Phylum Platyhelminthes (Flatworms)

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Polyclad flatworm, from Halstead, 1965. The phylum Platyhelminthes (flatworms ) traditionally includes four classes, the Trematoda and Monogenea (both commonly called flukes ), the Cestoda (tapeworms ), and the Turbellaria . Recent cladistic analyses retain these groups, but they are unequal in rank. The first three groups are parasitic , and are often found as parasites of various marine organisms (as well as many freshwater and terrestrial organisms). There are approximately 4,500 species of Turbellarians , about 9,000 species of flukes , and 5,000 species of Cestodes . The Turbellaria are free-living flatworms, and are probably a polyphyletic taxon. The free-lving flatworms can frequently be found under rocks or in seaweed in the intertidal and shallow water zones. A few species are pelagic , and many of the smaller forms live within sand and mud substrates. They range in size from meiofaunal forms to individuals over 60 cm. long (the largest tend to be terrestrial forms), however, most species are only about a centimeter in length. Some species are brightly colored, others tend to be a drab brown or similarly muted color. Usually carnivorous , a wide range of living and dead organisms (including protozoa , small crustaceans , annelids , molluscs , and even barnacles and oysters ) are preyed upon by various species of Turbellarians . Some species feed on algae (especially diatoms ), either as adults or when young. Three of the body plans of some comon forms of Turbellarians (there are several other groups not treated here): A. order Rhabdocoela B. order Polycladida C. order Tricladida For more information on sponges, follow these links Platyhelminthes Basics (Univ. Calif. Berkeley) Platyhelminthes basics (Univ. Michigan) Platyhelminthes basics (Iowa State univ.) Systematics (Tree of Life) Systematics and ecology (Univ. Michigan) Systematics (Univ. Alberta, Canada) Diagrams of Platyhelminths (Univ. Michigan) Pictures of Platyhelminths (Univ. Michigan) For more information beyond this site, search the web! Suggestions for improving, changing, or adding to this website and etextbook should be emailed to: References: Brusca, R., and G. Brusca. 2003. Invertebrates. Second edition. Sinauer Assoc. Inc. Sunderland, MA, USA. Castro, P., and M.E. Huber. 2005. Marine Biology. 5th edition. McGraw-Hill, New York. N.Y. Meglitsch, P.A., and F.R. Schram. 1991. Invertebrate Zoology. Third edition. Oxford University press, New York., Oxford. 623 pp. Pechenik, J.A. 2005. Biology of the Invertebrates. Fifth edition. McGraw Hill, 590 pp. Ruppert, E.E., R.S. Fox, and R.D. Barnes. 2004. Invertebrate Zoology, A Functional Evolutionary Approach. Seventh edition. Brooks/Cole, 961 pp.

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