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.