Phylum Platyhelminthes : General Characters with Examples | Flat worms | Animal Kingdom

Phylum Platyhelminthes (Gr.platys, flat+ helminthes, worm) includes worms with a flat body. This phylum includes about 13,000 species. Animals with elongated bodies but no conspicuous appendages are called “worms”. They are the first group of animals with bilateral symmetry. This is the first group of animals with their anterior ends develops to a head. The process of formation of head is called cephalisation. Flat worms vary in size from microscopic fluke to the large tape worms that may become 12 metres in length. Some of them are them are free living (e.g. Planaria) in fresh, or salt waters or in moist places on land and others are parasites (e.g. Tapeworms).

🎯General Characters:

  • Flat worms are triploblastic i.e. having three germ layers. They are the first organ grade of organisation.
  • Body is dorsoventrally flattened and has a definite anterior end, differentiated as a ‘head’.
  • They are acoelomate animals. The space between the body wall and gut are filled with parenchyma composed of loose cell masses.
  • Body is devoid of true metameric segmentation.
  • Alimentary canal is usually highly branched; it opens out by a single opening the mouth. Anus is absent.
  • Circulatory and respiratory system are absent.
  • Nervous system is ladder like. It is formed of brain and two main longitudinal nerve cords connected at intervals by transverse commissures.
  • Excretion is performed by specialized cells called flame cells. They lead into tubules which open out by one or more excretory pores.
  •  Reproductive system is well organized. Most of them are hermaphrodites.(Hermaphroditism is a phenomenon of occurrence of both male and female sex organs in one animal.)
  • Life history often includes larval stages. In some forms asexual reproduction is transverse fission takes place. Many flat worms reproduce asexually from a part of their body by regeneration.

🎯Classes of Platyhelminthes with Examples : Turbellaria, Trematoda, and Cestoda.

Phylum Platyhelminthes

    i. Class: Turbellaria 
  • This class includes mostly freee living flat worms 
  • Example: Planaria

ii. Class: Trematoda
  •  This class includes a group of parasites that infect various animals including man. Body is highly specialixzed for parasite mode of life.
  • Example: Fasciola (Liver Fluke) and Schistosoma (Blood Fluke)

iii. Class: Cestoda
  • This class include tape worms which are the intenstinal parasites of vertebrates. 
  • Example: Taenia solium and Taenia saginata

Flat worms are very advanced than Porifera and Cnidaria because of the development of mesoderm as third layer, development of brain, well organised reproductive system and bilateral symmetry.
🎯Key Points Phylum Platyhelminthes (Flat worms)

They have dorso-ventrally flattened body, hence are called flatworms. These are mostly endoparasites found in animals including human beings. Flatworms are bilaterally symmetrical, triploblastic and acoelomate animals with organ level of organisation. Hooks and suckers are present in the parasitic forms. Some of them absorb nutrients from the host directly through their body surface. Specialised cells called flame cells help in osmoregulation and excretion. Sexes are not separate. Fertilisation is internal and development is through many larval stages. Some members like Planaria possess high regeneration capacity.

🎯Example of Phylum Platyhelminthes

  • Free living: Planaria
  • Fasciola (Liver fluke): found in the bile duct and liver of sheep.
  • Schistosoma: Found in the veins of man
  • Taenia solium : the pork tapeworm.

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