Chp 6

• Platyhelminthes (Flat worms)


  • Live in marine, freshwater and humid terrestrial environments
  • Acoelomates
  • Ribbon-shaped, soft-bodied, flattened dorsoventrally
  • Most are parasitic, some are scavengers and carnivores
  • Those with a digestive cavity have an incomplete gut (only one opening: lack anus)
  • Excretory system - small tubules lined with ciliated flame cells (move water and waste into tubules and out of body)
  • No circulatory system – oxygen and food must diffuse to all cells
  • Some have nerve cords and simple central nervous system
  • Reproduction: most are hermaphroditic. Most have internal fertilization. Some with asexual reproduction
• Classes of Platyhelminthes


  • Tubellaria
  • Trematoda
  • Cestoda
• Class Turbellaria


  • Only free-living flatworms
  • Abundant in aqueous environments. Some occur in moist terrestrial areas
  • Use ciliated epithelial cells for movement
  • Have eyespots; usually move away from light
  • Have sensory pits or tentacles for detecting food, chemicals, and nearby movement
  • The digestive system may lack intestine. The mouth opens in the midventral part of the body. The intestine, when present, may be composed of one, two, three or many branches with or without diverticula (pouches)
  • Include Planaria
• Class Trematoda (flukes)


  • All parasitic mostly in vertebrates
  • Covered externally by a syncytial tegument which provides resistance to digestive enzymes and host immune responses. Beneath the tegument are consecutive circular, longitudinal and diagonal layers
  • Use mouth to feed. The mouth is anterior usually with sucker. Attach via suckers, anchors or hooks
  • They have usually a λ-shaped digestive system, with two main trunks having smaller branches
  • They have complex muscle layers. With excretory organs, and nervous system
  • Their food consists of the tissues or body fluid of the host, sucked by the muscular pharynx
  • Usually have two or more hosts (larvae almost always in snails, final host almost always a vertebrate)
  • Important pathogens of humans: Human liver fluke. Fasciola hepatica, Blood flukes of genus Schistosoma cause schistosomiasis
• Fasciola hepatica (sheep liver fluke)


  The common liver fluke of sheep inhabits the bile ducts and sometimes invades other organs, producing the disease known as liver rot
• Fasciola hepatica


  • Leaf-shaped body up to 30mm long, rounded anteriorly and bluntly pointed behind
  • The anterior sucker is terminal, surrounding the mouth and close behind is the ventral sucker, which serves for attachment in the host. Between them are the genital openings
  • The digestive system comprises the mouth, the muscular pharynx, short esophagus, and two branched intestine with many subdivisions throughout the body
  • The muscles are complex, and mesenchymal layer fills the spaces between the internal organs
  • The excretory system has many flame cells joined to one main canal opening in a single posterior bladder and posterior pore
  • The nervous system includes a double ganglion near the esophagus, 2 lengthwise nerve cords and various nerves and commissures
• The animal is monoecious. Copulation is mutual and cross fertilization is the general rule. During copulation, the cirrus or penis is inserted into the female genital opening of the other worm, and sperm is ejaculated.
• Life cycle of Fasciola hepatica

  1. Eggs emerge from the genital pore, pass through the bile duct and intestine of the sheep, and are excreted with feces

  2. In moist warm surroundings, they develop in 9 days or more; development is retarded at lower temperatures

  3. From each egg emerges a ciliated larva: the miracidium

  4. A snail may ingest an egg containing a miracidium or the hatched, ciliated free-swimming miracidium, or the larva may penetrate the snail's epidermis

  5. Inside the snail, the miracidium, which loses its cilia when it enters the host lymphatic vessels, begin a second developmental stage called a sporocyst

  6. Within the sporocyst are germ cells, each of which can develop by parthenogenesis (without fertilization) into another larval stage called the redia

  7. Each sporocyst produces 3 to 8 elongate saclike rediae, with a mouth and a short gut. Within 8 days, they burst from the sporocyst and migrate, usually to the liver

