Platyhelminthes

• Phylum Platyhelminthes


  • Acoelomate with organ-system level of organization
  • Simplest animals with Bilateral Symmetry
  • Triploblastic
  • Cephalization: with head region, eye spots, and nerve cells = response to environment
  • Body flattened dorsoventrally in most: ribbon-like body to glide easily through narrow spaces
  • Oral and genital apertures in most oral and genital surface: found on the ventral surface
  • Muscular system of mesodermal origin
  • Digestive system is incomplete, absent in some: Mouth fairings and intestine
• Parasitic flatworms


  • Evolved with specialized structures to attach to host and evade their immune system: Hooks, Suckers
  • Complex life cycles with multiple hosts
• Nervous system


  • Pair of anterior ganglia (or cluster of nerve cells) with 2 longitudinal cords
  • Simple sense organs
• Excretory system


  • Flame cells: elimination of of metabolic waste (ammonia) through flame cells (ciliated cells) to expel waste from the body
• Lacks respiratory and circulatory systems
• Regenerative ability


  • They can regenerate lost body parts including the head and the tail
• Reproduction


  • Most are monoecious or hermaphroditic: can produce both eggs and sperm in an individual
  • Asexual reproduction - fragmentation or fission; regeneration once broken into two pieces
• Incomplete Digestive System


  Intracellular and extracellular digestion
• Absorption of nutrients


  Through the tegument (specialized outer covering) in tapeworms
• Tubellarians digestive system


  Pharynx opens posteriorly and can extend throughout the mouth
• Flukes digestive system


  Mouth is usually at the anterior end and the pharynx is not protrusible
• Excretory and osmoregulatory system

  • Canals with tubules that end in flame cells
  • Negative pressure from flagella draws fluid to the lumen
  • Lumen continues to collecting duct and opens to the outside by pores
  • Microvilla in duct walls could function for reabsorption
  • Metabolic wastes removed largely via diffusion through the body wall
• Reproduction


  • Asexual - through fission or outgrowth of new individuals
  • Sexual - external fertilization; release of gametes into the environment
  • Some have internal fertilization
  • Copulatory organs - specialized structure to transfer sperm
• Life cycle


  • Complex and involve multiple hosts
  • Tapeworms require both intermediate and definitive hosts
• Larval forms


  • May differ morphologically and ecologically
  • No parental care
  • Some species protect eggs from predators
• Regeneration


  • Neoblast - specialized pluripotent stem cells responsible for regeneration
  • Can regenerate head, tails, and internal organs
  • Small fragments can regenerate into a complete organism
  • Complex signaling pathways and cell interactions
• Class Turbellaria


  • Mostly free-living; marine, freshwater, and terrestrial
  • Flattened ribbon-like body with head and tail regions
  • Cilia on underside for gliding and swimming
  • Incomplete digestive system: mouth, pharynx, and intestine
  • Flame cells for osmoregulation
  • Nervous system with ganglia
  • Specialized sensory structures: Eyespots and auricles
  • Hermaphroditic or monoacious
  • Some have asexual fission
  • Ability to regenerate lost body parts
  • Size range 5 mm - 50 cm
  • Locomotion via muscular and ciliary movements
• Class Trematoda

  • All parasitic flukes
  • Almost all found as endoparasites of vertebrates
  • Life cycle involve mollusk as a primary host
  • Structural adaptations: Penetration glands, Suckers and hooks, Flattened leaf shape body, Tegument (specialized outer covering)
  • Reduced digestive system: mouth, pharynx, and branch intestine
  • Monoecious, except for Schistosoma species
  • Require two or more hosts to complete life cycle
  • Adult trematodes live in digestive system of host and release eggs in feces
• Subclass Digenea


  • Shelled embryo passes to water, hatches to Miracidium, penetrates snail to become Sporocyst, reproduces asexually to Rediae, which produce Cercariae that penetrate intermediate host to become Metacercariae/Juvenile Flukes, then eaten by definitive host to grow to adult
• Schistosoma japonicum


  • Eggs eliminated in feces/urine, hatch to Miracidium that penetrates snail, develops to Cercariae that penetrate human skin, develop to Schistosomulae that migrate to liver, mature and reside in mesenteric venules
• Fasciola hepatica


  • Immature eggs discharged in feces, embryonate in freshwater, release Miracidia that invade snail, develop through sporocyst, rediae and cercariae stages, release Metacercariae that are ingested by humans, penetrate intestine, migrate through liver to mature in biliary ducts
• Class Monogenea


  • Mostly ectoparasites on fish gills and skin
  • Flattened elongated body with distinct head and tail
  • Specialized attachment structure (Opisthaptor) with hooks and suckers
  • Simple digestive system: mouth and intestine
  • Monoecious or hermaphroditic, can switch between sexual and asexual
  • Direct life cycle - eggs hatch to free-swimming larvae that infect host directly
• Class Cestoda


  • Tapeworms
  • Long flat body with scolex (hooks and suckers) for attachment, followed by many reproductive proglottids
  • Nearly all monoecious, produce many eggs in mature proglottids
  • No digestive system, absorb nutrients directly through body surface
  • Complex life cycle with intermediate and definitive hosts
  • High host specificity