Kingdom Animalia: Phylum Platyhelminthes

Overview

• Platyhelminthes includes flatworms.
• Characteristics:
  • Unsegmented and laterally symmetrical.
  • First and simplest organisms with an organ system.
  • No skeleton, but has protective cuticle.

Anatomy and Physiology

• Digestive System:
  • Highly branched gut with a mouth; anus is absent.
  • Muscular pharynx for ingesting food.
• Cephalization:
  • First phylum to show cephalization.
  • Development of sensory organs at the head, e.g., ocelli (eye spots in planaria).
• Respiration:
  • Non-parasitic flatworms respire through the body surface.
  • Parasitic flatworms are mostly anaerobic.
• Nervous System:
  • Central nervous system with head ganglion and interconnected nerve cords.
• Excretion and Osmoregulation:
  • Managed by flame cells, which resemble flickering flames.

Reproduction

• Capable of both sexual and asexual reproduction.
• Hermaphroditic: both sexes in one individual, except planarians (monoecious).
• Development:
  • Direct: Eggs hatch into miniatures of adults.
  • Indirect: Involves larval stages.

Classification

1. Turbellaria
   • Non-parasitic, aquatic flatworms.
   • High regenerative capacity.
   • Example: Planaria.
2. Trematoda
   • Parasitic flatworms, commonly known as flukes.
   • Example: Fasciola hepatica (sheep liver fluke).
3. Cestoda
   • Known as tapeworms; all are internal parasites.
   • Possess hooks and suckers for anchoring to the host.
   • Example: Taenia.

Ecology and Impact

• Majority are endoparasites.
• Cause diseases in humans and domesticated animals like sheep, goats, and dogs.

Evolutionary Note

• Most primitive flatworms resemble the planula larva of cnidarians.