Earthworm Anatomy and Physiology

Segmentation in Annelids

  • Segmentation supports:
    • Greater flexibility.
    • Movement efficiency.
    • Specialization of body regions.
    • Independent movement of body parts.
    • Easier regeneration and damage control.

Earthworm Classification

  • Genus: Lumbricus
  • Species: terrestris
  • Full scientific name: Lumbricus terrestris

Other Annelids

  • Leeches (e.g., Hirudo medicinalis)
  • Polychaetes (marine worms with bristles, e.g., Nereis)
  • Other types of earthworms in different genera

Symmetry of Annelids

  • Annelids exhibit bilateral symmetry.
    • The body can be divided into two mirror-image halves.

Coelom Definition

  • A coelom is a fluid-filled body cavity completely lined by mesoderm tissue.
  • Functions:
    • Acts as a hydrostatic skeleton.
    • Provides space for organs to grow and move.

Basic Body Plan of Segmented Worms

  • Segmentation: Repeated sections (metameres).
  • Head region with sensory structures.
  • Digestive system: mouth → pharynx → esophagus → crop → gizzard → intestine → anus
  • Coelom in each segment.
  • Closed circulatory system.
  • Setae (bristle-like structures for movement).
  • Nervous system with a ventral nerve cord and ganglia.

Earthworm Development

  • Time for young earthworms to emerge from eggs: approximately 3 to 4 weeks, sometimes up to 5 weeks depending on conditions.

Earthworm Feeding

  • Earthworms ingest soil containing organic matter (decaying leaves, microorganisms).
  • They digest the organic material.
  • They expel the rest as castings.

Benefits of Earthworms to Soil and Plants

  • Aerating the soil through burrowing.
  • Improving drainage.
  • Breaking down organic matter into nutrients.
  • Mixing soil layers.
  • Producing castings rich in nitrogen, phosphorus, and potassium, which fertilize plants.

Clitellum Function

  • The clitellum is a thick, band-like structure near the front of the worm.
  • Main function: produce mucus and a cocoon for reproduction.
  • During mating, it secretes a mucus ring that collects eggs and sperm, forming a protective cocoon where baby worms develop.

Nephridia

  • Nephridia are excretory organs, similar to tiny kidneys.
  • Each segment (except the first few and the last) has a pair of nephridia.
  • Function: remove waste from the coelomic fluid and blood.
  • Helps the worm get rid of excess nitrogen and maintain internal balance.

Systems Visible in a Dissected Worm

  • Digestive system (mouth, pharynx, esophagus, crop, gizzard, intestine).
  • Circulatory system (dorsal and ventral blood vessels, aortic arches or "hearts").
  • Nervous system (ventral nerve cord, simple brain/ganglia).
  • Reproductive system (clitellum, seminal vesicles, testes/ovaries in mature worms).

Earthworm Digestive Tract

  • Earthworms have a complete digestive tract with two openings: a mouth and an anus.
  • It runs from the anterior mouth to the posterior anus, passing through specialized digestive organs.

Order of Digestive Structures

  • Mouth
  • Pharynx
  • Esophagus
  • Crop
  • Gizzard
  • Intestine
  • Anus

Pharynx Function

  • The pharynx acts as a muscular pump.
  • Pulls in food (soil and organic matter) from the mouth.
  • Pushes it into the esophagus.
  • It does not chemically digest food, but it helps move it along.

Crop Function

  • The crop is a thin-walled, expandable organ.
  • Temporarily stores food before it passes to the gizzard.
  • It allows the worm to eat a lot quickly and digest it slowly.

Location of the Brain

  • The simple brain, or cerebral ganglia, is located above the pharynx in the anterior segment, close to the mouth.

How to See the Ventral Nerve Cord

  • Place the worm dorsal side up and cut through the dorsal side.
  • Pin back the skin and body wall.
  • Move internal organs (like the intestine) aside or remove them.
  • This exposes the ventral nerve cord running along the bottom (ventral) inside surface.

Gizzard Function

  • The gizzard is a muscular organ.
  • It grinds food using small bits of sand and soil the worm has swallowed.
  • This breaks down the food before it enters the intestine for absorption.

Intestine Function

  • In the intestine, nutrients are absorbed into the bloodstream through the intestinal walls.
  • These nutrients are then transported throughout the worm’s body to provide energy and support growth and repair.