The phylum Platyhelminthes (flatworms) is divided into four classes: the Turbellaria, or non parasitic flatworms; the Monogenea, or parasitic flatworms; the Trematoda, or flukes; and the Cestoda, or tapeworms. As their common name suggests, platyhelminthes are flattened, soft-bodied organisms. Mostturbellarians are free-living. The members of the other three classes are exclusively parasitic. Most flukes are microscopic. Intestinal tapeworms, on the other hand, may grow up to 100 feet (30 meters) long.
General features
Compared with the simpler cnidarians, flatworms show several advanced characteristics. For example, the body is bilaterally symmetrical and has a definite head end. Free-living flatworms are active animals, and in many species the head carries pairs of eyes, as well as organs that sense chemicals. There is also a concentration of nerve cells at the front end of the body that forms a primitive brain, in addition to a nerve net similar to that of the cnidarians.
Turbellarians
The turbellarians include all free-living flatworms. Some species grow up to 25 inches (60 centimeters) long, but most are about one-half inch (13 millimeters) long. Most freshwater species are drab and inconspicuous, whereas tropical marine species may be very colorful.
Flukes
These flatworms occur as parasites that live inside other organisms (order Digenea) and parasites that live outside their hosts (class Mono-genea). Flukes generally have leaf-shaped bodies with suckers or, rarely, hooks to attach to their hosts. As a result of their parasitic habits, adult flukes lack the sense organs and layer of cilia that are found in turbellarians.
Tapeworms
The adults of this parasitic group are found as internal parasites in the digestive tract of vertebrates. Like the flukes, tapeworms have no obvious sense organs, and their complex life cycle usually involves two more hosts. The beef tapeworm, for example, has two hosts— cattle and man.
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