Phylum Platyhelminthes

Phylum Platyhelminthes -The Flatworms

Phylum Platyhelminthes, commonly known as flatworms, represents a diverse group of invertebrate animals.

These organisms are characterized by their flattened, ribbon-like bodies.

Despite their simple appearance, flatworms exhibit a range of adaptations and complexities that make them interesting subjects for study.

The word Platyhelminthes is derived from Greek which means “the flat”.

The name Platyhelminthes has been given to the group of flatworms and the body of these animals is soft and dorsoventrally compressed.

General Characteristics of Phylum Platyhelminthes

Grade of organization: They are multicellular organisms and are triploblastic acoelomates i.e. they contain ectoderm and endoderm plus the development of mesoderm.

Shape and Symmetry: The Phylum Platyhelminthes have bilateral symmetry and these organisms have an unsegmented body. The size ranges from a few millimeters (planaria) to several meter long animals(tapeworm).

Habitat: Some members of Phylum Platyhelminthes are free-living, some live in freshwater and some are found in marine water.

Special Body cells:

Flame cells: Flame cells are specialized excretory cells found in the flatworms. These are bulb-like cells. A flame cell has a tuft of cilia, whose beating propels interstitial fluid into the tubular system. The beating of cilia looks like a flickering flame; therefore, these cells are termed flame cells.

Sac-like digestive system:  The body space is mostly occupied by a branching sac type digestive system. The digestive system is poorly developed in some species or maybe absent as in the tape-worms.


Organs and Organ systems: They have sense organs at the anterior end but respiratory and circulatory systems are not present.


Nervous system: There is a well developed nervous system but it may be in the form of a network f nerves or ganglia.

Movement: The free-living kinds are motile. They shoe movement by cilia present in their undersides (Planaria). In parasitic types the motion is limited.

Body in Phylum Platyhelminthes: The physique is dorsoventrally flattened and is unsegmented and looks like a leaf.

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Mode of nutrition: With few exceptions, the Platyhelminthes are all parasites, largely endoparasites, i.e., reside within their hosts. The most frequent cases include Taenia solium (tapeworm), Fasciola hepatica (liver fluke), and Schistosoma (blood fluke). The parasites are more prevalent in tropics. A few of them cause diseases in people.

Reproduction in Phylum Platyhelminthes

In Platyhelminthes reproduction takes place by asexual as well as sexual means:

Asexual reproduction:

Asexual reproduction is by fission where the organism constricts from the centre into two fragments, all fragments will regenerate the lost component.

Sexual reproduction:

The sexually reproducing species are hermaphrodite, i.e., both female and male reproductive organs exist in the same individual. Larval kind is occasionally present.

Life cycle:

Parasitic platyhelminths undergo quite intricate life cycles, and often include many larval stages in different organisms — the intermediate hosts; these hosts might be invertebrate or vertebrate.

Classification of Phylum Platyhelminthes

Class Turbellaria

The Turbellaria includes chiefly free-living, marine species, though some species reside in freshwater or moist temperate environments. Hookers and suckers are not present.

Examples: Planaria, Otoplana

Class Cestoda

The cestodes, or tapeworms, are internal parasites, chiefly of vertebrates. Hooks and suckers are found for sucking digested substance from host species therefore digestive tract are also absent.

Example: Taenia

Class Trematoda

The trematodes, or flukes, are parasites of mollusks and a variety of different organisms, such as humans. They also have suckers as they are parasites and obtain food by sucking substances from the host.

Example: Liver fluke

Examples of Phylum Platyhelminthes

Dugesia (Planaria): A free-living flatworm with cillias.


Fasciola (Liver luke): An endoparasite in sheep and sometimes in humans. It lives in the bile duct of its hosts.


Taenia (Tapeworm): An endoparasite of humans, cattle, and pig.


FAQs about Phylum Platyhelminthes – The Flatworms

  1. What does the name “Platyhelminthes” mean?
    • The word “Platyhelminthes” is derived from Greek and means “the flat.”
  2. How is the body of flatworms described?
    • The body of flatworms is soft and dorsoventrally compressed, giving them a flattened appearance.
  3. What is the grade of organization in Phylum Platyhelminthes?
    • Platyhelminthes are multicellular organisms with triploblastic acoelomate organization, meaning they have ectoderm and endoderm, and the development of mesoderm.
  4. Do flatworms exhibit bilateral or radial symmetry?
    • Flatworms exhibit bilateral symmetry, and their bodies are unsegmented.
  5. What is the habitat range of Phylum Platyhelminthes?
    • Some flatworms are free-living, while others live in freshwater or marine environments.
  6. What are flame cells, and where are they found?
    • Flame cells are specialized excretory cells found in flatworms. They are bulb-like cells with cilia that propel interstitial fluid into the tubular system.
  7. Describe the digestive system of flatworms.
    • Flatworms have a sac-like digestive system, with the body space mostly occupied by this branching system. However, it may be poorly developed or absent in some species.
  8. Do flatworms have a respiratory or circulatory system?
    • No, flatworms lack respiratory and circulatory systems.
  9. How do free-living flatworms move?
    • Free-living flatworms, like Planaria, show movement through cilia present on their undersides.
  10. What is the mode of nutrition in most Platyhelminthes?
    • With few exceptions, Platyhelminthes are parasites, mainly endoparasites, living within their hosts and causing diseases in some cases.
  11. How does reproduction occur in Platyhelminthes?
    • Reproduction in Platyhelminthes can be both asexual, through fission, and sexual, where they are hermaphrodites with both male and female reproductive organs.
  12. What are examples of parasitic Platyhelminthes?
    • Examples of parasitic Platyhelminthes include Taenia solium (tapeworm), Fasciola hepatica (liver fluke), and Schistosoma (blood fluke).
  13. Which class includes mainly free-living marine species?
    • Class Turbellaria includes mainly free-living, marine species, such as Planaria and Otoplana.
  14. Name an example of an internal parasite in Class Cestoda.
    • Taenia is an example of an internal parasite (tapeworm) in Class Cestoda.
  15. What are examples of parasitic Platyhelminthes?
    • Examples include Dugesia (Planaria), Fasciola (Liver fluke), and Taenia (Tapeworm).
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Wrap Up: Phylum Platyhelminthes

In summary, Phylum Platyhelminthes, characterized by their flat and soft bodies, exhibit a diverse range of organisms, from free-living to parasitic species. The name “Platyhelminthes” reflects their flattened appearance. These multicellular organisms are triploblastic acoelomates with bilateral symmetry and an unsegmented body.

Their habitats vary, encompassing free-living in freshwater or marine environments. Noteworthy characteristics include flame cells for excretion and a sac-like digestive system, though some species may lack a well-developed digestive system.

While they possess a well-developed nervous system, respiratory and circulatory systems are absent. Movement in free-living flatworms is facilitated by cilia.

Platyhelminthes engage in both asexual and sexual reproduction, often exhibiting complex life cycles, with some species being hermaphrodites. Parasitic members of this phylum, such as tapeworms and liver flukes, showcase intricate adaptations for their lifestyle.

The classification includes classes like Turbellaria (mainly free-living), Cestoda (tapeworms), and Trematoda (flukes). Examples like Planaria, Liver fluke (Fasciola), and Tapeworm (Taenia) illustrate the diversity within this phylum.

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In conclusion, the flatworms of Phylum Platyhelminthes play diverse ecological roles, from contributing to ecosystems as free-living organisms to causing diseases as parasitic forms. Their unique features and adaptations make them a fascinating group within the animal kingdom.