PLATYHELMINTHES

 

Platyhelminthes (Flatworms)

General Characteristics

  • These are flat-bodied, bilaterally symmetrical, triploblastic animals (have three body layers).

  • They have no body cavity (acoelomates).

  • Their bodies are soft, unsegmented, and dorsoventrally flattened—flat from top to bottom.

  • Mostly parasitic, but some are free-living (like Planaria).

  • They have cephalization (a distinct head with sensory organs).

  • Found in marine, freshwater, and moist terrestrial habitats.


Classification of Platyhelminthes

Platyhelminthes are divided into three main classes:

  1. Turbellaria – Mostly free-living flatworms.

    • Example: Planaria

  2. Trematoda – Parasitic flukes.

    • Example: Fasciola hepatica (liver fluke)

  3. Cestoda – Parasitic tapeworms.

    • Example: Taenia solium (pork tapeworm)

Body Systems in Platyhelminthes

  • Nervous system: Ladder-like structure with two main nerve cords and transverse connections.

  • Excretory system: Has protonephridia made up of flame cells that help remove waste and excess water.

  • Digestive system: Incomplete – only one opening (mouth) and no anus. Cestodes (tapeworms) don’t have a digestive system at all—they absorb nutrients directly.

  • Circulatory and respiratory systems: Absent. Exchange of gases and nutrients happens through body surface by diffusion.

Feeding in Platyhelminthes

  • Free-living forms (like Planaria): Have a muscular pharynx that comes out to suck in food.

  • Parasitic forms: Absorb nutrients from the host. Tapeworms absorb digested food through their skin (cuticle) since they have no digestive organs.

Diseases Caused by Platyhelminthes

These parasites can cause serious diseases:

  • Taeniasis – Caused by Taenia solium (pork tapeworm) and Taenia saginata (beef tapeworm).

  • Fascioliasis – Caused by Fasciola hepatica, infects liver and bile ducts.

  • Schistosomiasis (Bilharzia) – Caused by blood flukes (Schistosoma species), affects urinary tract and intestines.

Parasitic Adaptations in Flatworms

To survive inside hosts, flatworms have special features:

  • Tough outer covering (tegument) to resist host’s digestive juices.

  • Hooks and suckers for attachment.

  • Highly developed reproductive system – produce a huge number of eggs to increase survival chances.

  • Lack of digestive system in tapeworms – they absorb nutrients directly from the host’s intestine.

  • Reduced locomotion and sense organs, as they live inside stable environments (hosts).

Protonephridia and Flame Cells

  • Protonephridia are the excretory structures in Platyhelminthes.

  • Made up of a network of tubules with flame cells at the ends.

  • Flame cells have cilia that beat like a flickering flame—this movement pushes waste fluid through tubules to the outside.

  • They help in osmoregulation (maintaining water balance) and excretion.

In Simple Language:

Flatworms are thin, soft-bodied animals. Some live freely, but most are parasites that live inside other animals, like humans and cattle. They don’t have blood or lungs. Instead, they absorb food and oxygen through their skin. Their excretory system works using little "flame cells" that help remove waste. Some of them cause serious diseases, but they are really good at surviving inside their hosts.

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