Biology 11: Annelida Notes

• Annelids


  • Segmented bodies
  • True coelom lined with mesoderm tissue
  • Tube within a tube digestive tract that passes food from Mouth to Anus
• Feeding & Digestion in Annelids

  • Mouth
  • Pharynx
  • Esophagus
  • Crop
  • Gizzard
  • Intestine
• Annelid feeding


  Range from filter feeders to predators
• Carnivorous annelids


  • Pharynx can hold 2-3 sharp jaws to attack prey
• Earthworm feeding

  • Pharynx pumps food and dirt into esophagus
  • Food stored in crop
  • Ground in gizzard
  • Absorbed in intestines
• Annelid circulatory system

  • Closed circulatory system with blood in network of blood vessels
  • Dorsal and ventral blood vessels
  • Connecting vessels in each segment
• Earthworm blood circulation

  • Dorsal vessel moves blood towards head
  • Ventral vessel moves blood from head to tail
• Dorsal blood vessel


  Acts as a heart, contracting rhythmically to pump blood
• Aquatic annelid respiration


  • Breathe through gills
• Earthworm respiration


  • Take in oxygen and release carbon dioxide through moist skin
  • Secrete mucus coating to keep skin moist
• Annelid excretion

  • Digestive waste excreted through anus
  • Cellular waste excreted by nephridia
• Annelid nervous system

  • Well-developed with brain and nerve cords
  • Sense organs more developed in free-living marine species
• Annelid sense organs


  • Sensory tentacles
  • Chemical receptors
  • Statocysts
  • Eyes
• Annelid movement


  • Longitudinal muscles contract to shorten and fatten
  • Circular muscles contract to elongate
  • Setae prevent slipping
• Setae

  Stiff hair-like structures on annelid bodies
• Earthworm movement

  • Setae help cling to burrow walls
  • Circular muscles contract to elongate
  • Longitudinal muscles contract to fatten
• Annelid reproduction

  • Most reproduce sexually
  • Earthworms and leeches are hermaphrodites
• Earthworm reproduction


  1. Worms attach and exchange sperm

  2. Sperm stored in special sacs

  3. Eggs fertilized when released into mucus ring secreted by clitellum

  4. Mucus ring forms protective cocoon
• Clitellum


  Thickened band of segments on earthworm
• Annelid groups


  • Oligochaetes (earthworms and relatives)
  • Leeches
  • Polychaetes (marine paddle-like annelids)
• Oligochaetes


  • Streamlined bodies with fewer setae than polychaetes
  • Live in soil or fresh water
• Leeches


  • External parasites that suck blood and body fluids
  • Have powerful suckers at both ends
• Polychaetes


  Marine annelids with paired paddle-like appendages tipped with setae
• an earthworm has more light sensing cells in its anterior and posterior segments than in other parts of its body, how is this advantageous to them?


  This feature helps them detect if something is in front of them (like a predator closing in) or coming from the back, how close they are to the surface if they want to resurface.