FLATWORM

also called Platyhelminth, any of a phylum (Platyhelminthes) of soft-bodied, usually much-flattened worms, including both free-living and parasitic species. Flatworms occur in a variety of marine, freshwater, and terrestrial habitats and are widely distributed throughout the world. Flatworms, which range in length from a fraction of a millimetre to 15 m (50 feet), are of three main types: turbellarians, which include the planarian (see planarian), trematodes (see fluke), and cestodes (see tapeworm). Flatworms are bilaterally symmetrical and are usually flattened in appearance; they lack respiratory, skeletal, and circulatory systems and a body cavity (coelom). Spaces between the organs of the nonsegmented body are filled with solid connective tissue (mesenchyme). Flatworms of the class Turbellaria are mostly free-swimming; some live in or on a host, usually in an aquatic environment. Many are broad and leaflike, but some are cylindrical. All are covered with cilia (tiny hairlike structures) that are in constant motion. All trematodes are parasitic. Fish are the most common hosts, but all vertebrate classes are parasitized by them. About 35 species are known to parasitize humans. Some have one host during their lifetime, others two or more. All cestodes are internal parasites. They have no mouth or gut, and food is absorbed through the body wall. also called platyhelminth any of the phylum Platyhelminthes, a group of soft-bodied, usually much flattened invertebrates. The flatworms are free-living as well as parasitici.e., living on or in another organism and securing nourishment from it. They are bilaterally symmetrical (i.e., the right and left sides are similar) and lack respiratory, skeletal, and circulatory systems; no body cavity (coelom) is present. The body is not divided into true segments; spongy connective tissue (mesenchyme) constitutes the so-called parenchyma and fills the space between organs. Flatworms, generally hermaphroditicfunctional reproductive organs of both sexes occurring in one individualare the lowest invertebrates to possess three embryonic layersendoderm, mesoderm, and ectodermand to have a head region that contains concentrated sense organs and nervous tissue (brain). The phylum consists of five classes: Trematoda (flukes), Cestoda (tapeworms), Turbellaria (planarians), Monogenea, and Aspidocotylea (or Aspidobothria). Members of all classes except Turbellaria are parasitic during all or part of the life cycle. Most turbellarians are exclusively free-living forms. Approximately 13,000 species of flatworms have been described. Additional reading Classic studies of platyhelminths are Libbie Henrietta Hyman, The Invertebrates, vol. 2, Platyhelminthes and Rhynchocoela (1951); S. Yamaguti, Systema Helminthum (1958 ), taxonomic works with detailed keysvol. 1 was revised as Synopsis of Digenetic Trematodes of Vertebrates (1971); and Plathelminthes, msozoaires, acanthocphales, nmertiens, in P.P. Grasse (ed.), Trait de zoologie, vol. 4 (1961). Nathan W. Riser and M. Patricia Morse (eds.), Biology of the Turbellaria (1974), provides more current information. Collections of papers examining various aspects of turbellarians are found in Ernest R. Schockaert and Ian R. Ball (eds.), The Biology of the Turbellaria (1981); and Peter Ax, Ulrich Ehlers, and Beate Sopott-Ehlers (eds.), Free-Living and Symbiotic Plathelminthes (1988). Ben Dawes, The Trematoda (1946, reissued 1968), is a classic work but somewhat out-of-date; it may be updated by two newer works, David A. Erasmus, The Biology of Trematodes (1972); and J.D. Smyth and D.W. Halton, The Physiology of Trematodes, 2nd ed. (1983), a detailed monograph for students. Also useful is G.R. La Rue, The Classification of Digenetic Trematoda: A Review and a New System, Experimental Parasitology, 6:306344 (1957). Cestodes are examined in Robert A. Wardle and James Archie McLeod, The Zoology of Tapeworms (1952, reissued 1968), a classic work, largely taxonomic, but somewhat out-of-date; J.D. Smyth, The Physiology of Cestodes (1969), a detailed study for students; Robert A. Wardle, James Archie McLeod, and Sydney Radinovsky, Advances in the Zoology of Tapeworms: 19501970 (1974); and J.D. Smyth and D.P. McManus, The Physiology and Biochemistry of Cestodes (1989). Guides to identification include Ian R. Ball and T.B. Reynoldson, British Planarians, Platyhelminthes, Tricladida (1981); and Gerald D. Schmidt, CRC Handbook of Tapeworm Identification (1986). James Desmond Smyth The Editors of the Encyclopdia Britannica