(Greek; platys= Flat; helmins = worms); The flatworms.
Phylum Platyhelminthes includes 13000 species. These are found as free-living as well as parasitic life forms. These are also called Flatworms because of having a dorso-ventrally flattened body.
Characteristics features of Phylum Platyhelminthes
The Platyhelminthes are mostly parasitic and live on or inside the host. A few are free-living and aquatic. The body of flatworms is thin, soft, unsegmented, and dorsoventrally flattened. They may be leaf-like or ribbon-like animals.
- These are mostly endoparasites, found in various animals including human beings.
- Symmetry: These are Bilaterally symmetrical.
- Germinal layer: triploblastic animals.
- Triploblastic– In a developing embryo, the third germinal layer called mesoderm is found in between ectoderm and endoderm. These animals are called triploblastic animals
- The body cavity: Acoelomate animals
- Level of organization: The organ level of organization is found.
- In parasites the hooks and suckers are present.
- Some parasitic organisms absorb nutrients directly from the host body’s surface.
- Hermaphrodites ( bisexual or monoecious) Sexes are not separate, internal fertilization.
Flame cells are the specialized in this phylum which helps in osmoregulation and excretion. Chief excretory waste is ammonia in Platyhelminthes.
Power of regeneration:
Some members of this phylum possess a high regeneration capacity. e.g., in Planaria.
Larval statges in Platyhelmithes
- The development in Platyhelminthes occurs through many larval stages. for example, the life cycle of fasciola includes miracidium larva, sporocyst larva, redia larva, cercaria larva, and metacercaria larva.
- The larval forms of Taenia are hexacanth larva and cysticercus larva.
Examples of Platyhelminthes:
Parasites: Fasciola hepatica (Liver fluke), Taenia (Tapeworm), Schistosoma (Blood fluke)
Taenia solium: also called pork tapeworm and found in all countries where the consumption of pork occurs. The larva of this organism is found in the pig’s muscles. Taenia solium lives as a parasite in the intestine of the human. It is a hermaphrodite organism and it undergoes self – self-fertilization.
Fasciola: Also called the liver fluke, it lives in the bile duct of sheep and goats. It is also hermaphrodite but in these organisms cross-fertilization takes place.
Schistosomes: These are called blood flukes because found in mesenteric blood vessels and the hepatic portal system of humans. It shows well-marked sexual dimorphism. They spread many diseases by contaminating the water.
planaria- It is found in freshwater. The Body comprises cilia and has the power to regenerate the lost part. They possess a pair of eyes and two lateral lobes.
Classification of Phylum Platyhelminthes:
The Platyhelminthes can be divided into:
Characteristic features of Tuberllaria:
- These are mostly found in fresh-water
- These animals are free-living
- Hooks and suckers are absent
- For example; Planaria, Otoplana
Characteristic features of Trematoda:
- The animals are mostly parasitic
- Presence of Hooks and suckers
- Example: Fasciola hepatica, Diplozoon
Characteristic features of Cestoda:
- The members are the exclusively parasitic
- presence of hooks and suckers
- Example: Taenia sps. Convoluta
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