PHYLUM - PLATYHELMINTHES ( પૃથુકૃમિ )
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Platyhelminthes was introduced by GEGENBAUR.
They are commonly called as flatworms.
There are about 13,000 species of flatworms known today.
Shape of the body is generally dorsi-ventrally flat, Hence the name flatworm.
This is the first group of animals in which anterior part is developed into a head.
Important characteristic features:
- First group of animals to have an organ system level of organization.
- Parasitic adaptations: Presence of hooks and suckers.
- Bilaterally symmetrical with dorsal and ventral surfaces. First group of animals that show bilateral symmetry.
- Triploblastic animals: it is the first group of animals to possess 3 germ layers.
- Acoelomate animals: The space between the body cavity and alimentary canal is filled with loose parenchyma cells (mesenchyma cells) originated from mesoderm.
- Alimentary canal: It is branched. It opens out by a single opening called mouth. Anus is absent. In parasitic flat worms, alimentary canal is absent. They absorb nutrients through their body wall while living in the host intestine.
- • Excretory system: it consists of branching tubes which end in specialized cells called flame cells (protonephridia). The functions of these cells are osmoregulation and excretion.
- • Reproduction: Most of them are hermaphrodites. They reproduce asexually and sexually. Asexual reproduction takes place by fragmentation and regeneration. Some members possess high regeneration capacity.
Classificationof Platyhelminthes
Class I: Turbellaria (free living flatworms, eg : Planaria)
Class II: Trematoda (ecto or endoparasites with oral and ventral suckers, eg : liver fluke, blood fluke, etc).
Class III: Cestoda (exclusively endoparasites with scolex, eg: Taenia solium (pork tapeworm), Taenia saginata (beef tapeworm), etc).
• Fertilisation: It takes place inside the body and development involves larval stages (amphiblastula and parenchymula).
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