Annelida

• Coelomate


  Having a body cavity
• Phylum Annelida


  • Segmented worms
  • Body built of repeated units
  • Allows for specialization
• Body plan of Annelids

  1. Head has well-developed cerebral ganglion

  2. Sensory organs in ring like segments

  3. Many species have eyes

  4. Segments divided internally by septa

  5. Each segment has a pair of excretory organs, a ganglion, and locomotory structure

  6. Closed circulatory system

  7. Ventral nerve cord

  8. Hydrostatic skeleton
• Hydrostatic skeleton


  Skeleton cushioned on fluid
• Annelid locomotion


  1. Coelomic fluid creates a hydrostatic skeleton

  2. Alternating muscle contractions allows complex movements

  3. Chaetae (setae) - bristles of chitin found in most groups provide traction
• Annelid classes


  • Polychaeta (Polychaetes)
  • Oligochaeta
  • Hirudinea (Leeches)
• Polychaetes


  • Have paired parapodia on most segments
  • Fleshy, paddle-like lateral projections used in locomotion or gas exchange
  • Chaetae on parapodia
  • Most Dioecious (separate sexes)
  • External fertilization
  • Trochophore larva
• Oligochaetes

  • Clitellum found in all members
  • Earthworms have head not well differentiated, no parapodia, few chaetae project from body wall, hermaphroditic but cross-fertilize, clitellum secretes mucus cocoon
• Leeches


  • Occur mostly in freshwater
  • Usually flattened dorsoventrally
  • Hermaphroditic and cross-fertilization
  • Clitellum develops only during breeding season
  • Coleom reduced, not divided into segments
  • Suckers at both ends of body
  • No chaetae (except for one species)
  • Some eat detritus, others suck blood