Annelida

• Annelida


  Segmented worms

  
• Classes of Annelids


  • Polychaeta
  • Oligochaeta
  • Hirudinea

  
• Body cavity


  Space between outer body wall and digestive tract (Coelom)
• Peritoneum


  Covers the coelom and all organs within it

  
• Characteristics of Annelida


  • Segmentation
  • Chaetae (Hairs made of Chitin)
  • 3 Cell Layers
  • Head develops first (Protostomal)
• Polychaeta


  • Live in the ocean (marine)
  • Many chaetae (chitinous hairs)
  • Movement: Paired parapodia for swimming/crawling, may burrow using peristalsis
  • Feeding: Deposit feeders, raptorial predators, herbivores/scavengers, filter feeders
  • Reproduction: Dioecious, segmentation allows regeneration and epitoky

  
• Oligochaeta


  • Live in freshwater, land, ocean
  • Few chaetae (chitonous hairs)
  • Movement: Burrow with peristalsis, use chaetae as anchors
  • Feeding: Deposit feeders, herbivores/scavengers
  • Reproduction: Hermaphroditic
• Hirudinea


  • Mostly freshwater, some marine
  • No chaetae
  • Movement: Mostly using suckers on both ends
  • Feeding: 3/4 blood-sucking ectoparasites, 1/4 predators
  • Reproduction: Hermaphroditic
• The phylum Annelida is made up of segmented worms, numbering about 15,000 species
• Protostome


  Annelids have a coelom made from cell masses

  
• Metamerism


  Coelom is divided into a series of repeated parts, each segment is called a metamere
• Except for the head and tail region, each segment in an annelid is ring like and very similar
• Coelomate


  Annelids normally have long thin bodies composed of a series of identical segments
• Growth in Annelids


  1. Lateral enlargement of segments during juvenile stages

  2. Addition of new segments produced by the foremost section of the pygidium
• Characteristics of Annelida


  • Bilaterally symmetrical and vermiform
  • Body has more than two cell layers, tissues and organs
  • Body cavity is a true coelom, often divided by internal septa
  • Body possesses a through gut with mouth and anus
  • Body possesses 3 separate sections: prosomium, trunk, pygidium
  • Has a nervous system with anterior nerve ring, ganglia and ventral nerve chord
  • Has a true closed circulatory system
  • Has no true respiratory organs
  • Reproduction normally sexual and gonochoristic or hermaphoditic
  • Feed a wide range of material

  
• Metamere


  A section of the body wall and a compartment of the body cavity with its internal organs

  
• Pygidium


  Unsegmented terminal region of the annelid body
• Epidermis


  Covers the body wall, overlaid with a thin, pliant cuticle
• Setae/Chaetae


  Short external bristles composed of chitin, used for gripping, holding, and increasing surface area
• Digestive system


  Unsegmented gut running from mouth to anus

  
• Circulatory system


  Usually closed, with blood containing hemoglobin or other pigments, flowing towards head and returning to terminal region

  
• Respiration


  Some have thin-walled, feathery gills, but most respire directly through the body wall
• Nervous system


  Primitive brain in head region, connected to ventral nerve cord with lateral nerves and ganglia
• Sense organs


  Eyes, taste buds, tactile tentacles, statocysts (organs of equilibrium)
• Reproduction in Annelids


  • Sexual or asexual (fragmentation, budding, fission)
  • Hermaphrodites common, but most have separate sexes
  • Fertilized eggs develop into free-swimming larvae or hatch as miniatures
  • Ability to regenerate lost body parts highly developed in many
• Polychaetes reproduce sexually, with separate sexes. Fertilization is usually external, resulting in free-swimming larvae. Some undergo extreme changes and swarming behaviour at spawning time.
• Oligochaetes include about 3,500 species of earthworms and freshwater worms, mostly burrowers.

  
• Hirudinea includes 500 species of leeches, flattened predatory or parasitic annelids with suckers. Most are freshwater, some marine.

  
• Annelids have two main modes of existence: quiet life in holes, or more active lives.
• Segmentation allows flexibility and mobility, and evolved twice - in protostomes (Annelida, Arthropoda) and deuterostomes (Chordata).