Earthworm Anatomy and Physiology

Nervous System

• Main Function: Controls body activities and responses to stimuli.
• Parts & Functions:
  • Cerebral Ganglia (Brain): Small, paired lobes above the pharynx; coordinate sensory input and movement.
  • Subpharyngeal Ganglia: Relay center located below the pharynx.
  • Ventral Nerve Cord: Runs along the belly; sends and receives signals to/from body segments.
  • Segmental Ganglia: Mini "brains" in each segment that control muscles locally.

Circulatory System

• Main Function: Transports oxygen, nutrients, and waste.
• Type: Closed circulatory system (blood stays in vessels)
• Parts & Functions:
  • Dorsal Blood Vessel: Main vessel on the back; carries blood forward.
  • Ventral Blood Vessel: On the belly side; carries blood backward.
  • Aortic Arches (“Hearts”): Usually 5 pairs; pump blood between dorsal and ventral vessels.
  • Capillaries: Tiny vessels in the body wall; site of gas and nutrient exchange.

Respiratory System

• Main Function: Gas exchange (oxygen in, carbon dioxide out).
• Parts & Functions:
  • Skin: Earthworms breathe through their skin. It must stay moist for diffusion of gases.
  • Capillaries in Skin: Absorb oxygen and release CO_2.

Digestive System

• Main Function: Breaks down food and absorbs nutrients.
• Parts & Functions (in order of food movement):
  1. Mouth: Takes in food (soil containing organic matter).
  2. Pharynx: Muscular structure that swallows food and lubricates it with mucus.
  3. Esophagus: Narrow tube that passes food from the pharynx to the crop.
  4. Crop: Temporary storage area for food.
  5. Gizzard: Grinds food using sand/soil particles.
  6. Intestine: Digests food chemically and absorbs nutrients.
     • Typhlosole: Internal fold in the intestine that increases surface area for absorption.
  7. Anus: Removes undigested waste (casts).

Excretory System

• Main Function: Removes metabolic waste (like nitrogenous waste).
• Parts & Functions:
  • Nephridia (1 pair per segment): Excrete urea and other wastes from the blood/body fluid.
  • Nephridiopores: External openings for excretion.

Muscular System

• Main Function: Movement and flexibility.
• Parts & Functions:
  • Circular Muscles: Contract to make the body longer and thinner.
  • Longitudinal Muscles: Contract to make the body shorter and fatter.
  • Setae (tiny bristles): Anchor parts of the worm to the ground during movement.

Skeletal System

• Main Function: Support and movement.
• Type: Hydrostatic skeleton
  • No bones. Body is supported by fluid pressure in each segment.
  • Muscles work against this pressure to produce movement.

Reproductive System

• Main Function: Produces gametes and enables reproduction.
• Parts & Functions:
  • Clitellum: Thickened segment that secretes mucus and forms the cocoon for eggs.
  • Seminal Vesicles: Store and mature the worm’s own sperm.
  • Seminal Receptacles: Store sperm received from another worm.
  • Testes: Produce sperm.
  • Ovaries: Produce eggs.
  • Cocoon: Formed around the clitellum where fertilization and embryonic development occur.

Integumentary System

• Main Function: Protection, respiration, sensation.
• Parts & Functions:
  • Epidermis: Outer skin layer.
  • Cuticle: Thin, protective layer secreted by epidermis; must stay moist.
  • Mucus Glands: Keep skin moist for gas exchange.

Endocrine System (Simple Hormonal Control)

• Main Function: Regulates growth, regeneration, and reproduction.
• Parts & Functions:
  • Neurosecretory Cells in Brain: Release hormones to regulate body functions like regeneration and gamete production.