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Phylum Platyhelminthes: General characteristics, Parasitic adaptations, Excretion, Reproduction, Examples and Classification

Whenever you think about the word ‘animal’, something robust three-dimensional figures must be striking your mind! Have you ever imagined an animal in the form of a cellotape? Yes, there are such animals also on our planet. These animals belong to the phylum Platyhelminthes, another group under kingdom Animalia. 

Platyhelminthes has een derived from two words ‘platys’ meaning broad or flat and ‘helmin’ meaning worm. Hence, members of phylum Platyhelminthes are also called flatworms.b

Table of Contents:

Habit

Platyhelminthes are mainly endoparasites (E.g. flatworms) with a few of them being free-living (E.g. Planaria). These are also known as flatworms because of their dorso-ventrally flattened body

Symmetry

Flatworms are bilaterally symmetrical as the body can be divided into two equal and identical halves, i.e., right half and left half, through only one plane passing through the centre.

bilaterally symmetrical in Flatworms

Level of Organisation

Flatworms have an organ level of body organisation. Their tissues performing similar functions aggregate to form organs which perform respective physiological functions.

organ level organization of Platyhelminthes

Germ Layers and Coelom

Platyhelminthes are triploblastic in nature. This is because they have cells arranged in three germinal layers, outer ectoderm, inner endoderm and middle mesoderm. 

However, the body cavity is absent between their body wall and gut wall. Hence, they are Acoelomates.

Triploblastic organization in Platyhelminthes

Parasitic Adaptations of Platyhelminthes

Parasitic forms of Platyhelminthes possess the following specialised structures:

  • Hooks: These attach parasites to the body of the host.
  • Suckers: These help in the absorption of nutrients from the host’s intestine.

Parasitic adaptations in Platyhelminthes

A few parasitic forms obtain nutrition directly from the host through their body surface.

Osmoregulation and Excretion in Platyhelminthes

The process of osmoregulation and excretion are conducted by specialised cells called Flame cells. These cells have a cluster of cilia, resembling a flickering flame. The fluids get filtered through the cilia in the flame cell. The waste gets collected by the collecting tubules which is subsequently expelled from the excretory pore.

flame cells in platyhelminthes

Modes of Reproduction and Development of Platyhelminthes

Asexual mode

Regeneration ability is well-marked in certain flatworms like Planaria. The body gets splitted into pieces, each subsequently developing into new individuals.

s

Sexual mode

Flatworms are hermaphrodites, i.e., the sexes are not separate. However, fertilisation is usually cross (male gamete of one flatworm fertilises the female gamete of another flatworm). Self fertilisation can be observed in certain tapeworms. Since fertilisation occurs inside the body, hence, it’s considered as Internal fertilisation.

planaria

The embryos undergo indirect development through intervening larval stages. The larval forms of liver flukes include miracidium, sporocyst, redia, cercaria, metacercaria. The larval stages of tapeworms include oncosphere, hexacanth and cysticercus larvae.

Examples of Platyhelminthes

  • Tapeworm (Taenia solium)
  • Liver fluke (Fasciola hepatica)
  • Planaria

tapeworm and planaria

Classification of Platyhelminthes

On the basis of mode of living, phylum Platyhelminthes can be divided into three classes as follows:

Class Turbellaria

The word Turbellaria is derived from a latin word ‘turbella’ which means stirring. Most of the organisms under this class are free-living. For example: Dugesia.

Class Trematoda

This class of Platyhelminthes includes organisms which are ectoparasitic or endoparasitic. For example: Fasciola.

Class Cestoda

This class of Platyhelminthes includes organisms which are mainly endoparasitic. Body of members of class Cestoda is divided into false segments known as proglottids. For example: Taenia.

Practice Problems of Phylum Platyhelminthesis

Q1. Read the following statements about ‘Y’ carefully.

The parasitic forms have hooks and suckers in them. Some of the animals derive nutrients from the host directly through their body surface.

Identify the phylum to which ‘Y’ belongs:

  • Annelida
  • Platyhelminthes
  • Porifera
  • Mollusca

Solution: Certain members of phylum Platyhelminthes are endoparasites that have specialised structures called Hooks and Suckers. These help them to adhere to their respective hosts and acquire food. For example, liver fluke. 

Members like Taenia solium, living in the gut intestine of humans, can directly absorb the flowing liquid nutrients, with the help of their body surfaces.

Hence, the correct option is b.

Q2. Excretion in Planaria occurs by

  • Flame cells
  • Renette cell
  • Nephridia
  • Kidneys

Solution: Excretion and osmoregulation in Planaria takes place with help of specialised cells known as Flame Cells. Numerous flame cells combine to form a bundle-like structure known as protonephridia. Their function is to propel out waste products with the help of cilia.

Hence, the correct option is a.

Q3. Which among the following is incorrect?

  • Taenia is triploblastic
  • Bilateral symmetry is found in adult Cucumaria
  • Mesoglea is present in Physalia
  • Fasciola is acoelomate

Solution: Cucumaria (commonly known as sea cucumber) belongs to the phylum Echinodermata. The adult members exhibit radial symmetry (the type of symmetry in which the body of the organism can be divided into two equal halves along the central axis in any plane). 

Hence, the correct option is b.

Q4. During identification of a specimen in a zoology laboratory, Smita spotted that one of the specimens was flattened, resembling a sheet of paper. Which of the following can be the probable phylum to which the specimen belongs to?

  • Phylum Porifera 
  • Phylum Coelenterata 
  • Phylum Cnidaria 
  • Phylum Platyhelminthes

Solution: Platyhelminthes is a phylum including organisms having dorso-ventrally flattened bodies. They are also called flatworms. Hence, the specimen spotted by Smita likely belongs to phylum Platyhelminthes.

Hence, the correct option is d.

FAQs of of Phylum Platyhelminthesis

Question1.- What is the common method of fertilisation shown by flatworms?

Answer. Flatworms are members of phylum Platyhelminthes. They are hermaphrodites. However, cross fertilisation is the common method of fertilisation in which the male gamete of one flatworm fertilises the female gamete of another flatworm. 

Question2.- Are members of phylum Platyhelminthes harmful to humans?

Answer. Certain platyhelminths cause diseases in humans. Some of them are mentioned below:

  • Tapeworm: Causes taeniasis.
  • Bladder worm: Causes cysticercosis

Question3.- What are the three examples of Platyhelminthes?

Answer. Three examples of Platyhelminthes are as follows:

  • Planaria
  • Liver fluke (Fasciola hepatica)
  • Tapeworm (Taenia solium)

Question4.- Define regeneration with an example.

Answer. The process involving development of a full organism from its body part is called regeneration. It is well marked in Planaria, a member of phylum Platyhelminthes.

Other Related Topics

The Living World Biological Classification Plant Kingdom
Animal Kingdom Morphology of Flowering Plants Anatomy of Flowering Plants
Structural Organization in Animals Cells: The Unit of Life Biomolecules
Cell Cycle and Division Transport in Plants Mineral Nutrition
Photosynthesis in Higher Plants Respiration in Plants Plant Growth and Development
Digestion and Absorption Breathing and Exchange of Gases Body Fluids and Circulation
Excretory Products and their Elimination Locomotion and Movement Neural Control and Coordination
Chemical Coordination and Integration

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