Major Worm Phyla: Flatworms, Annelids, and Nematodes

Phylum Platyhelminthes: Flatworms

General Characteristics

• Most are hermaphroditic.
• Possess bilateral symmetry and a defined head region.
• The nervous system is present.
• Includes the parasitic tapeworm.

Movement

• Controlled primarily by longitudinal muscles.
• Some species move along slime trails.

Feeding and Digestion

• Free-Living Species: Carnivores or scavengers with a complex digestive system.
• Parasitic Species: Feed on blood or fluids; lack a digestive system, absorbing nutrients directly from the host.

Sense Organs

• Possess chemoreceptors, balance receptors, and receptors that sense water movement.
• Some species show sensory development in the head region.

Classes of Flatworms

Turbellaria (e.g., Planaria)

• Live primarily in fresh water.
• Are hermaphrodites.
• Reproduce internally by fission and can regenerate lost parts.
• Key Structures:
  • Brain (Ganglia): Processes environmental information.
  • Pharynx: Used for sucking food.
  • Eyespot: Simple eye that can detect light.
  • Flame Cells: Used for excretion, located in the lateral edges.
  • Intestine: Used for digestion.

Cestoda (Tapeworms)

• Long, flat, parasitic flatworms.
• Live in the intestines of vertebrates and can grow up to 10 meters.
• The body is composed of a head (the scolex) and sections called proglottids, which can contain up to 100,000 eggs.

Trematoda (Flukes)

• Parasitic flatworms (flukes).
• Live in the mouth or digestive tracts of hosts.
• Feed on host fluids, such as blood.
• Have complex life cycles, often involving asexual reproduction.
• Example: Schistosoma mansoni
  • Requires multiple hosts (human and snail).
  • Causes the disease Schistosomiasis.
  • Prevalent in tropical areas with poor sanitation, particularly South and Central America.

Phylum Annelida: Segmented Worms

Includes earthworms, leeches, and bristleworms. The name "Annelida" means "tiny rings."

Characteristics

• Bilateral symmetry.
• Two body openings (complete digestive tract).
• Some species have a larval stage.

Segmentation

• Cylindrical bodies with internal and external segmentation.
• Segmentation allows for specialization of body parts.

Habitat and Distribution

• Found everywhere except polar and desert regions; many are aquatic.
• Approximately 9,000 species of segmented worms exist.
• Often nocturnal, living in burrows.

Reproduction

• Are hermaphrodites, but two worms typically exchange sperm.
• Each worm produces a capsule for eggs, which hatch in 2 or 3 weeks.

Class Hirudinea (Leeches)

• Segmented, flattened bodies.
• Lack bristles.
• Live in fresh water.
• Suck blood or other body fluids.

Phylum Nematoda: Roundworms

Includes Trichinella, pinworms, hookworms, and Ascaris lumbricoides.

General Features

• Some are free-living, while others are parasitic, affecting all plants and animals.
• Live primarily in soil.
• Have a thick outer covering that protects them from being digested by hosts.
• Possess a one-way digestive tract.

Nematode-Caused Diseases

Elephantiasis

A disease caused by microscopic parasitic worms, including Wuchereria bancrofti, Brugia malayi, and B. timori. It is transmitted by mosquitoes.

Specific Parasitic Nematodes

Trichinella

• Transmitted through the consumption of contaminated meat, often from hogs that have eaten raw meat scraps.

Hookworm

• Infects humans in warm climates.
• Contracted by walking barefoot on contaminated soil.

Ascaris lumbricoides

• Commonly infects children.
• Transmission occurs when dirty hands or unwashed vegetables introduce eggs into the mouth.
• Eggs hatch in the intestine and move to the bloodstream.

Pinworms

• Only humans are affected; common in humans.
• Invade the intestinal tract, often through ingestion of contaminated soil.
• Females lay eggs near the anus, causing intense itching.