Notes on Platyhelminthes | Grade 11 > Biology > Introduction to kingdom Animalia | KULLABS.COM

Platyhelminthes

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Examples of Phylum Platyhelminthes
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fig:Examples of Phylum Platyhelminthes

Platyhelminthes is made up of two words i.e. Platy which means flat and helminthes which mean worms. Gegenbaur coined the term Platyhelminthes.

General characteristics of phylum Platyhelminthes:

  • Most of them are parasites while some are free living in sea water, fresh water or moist soil.
  • Their body is dorsoventrally flattened and bilaterally symmetrical and soft leaf-like.
  • Organ system grade of body organization.
  • Triploblastic
  • Body is covered with distinct head with a cellular syncytial one layered partly ciliated epidermis while, in parasitic Trematodes and Cestodes the body is covered with cuticle, the epidermis is absent.
  • They possess adhesive structures like hooks, spines, suckers and adhesive secretions for attachment.
  • Parthenogenesis and polyembryony usually occur in trematodes and tapeworms.
  • Most of them are white or colorless, some derive color from the ingested food while the free living form shows brown, gray, black or brilliant color.
  • Endoskeleton and Exoskeleton are absent.
  • The digestive system is totally absent in some and some consists of mouth, pharynx and intestine. The anus is absent.
  • The circulatory and respiratory system is absent whereas the excretory system is present.
  • A simple nervous system is present and sense organs are greatly reduced in parasitic forms.
  • Asexual reproduction occurs by fission and sexual by the fusion of gametes.
  • Fertilization is internal.
  • Eyespots are present as a sensory organ.

Classification:

Platyhelminthes consists of about 10,000 to 15,000 species. They are divided into three classes on the basis of habitat, body shape and mouth position.

Bipalium
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fig:Bipalium

Turbellaria:

  • They are mostly free-living, aquatic, carnivorous, worms while few are commensals or parasitic.
  • Body is dorsoventrally flattened, unsegmented, covered with ciliated, cellular or syncytial epidermis containing secretory cells. The cuticle is absent.
  • The life cycle is simple.
  • Hooks and suckers usually absent.
  • Example: Planaria, Bipalium.

Trematoda:

  • The body is covered with thick cuticle layer. There are no epidermis and cilia.
  • The body is unsegmented and dorsoventrally flattened.
  • They remain attached to their host body by means of suckers and sometimes by hooks.
  • The life cycle is complex.
  • Examples: Fasciola hepatica.

Convoluta
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fig:Convoluta

Cestoda:

  • The body is covered with cuticle, ciliated, the epidermis is absent.
  • They are found as endoparasitic in the intestine of vertebrates.
  • Digestive system and sense organs are absent.
  • Lifecycle is complicated and involves two or more hosts.
  • Example: Taenia solium, Convoluta.



  • Platyhelminths is made up of two words i.e. platy which means flat and helminthes which means worms.
  • Most of them are parasites while some are free living in sea water, fresh water or moist soil.
  • Their body is dorsoventrally flattened and bilaterally symmetrical and soft leaf like.
  • They posses adhesive structures like hooks, spines, suckers and adhesive secretions for attachment.
  • Circulatory and respiratory system is absent whereas excretory system is present.
  • Eyespots are present as sensory organ.
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