The word Platyhelminthes is formed from the Greek words platy meaning flat and helminth meaning worm. Animals of this group are known as flatworms or tapeworms. The number of species in this category is about 29,487. The group Platyhelminthes was named by the scientist Gagenbaur (Gagenbaur, 1859). Some flatworms of the genus cause schistosomiasis or snail fever in humans.
Characteristics of Platyhelminthes
1. They are triploblastic animals. That is, the body has cell layers called ectoderm, endoderm and mesoderm.
2. The body is bilaterally symmetrical and flattened like a leaf or ribbon.
3. They are Asilomates. Between different organs are parenchyma or collenchyma cells.
4. Body skin is soft, without cilia or cuticles.
5. The mouth has suckers or hooks or proglottids.
6. The excretory system is made up of flame cells.
7. The nervous system is ladder-like and consists of ganglia and digitorum plexus.
8. Their epidermis is composed of syncytes.
9. Circulatory system and respiratory system are absent.
10. Alimentary system is incomplete, branched and consists of mouth, pharynx and intestine. There is no anus.
11. They self-destruct. The life cycle consists of larvae such as radia, cercoria, sporocysts, cysticercus etc.
[Key words to remember characteristics: triploblastic, flattened, parenchyma, autophagic, sucker, hook, flame cell, syncytial]
Classification of Platyhelminthes
The order Platyhelminthes is divided into 4 classes.
1. Turbellaria: Their bodies are flat like leaves. Cuticle is absent on the body. The epidermis is ciliated and rhabdite in nature. Suction is missing. Larval stage absent. They are known as Edewam. Such as Planaria (Dugesia tigrina).
2. Trematoda: They are cylindrical or flat like leaves. Cilia are absent on the body. The mouth has suckers or hooks. Larval stage present. They are known as Fluke. For example- liver worm (Fasciola hepatica).
3. Monogenia: These are small parasites. Their heads contain nerve cells and sensory organs. Alimentary canal is simple in nature. They are known as flatworms. Such as Planaria (Dugesia tigrina).
4. Cestoda: Their bodies are long and flattened like ribbons. Composed of 800-900 proglottid segments. Each segment has one or two sets of male or female reproductive organs. The mouth has suckers and curved hooks. Mouth opening and esophagus absent. They are known as tapeworms. For example, tapeworm (Taenia solium).
Flame cell
The cyst is a part of the excretory organ. Cells are bulbous with holes. It is attached to the tube of mesoderm. It has cilia in clusters. The cilia vibrate violently inside the tube, creating a current of water. As a result, the waste material is released through the excretory hole. Thus it helps in digestion.
Some animals of Platyhelminthes
Liver worm- Fasciola hepatica
Tape worm – Taenia solium
Dog tapeworm – Echinococcus granulosus
Free-living flatworm – Dugesia tigrina
Hammer head worm- Bipalium kewense
Human bloodworm – Schistosoma mansoni
Frog worm- Polystoma integrimum
Gyrodactylus- Gyrodactylus corti
Dactylogyrus alatus
Convoluta- Convoluta convoluta
Microstomum- Microstomum lineare
Thysanozoon – Thysanozoon brocchii
Sheep worm- Dicrocoelium dendriticum