatworms are generally highly modified with adaptations for their selfishly symbiotic life-style, the turbellarians exhibit the ancestral morphological conditions of the phylum. While flatworms retain an incomplete gut, similar to the gastrovascular cavity of cnidarians, they have three embryonic tissue layers, bilateral symmetry, and all the rights and privileges associated with cephalization. These include a central nervous system with an anterior concentration of ganglia (“a brain”) and the rudiments of typical protostome “ladder-like” nerve cords with lateral connectives. Common species belonging to the Order Triclada are large enough to be measured in mm or cm, but the majority of species are in the μm to mm range.

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Bibliography

• Artois, T., E. Schockaert & S. Tyler. 2013. World Checklist of Freshwater Turbellaria. World Wide Web electronic publication. 59 pp. [Available online]
• Zhang, Z.-Q. 2011. Animal biodiversity: An introduction to higher-level classification and taxonomic richness. Zootaxa 3148: 7-12. [Available online]
• Kolasa, J. & S. Tyler. 2010. Flatworms: Turbellarians and Nemerteans. [in] J.H. Thorp & A.P. Covich (eds.). Ecology and Classification of North American Freshwater Invertebrates, 3rd edition: 143-161.
• Watermolen, D.J. & P. Fojut. 2008. An introduced flatworm, Bipalium adventitum (Tricladida: Terricola), in Wisconsin and its potential impacts. WI DNR Miscellaneous Publications PUB-SS-1041. 18 pp. [Available online]
• Schockaert, E.R., M. Hooge, R. Sluys, S. Schilling, S. Tyler & T. Artois. 2008. Global diversity of free living flatworms (Platyhelminthes, “Turbellaria”) in freshwater. Hydrobiologia 595: 41-48. [Available online]
• Watermolen, D.J. 2005. Aquatic and terrestrial flatworm (Platyhelminthes, Turbellaria) and ribbon worm (Nemertea) records from Wisconsin. Research/Management Findings, Wisconsin DNR (55). 8 pp.
• Brusca, R.C. & G.J. Brusca. 2003. Invertebrates, 2nd edition. Sinauer Associates, Inc., Sunderland, Massachusetts. 936 pp.
• Smith, D.G. 2001. Platyhelminthes (free-living flatworms). Pennak's Freshwater Invertebrates of the United States, 4th edition: 77-108.
• Underhill, J.C., W.D. Schmid & D.E. Gilbertson. 1982. Taxonomic Keys to the Common Animals of the North Central States, exclusive of the parasitic worms, terrestrial insects, and birds, 4th edition. 205 pp.
• Kenk, R. 1972. Freshwater planarians (Turbellaria) of North America. U.S. Environmental Protection Agency, Biota of Freshwater Ecosystems Identification Manual 1. 81 pp. [Available online]
• Hyman, L.H. & E.R. Jones. 1959. Turbellaria. [in] W.T. Edmondson (ed.). Ward & Whipple Fresh-Water Biology, 2nd edition: 323-365.
• Ferguson, F.F. 1954. Monograph of the macrostomine worms of Turbellaria. Transactions of the American Microscopical Society 73(2): 137-164. [Available online]
• Pennak, R.W. 1940. Ecology of the microscopic Metazoa inhabiting the sandy beaches of some Wisconsin lakes. Ecological Monographs 10(4): 537-615. [Available online]

Student Essays

• "Biological Control of Mosquitos Through the Implementation of Flatworms" by Collin Rogers (BIOL/WATR 361, Spring 2015 )

Web Pages

• Animal Diversity Web
• Freshwater Animal Diversity Assessment
• Integrated Taxonomic Information System
• Systema Naturae 2000
• Tree of Life
• WikiSpecies
• Wikipedia
• World Register of Marine Species