Platyhelminthes: Characteristics and Classification

General Characteristics of Platyhelminthes

  • Platyhelminthes comprises the flatworms (Gr. platys = flat; helminth = worm).
  • Flatworms are triploblastic (all worms are triploblastic), bilaterally symmetrical, acoelomate.
  • Body dorsoventrally flattened, leaf like or tapelike.
  • Mostly parasites and few free-livings.
  • First animals to have organ system organization and cephalization (differentiation of heads).
  • Segmentation is absent i.e. body not metamerically segmented.
  • Digestive, excretory, nervous and reproductive systems are present.
  • Alimentary canal is incomplete or absent.
  • Skeletal, respiratory and circulatory systems are absent.
  • Anus is absent like coelenterates, with blind-sac body plan.
  • Excretory organs are protonephridia or solenocytes or flame cells.
  • Flame cells arise from mesoderm ; unicellular, uninucleate, ovoid.
  • Main excretory products are ammonia, fatty acids and CO2.
  • Respiration and nutrition through general body surface.
  • Anaerobic respiration in parasites like Taenia.
  • Nervous system with large, anteriorly situated ganglia and nerve cords running along the body.
  • Sense organs are better developed in free living forms like Planaria.
  • These are monoecious or hermaphrodite. Reproductive organs are well-developed. Eggs possess yolk and covered by shell.
  • Development is direct in free-living and indirect in parasitic forms.
  • Fertilization is internal.
  • Spiral cleavage ; life cycle completed with many larval stages.
  • Larva are free swimming.
  • Ootype is surrounded by Mehli’s glands / shell glands. The secretion of this gland lubricates the passage.


3 classes on the basis of mode of life, presence or absence of digestive system and cilia in epidermis.

(L;turbella = a string)
(Gr; trema = hole, eidos = form)
(Gr; kestos = girdle, eidos = form)
Mostly free living; ectocommensal or parasitic, body unsegmentedCommonly called Flukes. Ecto and endo parasites, body unsegmented.Commonly called tapeworms. Endoparasites, body strobilated (pseudometamerism)
Epidermis cellular or syncytial ; covered with cilia.Cilia and epidermis absent.Cilia and epidermis absent.
Hooks and suckers absent.Suckers present.Hooks and suckers present.
Digestive system mostly present.Digestive system well developed.Digestive system completely absent.
Development usually direct, with spiral cleavage.Development direct in ectoparasites and usually indirect in endoparasites.Development is indirect.

Planaria, Bipalium, Convoluta, Mesostoma


Fasciola hepatica (liverfluke),Schistosoma (Blood/lymphatic fluke), Paragorimus westermani (Lung fluke)


 Taenia solium (pork tapeworm), Taenia saginata (beef tapeworm), Echinococcus granulosus (dog tapeworm), Diphyllobothrium latum (fish tapeworm), Hymenolepis nana (Dwarf tapeworm)

Binod G C

I'm Binod G C (MSc), a PhD candidate in cell and molecular biology who works as a biology educator and enjoys scientific blogging. My proclivity for blogging is intended to make notes and study materials more accessible to students.

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