Phylum : Platyhelminthes (Flat worms), its classification and characteristics

  • multicellular, triploblastic, bilaterally symmetrical.
  • first animals to have organ-system grade of body organization.
  • body is dorsoventrally flattened so called flat worms.
  • size varies from less than mm and more than 10-15 metres.
  • mostly parasitic, few are free living and are found in water or damp soil.
  • parasitic forms have hooks and suckers to stick to the host body.
  • blind sac body plan.
  • alimentary canal is absent or incomplete without anus.
  • excretion occurs by flame cells, paired protonephridia or bulbs.
  • mostly hermaphrodite.
  • free living forms have photoreceptors, chemoreceptors and tango receptors.
  • respiration occurs through general body surface.
  • in many, fresh water asexual reproduction occurs by fission.
  • life cycle is simple or complex with alternation of one or more hosts.
  • development is direct in free living forms and indirect in parasitic forms.
  • fertilization is internal.
  • parthenogenesis and polyembryony usually occurs in Trematodes and tapeworms.


Flatworms are classified in three classes on the basis of habit. They are:

Class 1: Turbellaria

  • free living flatworms found in sea or fresh water.
  • commonly known as eddy worms.
  • cuticle is absent.
  • body is covered by ciliated epidermis.
  • hooks and suckers are absent.
  • tangoreceptors, chemoreceptors, photoreceptors are present.
  • mostly hermaphrodite, some reproduce asexually.
  • life cycle is simple.
  • examples: Planaria (Dugesia), Convoluta, Otoplana, Bipalium.

Information about Platyhelminthes

 Image source: microbiologynotes

Class 2: Trematoda

  • ecto or endo parasites.
  • body is flat, unsegmented, commonly known as fluke.
  • usually rounded or leaf-like.
  • epidermis and cilia absent.
  • suckers are present.
  • life cycle is complex.
  • usually hermaphrodite except few families of order Digenia.
  • examples: Fasciola hepatica (liver fluke), Schistosome (blood fluke), Monocotyle, Cotylapsis

LABORATORIO: Parte 1 Filo Platyhelminthes - ppt download

Image source: slideplayer

Class 3: Cestoda

  • exclusively endoparasitic.
  • flat, pseudo segmented, elongated and ribbon like.
  • hooks and suckers are present.
  • body covered with cuticle, ciliated epidermis absent.
  • digestive system and sense organs are absent.
  • pair of ganglia and two lateral longitudinal nerve cords present.
  • life cycle is complex, involves two or more hosts.
  • examples: Taenia solium (pork tapeworm), Echinococcus (dog tapeworm)




Phylum : Platyhelminthes (Flat worms), its classification and characteristics