The phylum name Platyhelminthes literally means "flatworms." Members of this phylum are soft, thin-bodied, leaf or ribbonlike worms, including the familiar planaria of ponds and streams, as well as the flukes and tapeworms parasitic in human and other animal bodies. Some defining characteristics of the phylum are that flatworms are acoelomate (they have no body cavity), triploblastic (the body has three tissue layers), and bilaterally symmetric (they have symmetric right and left sides and usually a definite head), and they have organ systems, including an excretory, digestive, reproductive, and nervous system, but no respiratory system.

The class Turbellaria includes all free-living members of the phylum, as well as a few parasites . It includes many marine forms, whose beautiful colors serve as a warning of their toxicity to would-be predators, as well as the more drab freshwater planarians ( Dugesia ). Some Turbellaria can swim by undulations of the body margins, but most of them glide gracefully over surfaces along a trail of mucus, pushed by cilia on their ventral surface.

The class Trematoda, commonly called flukes, are unsegmented parasitic flatworms that usually parasitize a snail as an intermediate host (in which they reproduce asexually) and a human or other vertebrate as a definitive host (in which the worms mate and lay eggs). Many species have other hosts between these two, such as fish or frogs. Trematodes usually have a pair of suckers for crawling and clinging to the host's tissues. Many humans are infected with blood flukes, liver flukes, lung flukes, and other trematode parasites of great medical importance.

The Cestoda, commonly called tapeworms, are segmented, ribbonlike parasites usually found as adults in the small intestines of vertebrate animals. Unlike the other classes, they have no digestive tract, for they can absorb predigested nutrients from the host's intestine. The body consists of a long chain of segments, each with its own reproductive system. The anterior end is a knoblike holdfast called a scolex, equipped with suckers and often hooks for attachment to the host's intestine. In general, tapeworm infections are not as medically serious as trematode infections, but some tapeworms can be lethal.

SEE ALSO Animalia ; Body Cavities ; Nematode ; Parasitic Diseases ; Symbiosis

Kenneth S. Saladin


Pechenik, Jan A. Biology of the Invertebrates, 4th ed. Boston: McGraw-Hill, 2000.

Schmidt, Gerald D., and Larry S. Roberts. Foundations of Parasitology, 6th ed. Dubuque, IA: McGraw-Hill Higher Education, 2000.


U.S. zoologist famous for her authoritative six-volume treatise on invertebrates, whose own specialty was hydras and flatworms. Hyman went to college despite her family's objections. In her last years, she lived off the royalties from her textbooks and worked at the American Museum of Natural History.

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