The phylum PlatyheIminthes is commonly known as the flatworms. Many of these organisms are animal parasites, and all of them are obliged to live in a moist environment because they cannot prevent themselves from drying out (desiccation). They have flattened bodies, hence their common name. They have a definite anterior end and a definite posterior end. The important classes belonging to this phylum are Trematoda and Cestoidea. The former are known as the flukes, and include the Liver Fluke and Bilharzia. The latter are the Tapeworms which are segmented flatworms.


Trematoda are the flukes. Some flukes are external parasites, while others are internal parasites, together these form two orders. The internal parasites belong to the order Monogenea. However, in this study only two examples from Digenea will be studied. Organisms in the order of Digenea have two hosts – one is a vertebrate host and the other is usually a mollusc. The vertebrate host is an animal with a back-bone, and the mollusc is a snail. In the vertebrate host the fluke will undergo sexual reproduction. In the snail the fluke undergoes asexual reproduction. The two examples to be studied in this lecture are:

  • The Liver Fluke
    • Bilharzia.

The Liver Fluke is a liver parasite of cattle and sheep. The adult fluke is about 1.3 cm wide and 2.5 cm long. The adult fluke is leaf-shaped and is very thin, like a leaf. Figure 1 shows an enlarged diagramme of an adult fluke. The adult is a hermaphrodite, i.e. it possesses both female and male sex organs. Despite this, however, a fluke will usually mate with another fluke. The adult possesses a primitive nervous system. At the posterior end is a small pore which is an excretory pore, where toxic waste is voided.

Figure 1: A Liver Fluke Adult (Fasciola hepatica)


The adult fluke lives in the bile duct of the animal host and produces about 20 000 eggs a day. The eggs pass with the bile into the digestive system of the host, ultimately passing out with the faeces. If the dung falls into a watery environment, a vlei or a pool of water the eggs and dung separate and after about nine days the eggs hatch.

Miracidia hatch from the eggs and these swim in search of the secondary host, the water snail (Lymnea natalensis). The complete life cycle of the liver fluke is shown in Figure 2.

Figure 2: The Life Cycle of the Liver Fluke (Fasciola hepatica)

The miracidia enter the snail through its skin. Once inside the snail’s blood, the outer skin is lost, and the miracidium becomes a Sporocyst. This Sporocyst goes to the snail’s liver or reproductive organs, where it develops many Redia inside itself and these are released. This Redia (many are produced from one Sporocyst) may follow one of two courses of development, namely:

  • Some may produce more Redia, this is however rare
  • Others produce 16 – 20 Cercaria each. These then leave the snail and enter the surrounding water. This Cercaria have tails to help them swim and they leave the snail from 4½ – 7 weeks after infection by miracidia. The Cercaria swims around, and eventually attach themselves to a blade of grass where they develop into a cyst-like phase known as a metacercaria. The latter are ingested by cattle or sheep and the outer case is dissolved away by the animals’ digestive juices, releasing a small immature fluke. The young flukes bore through the gut wall and migrate to the liver on which they feed, causing serious bleeding. Eventually, the flukes enter the bile duct and grow to maturity. Figure 2 gives the Life Cycle of the Fluke.

The liver fluke causes liver damage and the animal loses weight, their condition deteriorates and is generally debilitated.


Bilharzia is a severely debilitating tropical disease. Most adults have or will have the disease in Central Africa. The disease is caused by a parasitic flatworm of which there are two main species, the Schistosoma haematobia and the Schistosoma mansoni.

These are flukes but differ from Liver Fluke in that the males and females are different worms. Two types of diseases are produced: Bladder disease by s. haematobia and bowel disease by s. mansoni. The life cycle for both species is essentially the same and is as follows:


The adult worms may live up to 30 years in the veins of the urinary bladder or bowels. The female continuously lays eggs which are eventually deposited in the bladder or faeces. On contact with fresh water, the eggs hatch into Miracidia; these then enter a water snail, either as Physapsis spp, (S. haematobia) or Planorbis spp (S.Mansoni). The miracidea enter the snails liver where further development occurs. One miracidium will produce thousands of Cercaria which leave the snail and infect the human host. Cercaria can infect unbroken skin and enter the blood, shedding their tail in the process. They migrate in the blood stream via the heart and lungs to the liver, where they mature into adults. The sexes pair up and travel via the blood to the bladder or the bowels. The whole cycle is then repeated. Refer to Diagramme 3 below.

Figure 3: Bilharzia Life Cycle (Not to scale)


Tapeworms have long ribbon-like bodies which are segmented as can be seen in Figure 4 below. They are the main parasite of mammals. One host can usually support one adult tapeworm at a  time.

Figure 4: The Tape Worm


      Alimentary: relating to nourishment or sustenance.

Tapeworms consist of a head, known as the scolex. This is a small, spherical structure which possesses four suckers, and a ring of hooks below the Rostellum, which is used to anchor the parasite to the gut wall. The head has no mouth and the worm does not have an alimentary canal. The worm obtains its entire nutrient by absorbing them through its skin. The length of the tapeworm depends on how many segments it possesses. The segments contain a full complement of both male and female sex organs. The chain of segments behind the head

is called the Strobilla. Segments are continuously being added from the region immediately behind the head. The segments  at the end of the worm are mature and they become detached. A tapeworm segment is known as a Proglottid.


Taenia Solium: is commonly known as the pig tapeworm, as the pig is an intermediate host. Other species use cattle as

their intermediate hosts. The adult tapeworm parasitises man, and the life cycle can be regarded as beginning here (for convenience sake).

The mature segments, towards the end of the tapeworm, possess both male and female sex organs. The female ova can be fertilized by sperm from another segment or from the same segment. The fertilized ova then develop into eggs with yolk and resistant shells. Eventually all other organs in the segment degenerate, and it becomes a bag of tapeworm eggs. At this stage, it detaches from the worm and passes out in the faeces. The segment wall degenerates, releasing the eggs. The eggs are then ingested by the intermediate host, the pig. The digestive juices of the pig dissolve the egg shell and release an oncosphere. This then travels in the blood from the gut to the muscles of the animal, where it encysts to form a Bladder worm. The bladder worm cyst contains a small tapeworm head (Scolex). The pork is said to be measley, and the condition is thus known as Pork Measles. All meat is inspected for measles and as a result, people in urban areas are virtually tapeworm-free.

If the pork is not cooked properly and is eaten by man, the digestive juices of man dissolve away the cyst shell, and a small scolex is released. This then hooks onto the intestine and begins to produce a chain of segments behind it, and the whole cycle is repeated. See Figure 5 below. Tapeworms steal the host’s food and the host becomes lethargic and debilitated.

Figure 5: Life cycle of the Tapeworm