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
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.
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.
Flatworms have an organ level of body organisation. Their tissues performing similar functions aggregate to form organs which perform respective physiological functions.
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.
Parasitic forms of Platyhelminthes possess the following specialised structures:
A few parasitic forms obtain nutrition directly from the host through their body surface.
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.
Regeneration ability is well-marked in certain flatworms like Planaria. The body gets splitted into pieces, each subsequently developing into new individuals.
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.
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.
On the basis of mode of living, phylum Platyhelminthes can be divided into three classes as follows:
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.
This class of Platyhelminthes includes organisms which are ectoparasitic or endoparasitic. For example: Fasciola.
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.
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:
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
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?
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?
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.
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:
Question3.- What are the three examples of Platyhelminthes?
Answer. Three examples of Platyhelminthes are as follows:
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.
|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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