Animal Kingdom

Phylum Platyhelminthes

Did you know, potentially life-endangering parasites can live in the human body for up to several decades? These parasites, known as blood flukes, have the ability to regenerate themselves and so can survive in hostile environments. These worms belong to Phylum Platyhelminthes. Let us explore this phylum.

Suggested Videos

Phylum Platyhelminthes

Bedfords Flatworm

(Source: Wikipedia)

Platyhelminthes very commonly known as flatworms or tapeworms, these animals are soft-bodied invertebrate animals. There are around 20,000 species of these animals. A few of these live as parasites on humans and other animals. It is because of this parasitic nature that they do cause some amount of trouble for the host animal. A few species belonging to this phylum can be a major cause of certain diseases. Schistosomiasis, or bilharzia or bilharziasis, is a disease caused by these parasitic flatworms belonging to the family Schistosomatidae.

The most distinguishing feature of these invertebrates is their flat body. As the body does not have any cavity, they are flat. The body is also not segmented and they do not have specialized systems. Around eighty percent of the flatworms are parasitic in nature, while a few free-form flatworms are also present. The free-living species are scavengers or predators. The parasitic species feed on the tissues of the host organism in which they live.

The animals in this phylum have a diverse range in size. Some are microscopic, while a few go up to two feet long. They are also hermaphrodites, which mean that both the sexes are present in the same organism.

Learn more about Phylum Chordata here.

Characteristic features of Phylum Platyhelminthes

Planaria Nervous system

(Source: Britannica)

  • Their body is dorsoventrally flattened.
  • They exhibit bilateral symmetry.
  • Also, they are triploblastic, with three germ layers.
  • They do not have a body cavity and are acoelomate.
  • Body is soft and unsegmented.
  • They are mostly parasitic with a few free-living
  • They exhibit an organ system grade of organization.
  • The digestive system is incomplete or absent. There is a single opening which leads to a well-developed gastro-vascular cavity. The anus is absent. There is no true stomach structure. In a few species, the digestive system is completely absent.
  • Respiratory and circulatory systems are absent. Respiration generally occurs through diffusion through the general body surface.
  • The excretory system has protonephridia with the flame
  • There is primitive nervous system present.
  • These animals are hermaphrodites.
  • Sexual reproduction happens through gametic fusion.
  • Asexual reproduction also happens in a few species through regeneration and fission.
  • Fertilization is internal.
  • The life cycle of these organisms can be complex, especially if they are parasitic, as this may involve one or more host animals.

Learn more about Phylum Mollusca here.

Classification of Phylum Platyhelminthes

The different classes under this phylum are:

  • Turbellaria
  • Trematoda
  • Cestoda


  • Taenia (Tapeworm)
  • Fasciola (Liver fluke)
  • Taenia saginata (Beef tapeworm)
  • Echinococcus granulosus – The dog tapeworm
  • Planeria (freshwater flatworm)
  • Opistorchis

Learn more about Phylum Hemichordata here.

Solved Questions for You

Q: Which is the phylum, where the body is dorsoventrally flat without a body cavity?

Ans: Phylum Platyhelminthes.

Q: Give an example of a free-living

Ans: Planaria

Q: The digestive system is absent in which class of flatworms?

Ans: Class Cestoda.

Q: Phylum Platyhelminthes shows which grade of organization?

Ans: It is the first phylum to show organ system grade of organization along with bilateral symmetry.

Share with friends

Customize your course in 30 seconds

Which class are you in?

Leave a Reply

Notify of

Stuck with a

Question Mark?

Have a doubt at 3 am? Our experts are available 24x7. Connect with a tutor instantly and get your concepts cleared in less than 3 steps.

Download the App

Watch lectures, practise questions and take tests on the go.

Do you want

Question Papers

of last 10 years for free?

No thanks.