The flatworms (phylum Platyhelminthes) are divied into three classes: the non-parasitic faltworms (Turbellaria), the flukes (Turbellaria) and the tapeworms (Cestoda). Only the tubellarians are free-living; the other two are exclusively parasitic. As their common name suggests, flatworms are flattened, soft-bodied organisms. Most flukes are microscopic, but intestinal tapeworms may grow up to 65 feet (20 meters).

General Features

Compared with the simpler true jellies, sea anemones, sea pens, and their allies, flatworms show several advanced characteristics: 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 chemoreceptors. There is also a concentration of nerve cells at the front end of the body which forms a primitive brain, in addition to the diffuse nerve net similar to that of cnidarians.

The flatworm's body is composed of 3 layers of cells, whereas in coelenterates there are only two. Between the epidermis and gastrodermis (which lines the digestive cavity) there is layer of mesodermal cells. Oxygen reaches this middle cellular layer by diffusion because these animals ae so highly flattened that a specialized breathing system is not required. The mesoderm contains a complicated reproductive organs which are made up of different types of cells. This differentiation represents a higher level of organization than that found in coelenterates, which possess tissues but not organs.


The turbellarians include all free-living flatworms. Some species grow up to 25 inches, but most are less than 2 inches. Most freshwater species are drab and inconspicuous, whereas tropical marine species may be very colorful. Turbellarians do not swim freely, although they live in water, but creep along the bottom. The epidermis is equipped with minute, hair-like cilia and glandular cells which secrete mucus in which the cilia beat, enabling the flatworm to glide along. They also have bands of muscles which allow complex bending movements of he body.

Photo courtesy of Glenn and Martha Vargas © California Academy of Sciences)

Most turbellarians are carnivorous, feeding on small animals, or necrophagous (feeding on dead animals). Many secrete mucus and an adhesive to entangle their prey, before using the protrudible pharynx to break it up into small particles. In some cases the prey in ingested whole.

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Compared with parasitic flatworms, turbellarians have a simple life cycle. Most species are hermaphroditic, with male and female sex ogans in the same individual. Self-fertilization does not usually occur, however, and a partner is needed for fertilization to take place. Sperm is exchanged between the two individuals and both lay fertilized eggs in egg capsules or gelatinous masses. After 2 or 3 weeks, young flatworms resembling their parents hatch from the eggs. Some turbellarians also reproduce asexually.


These flatworms occur as intestinal parasites (order Digenea) and external parasites (order Monogenea). They 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. Another feature found in flukes and typicl of most parasites is a high reproductivity ability, which ensures that at least some offspring reach suitable hosts. Not only do the adult flukes of internal parasites produce large number of eggs, but the larvae of these flukes alsol reproduce within thir hosts.

External parasites are often found in the gill cavities of fish where they have to withstand strong water currents. They feed on blood and gill tissue. Internally parasitic flukes may be found in many vertebrates as well as in other invertebrates. The liver fluke (Fasciola hepatica) is a well-known parasite found in the bile passage of the liver of sheep and cattle where it inflicts fatal damage. This fluke has a complex life cycle which involves several hosts. The eggs leave the vertebrate host in the feces and hatch in winter into free-swimming larvae (miracidia). These larvae swim until they encounter a snail, an intermediate host, into which they burrow. Inside this hosts the larvae multiply and eventually leave the snail as another larval form, cercaria. Up to 600 cercariae may be produced from a single miracidium. These then encyst on grass where they are eaten by a vertebrate host, in which they develop into sexually mature flukes.


The adults of this parasitic group are found as internal parasites in the alimentary canal of vertebrates. Like the flukes, tapeworms have no obvious sense organs and their complex life cycle usually involves 2 more hosts. The beef tapeworm (Taenia saginata), for example, has 2 hosts, cattle and man. Tapeworms are a big part of dog waste pollution problem.

Tapeworms have no digestive system, they simply absorb nutrients in the gut of their hosts through their cuticle. These worms consist of a head, or scolex, from which segments or proglottids bud off. The head usually bears hooks and suckers for attachment to the gut lining of the host. The proglottids remain attached in a long chain as they mature, finally breaking off and leaving the host in the feces. The eggs hatch from the proglottids once they have been eaten by the intermediate host.

Host-Parasite Interaction

Helminth parasites generally establish long-term infections in their host, driving physiological and immunological changes that best accomodates the invader. They manipulate, inhibit or activate different host cells or pathways in order to maximise parasite success. The strategy of helminths is not to outpace the immune system through rapid multiplication or antigenic variation, but to manipulate immunity in order to defuse immune defenses, meaning the host fails to eliminate the parasites. Helminths essentially take hold by stealth, first inactivating host detection systems that would otherwise raise the alarm, and then effectively making the immune system "politically correct" to parasite antigens, and in doing so, preventing bystander antigens from attacking the invaders.

Helminth parasites produce compounds that enter host cells and cause profound biological effects. For this they use different mechanisms and cell structures. For example, Schistosoma mansoni secrete proteins that effectively block host cell protein synthesis. In other cases (Echinostoma caproni and Fasciola hepatica), the parasites exploit exosomes or extracellular vesicles that play a key role in cellular communication. They contain RNas and proteins. It has been discovered that parasitic helminths also produce exosomes which can be incorporated by the host cells into their cell environment.1


  1. Host parasite communications—Messages from helminths for the immune system. Gillian Coakley,a Amy H. Buck,a and Rick M. Maizelsb,⁎ Elsevier Sponsored Documents

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