  8. Germinal cells within the redia again develop into larvae called cercaria. This latter possesses a digestive tract, the 2 suckers, and a tail

  9. The cercaria leave the host and are free-swimming using their tail

  10. After a few hours, they settle on grass blades or other vegetation near the water surfaces, loose the tail , and become metacercaria in tough enclosing cysts

  11. When infested vegetation bearing metcercaria are eaten by a sheep, the cysts are digested off and the larva burrow through the intestinal wall to the body cavity to reach the liver. They damage the liver tissue before entering the bile duct to mature and live for years
• Schistosoma (blood-flukes)


  • Parasitic flatworms responsible for a highly significant group of infections in humans termed schistosomiasis
  • Adult flatworms parasitize blood capillaries of either the mesenteries or plexus of the bladder, depending on the infecting species
  • Unique among trematodes and any other flatworms in that they are dioecious with distinct sexual dimorphism between male and female
  • Thousands of eggs are released and reach either the bladder or the intestine (according to the infecting species), and these are then excreted in urine or feces to fresh water
  • Larvae must then pass through an intermediate snail host, before the next larval stage of the parasite emerges that can infect a new mammalian host by directly penetrating the skin
• Class Cestoda (Tapeworms)


  • All parasitic; resistant to digestive enzymes and host immune responses
  • Absorb food through superficial microvilla or microtriches (no mouth, no digestive tract)
  • The body consists of three regions. 1) Scolex - attachment organ with several suckers and possibly also hooks. 2) Neck – unsegmented; connects scolex to proglottids and 3) proglottids - complete hermaphroditic units, making sperm and eggs
  • Human pathogen – Taenia solium
• Taenia Solium (Tapeworm)


  • The scolex is the anterior region of the worm. It is provided with four suckers on the sides and a circle of hooks on the elevated tip or rostellum. Suckers and hooks serve to attach the scolex to the intestinal wall of the host
  • Behind the scolex is a narrow neck region or budding zone, which joins the scolex to the body or strobila. The latter consists of a series of up to 1000 proglottids
  • The neck produces the proglottids by transverse constrictions. The youngest proglottid is thus at the anterior end, and an increase in size and maturity toward the posterior end where they become detached
  • The tapeworm tegument plays an important role in food absorption (No digestive system)
  • Each proglottid contains muscles, mesenchyme, section of excretory canals, many flame cells, and nerves
  • The muscle layer consists of usual circular and longitudinal layers, in addition to a secondary parenchymal musculature of longitudinal, transverse and dorsoventral fibers
  • An anterior nerve mass lies in the scolex, and 2 lateral, longitudinal cords extend posteriorly through the strobilia, in addition to one ventral and one dorsal pair
  • A protonephridial excretory system is present: flame cells and tubules in the mesenchyme drain into four collecting canals
• Reproductive system of Taenia Solium

  • A complete reproductive system occurs within each proglottid
  • Cross-fertilization is probably the rule where there are adjacent individuals in the host's gut, but fertilization occurs between two proglottids in the same strobila. Even self-fertilization in the same proglottid is known to occur
  • The male system matures first
• Life cycle of Taenia Solium


  1. The egg consists of a shell enclosing a zygote and yolk cells. Eggs are released with the rupture of the ripe proglottid, which may occur in the host's intestine or after they leave with feces

  2. If these eggs are eaten by a pig, the shells are digested off in the pig's intestine and the six-hooked larva (oncosphere) burrow into blood or lymph vessels, to be carried finally to muscles, where they encyst. The cyst enlarges, becomes filled with fluid, and is then called a bladder worm or cysticercus

  3. When raw or imperfectly cooked pork meat containing such bladder worm is consumed, the flesh is digested and the bladder worm becomes free, the scolex evaginates, attaches to the intestinal epithelium through the hooks and matures within 5-12 weeks. Specimens of T. solium can live for 25 years

  4. The infection by the parasite causes abdominal pain, diarrhea, headache and nausea. Humans may be infected by the eggs of this parasite