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Flatworms
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The phylum Platyhelminthes (flatworms) are generally dorsally-ventrally flattened and elongate animals. These worms are generally simple and the digestive cavity, when present, is a single cavity with a single opening serving as both mouth and anus. Respiratory and vascular systems are absent, although a simple excretory system is present. Most of the flatworms are parasitic, although the Turbellaria are free-living in fresh water. The Platyhelminthes are divided into three classes: Turbellaria: The Turbellaria are free-living, freshwater flatworms commonly called planaria. They range is size from microscopic to several inches in length. Trematoda: The Trematoda consistute the flukes. The flukes are parasitic flatworms. Flukes characteristically possess suckers, usually two or more, used to attach themselves to their host. Much of a fluke's body cavity is occupied by reproductive organs. Flukes may be either external or internal feeders, and some fluke groups have extremely complicated life histories. Cestoda: The Cestoda are made up of the tapeworms. Tapeworms are internal parasites of vertebrates. The body is composed of a small head and neck, followed by a large number of segments (proglottids). Each segment is a reproductive sack and as it ripens it breaks off and pass to the outside of the host in its feces. To find out more about the Platyhelminthes, visit the links listed below.
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