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Phylum Platyhelminthes : General Characters

Updated on September 19, 2013

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).

Classes of Platyhelminthes: Turbellaria, Trematoda, and Cestoda.

Class: Turbellaria

Planaria
Planaria

Class: Trematoda

Liver fluke: Fasciola hepatica
Liver fluke: Fasciola hepatica
Blood Fluke: Schistosoma
Blood Fluke: Schistosoma

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.

Class: Cestoda

Pork Tape Worm: Taenia solium
Pork Tape Worm: Taenia solium

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.

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      maiga 19 months ago

      There some areas you have just gave the wrong information..."Body is devoid of true metameric segmentation."

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      Sandip Naskar 22 months ago

      Please update...this type mcq download pdf file.

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      Turgut 2 years ago

      I was going to say that I would teach kingdom, Phylum, genus and seiecps unless we were talking about classification, in which case I would include it all and use the organism as an example.But having read the above posts, I think put it all in because then each person has the opportunity to select only what they want.

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      Nayeni 2 years ago

      All as the comment above sutgesgs, if you are making resources for the full spectrum of schools, then put all of it! The teachers or even better the students will select what they need or want or can cope with. It is just nice that someone can produce a resource that will be a diversion from wikipedia! 10-16 is a wide range of ages some of my 11 year olds could cope with all that taxonomy as an extension, and some of my y11s could only deal with the kingdom and genus/species. It depends!

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      Shivani 2 years ago

      very nice

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      thax 3 years ago

      great info abt biodiversity

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      Tara thakur. 3 years ago

      very nice

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      shakir faiz 4 years ago

      thank u for the help me