Platyhelminthes

• They are Acoelomates because they lack body cavity (Coelom)
• This gives them a flat body structure (Flat worms)

  
• Flatworm Body Plans


  • They exhibit bilateral symmetry, acoelomate, and are triploblastic
  • They have Parenchyma tissue - made up of cells and intercellular spaces that fills the interior of their body in which several types of cells are lodged in their extracellular matrices

  
• CHARACTERISTICS


  • THEY EXHIBIT BILATERAL SYMMETRY
  • THEY HAVE THREE GERM LAYERS (TRIPLOBLASTIC)
  • THEY HAVE CENTRALIZED NERVOUS SYSTEM
  • SIMPLE EXCRETORY SYSTEM (Protonephridia)
  • COMPLEX REPRODUCTIVE SYSTEM - Most have more than one hosts
  • SIMPLE DIGESTIVE SYSTEM
  • MOSTLY PARASITIC (80% are parasitic)
  • RESPIRATION BY DIFFUSION - Flatness allows diffusion for gas exchange
• REPRODUCTION


  • Almost all are simultaneous hermaphrodites
  • Parasitic species (flukes and tapeworms) have complex life cycles, with various hosts and different larval stages
  • They have incredible powers of regeneration
• NERVOUS SYSTEM


  • Free-living species usually have a well-developed sensory system
  • Parasites generally have less elaborate systems because they rely on the host for many of their resources
  • They exhibit cephalization
• CEPHALIZATION


  The concentration of sense organs, nervous control, etc., at the anterior end of the body, forming a head and brain
• Flatworms are the most primitive animals to show Cephalization

  
• The anterior end of the animal become most likely to first encounter food, predator, and other features of the external environment

  
• Cephalization enhances complexity of animals
• CLASSIFICATIONS


  • Class Turbellaria (Flatworms)
  • Class Monogenea
  • Class Digenea (Flukes)
  • Class Cestoda (Tapeworms)

  
• Class Turbellaria


  • Most are free–living, tiny and inconspicuous
  • Primarily marine and freshwater
  • They crawl on substrate or swim using cilia
  • e.g. Planaria
• Digestion and Excretion


  1. Mouth is usually located at the anterior end or mid-body on ventral surface

  2. Excretory system contains flame bulbs known as flame cell

  
• Many species posses remarkable powers of regeneration and repair wounds
• Turbellarian Lifestyles: Most are benthic, Possess several nerve cords with a centralized ganglia (brain)
• Reproduction


  1. Reproduction occurs with the reciprocal exchange of sperm

  2. Fertilized eggs are released and usually develop directly into flatworms

  3. Muller's larva
• Class Monogenea


  • Ectoparasites of fish skin and gills
  • Only 1 host
  • Reproduction: Eggs → oncomiracidium larva → attach to fish
  • Larva similar to that of cestode (oncosphere)
• Class Digenea (Flukes)


  • All are internal parasites of other animals
  • Leech-like bodies with a sucker at each end
  • Have a gut and well-developed reproductive system, never segmented
  • Blood flukes (Shistosoma), Liver flukes (Fasciola) & Lung flukes (Paragonimus)
• Schistosomes (Blood Fluke)


  • Causes infection known as Schistosomiasis (Formerly called Bilharziasis)
  • They are dioecious (Sexes are separated)
  • Adult males carry females in a gynaecophoric canal
• Schistosome species – Diagnostic Features

  • S. mansoni - Snail intermediate host: Biomphalaria, Location in host veins: Large intestine, Type of eggs spine: Lateral spine
  • S. japonicum - Snail intermediate host: Oncomelenia, Location in host veins: Small intestine, Type of eggs spine: Small round knob
  • S. haematobium - Snail intermediate host: Bulinus, Location in host veins: Urinary bladder, Type of eggs spine: Terminal spine
• Digenea Lifecycles


  The lifecycle is complex with up to 4 different hosts and several larval types
• Trematodes of Concern


  • Schistosomiasis - Deadly prominent disease in many regions of the world, Bores into the skin of the definitive host and travels in the circulatory system to the heart the lungs and the kidneys where it feeds and grows, Inflammation is caused by eggs becoming trapped in the hosts tissues
  • Chinese liver fluke - Lives in the bile duct of humans, cats, and dogs (1-2 cm.), Goes through two intermediate hosts, Infection occurs from ingestion of raw fish
• Liver Fluke (Fasciola)


  • Causes fascioliasis or liver rot in sheep and dogs
  • Major examples : F. hepatica & F. gigantica
  • Snail intermediate host- Lymnaea spp
• Class Cestoda (Tapeworms)


  • Defining characteristics: Scolex, Proglottids
  • All parasitic
  • E.g. Taenia saginatta (beef tapeworm), Taenia solium (pork tapeworm)

  
• Proglottids


  Each animal can 3,000 – 4,000 proglottids, Highly efficient reproductive output, Each may contain several ovaries and 1,000 distinct testes
• Phylum Platyhelminthes: Relatively simple, Passive dispersal, Reproduction is primarily sexual
• Ecological and Economic Importance of Platyhelminthes

  • Many flatworms such as flukes and tapeworms are important parasites of humans and cattle resulting in different wasting diseases
  • Infestation of livestock (e.g. cattle, sheep) reduces market value of the food resulting in economic losses and public health concerns
  • Platyhelminthes such as Turbellarians are predators or scavengers of dead animals helping in enriching the environment with nutrients
• Platyhelminths are flatworms that lack segmentation, have bilateral symmetry, and possess an ectodermal nervous system.
• The body wall is composed of three layers: epidermis (outer layer), muscle layer, and gastrodermis (inner lining).
• Flatworms can be hermaphroditic or dioecious, with the male reproductive organ being called testes and the female reproductive organ being called ovaries.
• Flatworms have no coelom but instead have fluid-filled spaces called pseudocoels.
• The phylum includes three classes: Trematodes (flukes), Cestodes (tapeworms), and Turbellaria (free-living planarians).