Phylum Platyhelminthes : General Characters
Phylum Platyhelminthes (Gr.platys, flat+ helminthes, worm) includes worms with a flat body. This phylum includes about 13,000 species. Animals with elongated bodies but no conspicuous appendages are called “worms”. They are the first group of animals with bilateral symmetry. This is the first group of animals with their anterior ends develops to a head. The process of formation of head is called cephalisation. Flat worms vary in size from microscopic fluke to the large tape worms that may become 12 metres in length. Some of them are them are free living (e.g. Planaria) in fresh, or salt waters or in moist places on land and others are parasites (e.g. Tapeworms).
Classes of Platyhelminthes: Turbellaria, Trematoda, and Cestoda.
- Flat worms are triploblastic i.e. having three germ layers. They are the first organ grade of organisation.
- Body is dorsoventrally flattened and has a definite anterior end, differentiated as a ‘head’.
- They are acoelomate animals. The space between the body wall and gut are filled with parenchyma composed of loose cell masses.
- Body is devoid of true metameric segmentation.
- Alimentary canal is usually highly branched; it opens out by a single opening the mouth. Anus is absent.
- Circulatory and respiratory system are absent.
- Nervous system is ladder like. It is formed of brain and two main longitudinal nerve cords connected at intervals by transverse commissures.
- Excretion is performed by specialized cells called flame cells. They lead into tubules which open out by one or more excretory pores.
- Reproductive system is well organized. Most of them are hermaphrodites.
(Hermaphroditism is a phenomenon of occurrence of both male and female sex organs in one animal.)
- Life history often includes larval stages. In some forms asexual reproduction is transverse fission takes place. Many flat worms reproduce asexually from a part of their body by regeneration.
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