Phylum Platyheminthes

• • Platyhelminthes are flatworms
  • Some are free-moving active predators, others are parasitic
  • Class Turbellaria – Free-living flatworms that live in marine, freshwater or moist land environments
  • Class Trematoda are parasitic flukes
  • Class Cestoda are parasitic tapeworms
• Gland cells and Rhabdite cells form the epidermis

  o   Rhabdite cells are more specialised – the mucus produced from these cells are more viscous to trap and tangle prey

  o   Cilia helps to move along mucus

  o   Circular, parenchymal and longitudinal muscles are used for movement in large flatworms to produce undulation on their sides
• Flatworms have bilateral symmetry

  o   Animal has anterior and posterior ends

  o   Leads to cephalisation – concentration of brain, nerve cords and main sensory organs at the anterior end

  o   Flatworms have external and internal bilateral symmetry
• Concentration of nervous system at anterior end allows animal to monitor environment it is moving into

  o   Sensory input can be sent to the brain faster, therefore allowing animal to “see” changes in stimuli faster
• Flatworms move in a unidirectional forward manner which is intentional

  o   Move away from stimuli and predators

  o   Move towards prey
• Clustering of neurons to form brain and bundling of neurons to form nerve cord allows for interconnectivity of neurons

  o   More integration of signals to allow for more complex signals
• Flatworms have a thin and flattened body

  o   Increased surface area to volume ratio for increased rate of oxygen absorption from the environment

  o   Short diffusion distance for oxygen from surroundings to quickly diffuse into body cells
• How do flatworms feed?

  ·       Flatworms are attracted to food via chemotaxis (chemicals)

  ·       Flatworms extend their pharynx out

  ·       Food is ingested
• The pharynx (whiteish coloured structure in black and white images)

  • Middle of the ventral surface of the flatworm
  • Has an opening for the mouth and anus (flatworms have incomplete guts)
  • Protrusible structure is very muscular
  • Can extend out to catch food
  • Can engulf entire food items in one go
  • The pharynx can release enzymes from the opening
  • Enzymes produced by gland cells in GV cavity
  • Release enzymes to partially digest food first
  • Enzymes liquify the food, so pharynx takes up the food like a suction pump and pumps food into the gut
• Gastrovascular cavity

  • Acts as the circulatory system to transport digested nutrients around body
  • Flatworms don’t have a separate circulatory system
  • Extensively branched
  • Increase surface area for secretion of enzymes and absorption of digested nutrients
  • Decrease diffusion distance of nutrients into body cells
• Trematodes and cestodes are endoparasites of vertebrates
• Parasites have suckers on their head

  o   To securely attach itself onto host inner surface

  o   Prevent itself from being dislodged due to peristalsis or food moving through

  Some parasites have crown on hooks on their heads for similar purposes
• Parasites don’t contain a gut or mouth to conserve energy for other life processes (e.g., reproduction)

  • Host already digesting the food for the tapeworm by secreting enzymes to break down the food
  • Tapeworm can just absorb the digested nutrients from the host
  • Long body increases surface area to shorten diffusion distance for nutrient absorption
• Body of an adult tapeworm is divided into

  • Scolex
  • Narrow neck (growth zone with stem cells)
  • Strobila (segmented trunk)
• Narrow neck (growth zone with stem cells)

  • Connects scolex to strobila
  • Budding to form new segments to grow longer and to make up for continuous loss of proglottids at the posterior end
• Strobila (segmented trunk)

  • Each segment is called a proglottid which is produced by the neck region
  • Proglottids are continuously formed
  • New proglottid segments are progressively pushed back to posterior end where they mature
  • They become mature as they are displaced posteriorly
• Adult tapeworms

  Hermaphrodites – both sets of reproductive organs are present in each individual
• Tapeworms

  • Considered “ecological islands”
  • Mostly 1 tapeworm in 1 host
  • Important to carry out self-fertilisation (NOT asexual reproduction)
  • Self-fertilisation produces gametes which allows for genetic variation
  • Cross-fertilisation is also possible if another tapeworm is present
• Advantages of being hermaphrodites

  • Advantage 1: allows for doubling of eggs fertilised and offspring produced if another tapeworm is present as sperm transfer can occur both ways and eggs in both tapeworms can be fertilised
  • Advantage 2: Don’t need to worry about gender of the tapeworm when mating (if they find another tapeworm)
• Gravid proglottids are obtained after fertilisation

  o   The sex organs have been lost to make more space for the greatly branched and expanded uterus filled with fertilised eggs (can contained up to 10,000 eggs)
• The life cycle of the tapeworm involves 2 or more hosts - definitive host and intermediate host
• Intermediate host (e.g., cattle)

  • Ecology and behaviour of intermediate host increases chances of possible contacting and infecting of final host
  • Big role in transferring parasite to final host
  • It is a vector of disposal to increase infection rates and allow dispersal of parasite
• Definitive host (final host)

  • Provides proteins, shelter and other possible nutrients for tapeworm
  • Provides favourable environment for tapeworm
  • Tapeworm is sexually active and reproduces in definitive host due to high quantities and varieties of nutrients found in definitive host
• What happens when there are too many tapeworms in intestine

  o   Possible to block lumen of intestine so food cannot pass through

  o   Possible injury of intestinal walls by hooks causing inflammation and infection
• Cysticercosis can be caused by pork tapeworms
• Humans become the intermediate host


  Consuming food or water polluting by faeces containing tapeworm eggs
• Tapeworms enter human muscle


  Not receiving correct chemical cues and losing their way
• Tapeworms enter cardiac muscles


  Affect electrical signal conduction and affect cardiac muscle function over time
• Tapeworms enter aqueous humor of the eye


  Cysts can form in muscles behind the eyes or brain
• Cysts


  Evolve to evade and suppress immune system
• Immune system attacks cysts

  If tissue at the brain gets inflamed, it can be fatal
• Pork tapeworms can cause neurocysticercosis leading to seizures and eventual death
• Life cycle of tapeworms

  1. Fertilised eggs are released from gravid proglottid in faeces of host into environment

  2. Gravid proglottid degenerates and eggs enter environment

  3. Eggs are ingested by cattle during feeding and hatch inside the intestine of the cattle

  4. Larvae come out in intestine and burrow (using mechanical and chemical means) through the intestine wall into bloodstream then skeletal muscles of intermediate host

  5. Eggs are embedded into muscles to form cysts (known by chemical cues)

  6. Human eats raw uncooked beef with eggs in it

  7. Eggs go through the acid in stomach and alkaline in small intestine

  8. Protective barrier is present to resist acidic conditions

  9. If the larva accidentally hatches in the stomach, it will instantly die

  10. Acid and alkaline conditions are chemical cues for larva to evert and evaginate