Annelida

• 15,000 species

• triploblastic

• coelomate

• segmented

  • annel + ida = “little rings”

• 1 mm - 3 m

  • one of the advantages of worm form

Segmentation

• serial repetition of similar parts

• mesoderm is separated in segmental blocks of muscle

• separated by septa

  • what is the primary advantage?

    • locomotion

Anatomy

• segmentation and shape necessitate a circulatory system

• Closed circulatory system

Taxa

• class polychaeta

• class clitellata

  • subclass Oligochaeta

  • subclass Hirudinea

Polychaeta

• pairs of paddle-like appendages:

  • parapodia

    • used for locomotion

    • each appendage has numerous chaetae (namesake)

      • bristles made of chitin

    • many blood vessels allow them to function as gills too

Reproduction

• separate sexes

• external sperm transfer and fertilization

• planktonic trochophore larvae (like Mollusca)

Feeding

• mostly marine

• many dwell in the substrate or tubes

• other are mobile carnivores

Clitellata: Subclass Oligochaeta

• few chaetae

• include earthworms

• eat through the soil

  • extract nutrients as soil passes through gut

  • aerate soil and deposit “castings”

Reproduction

• heraphrodictic cross-fertilizers

• pairs exchange sperm

• sperm are stored temporarily

• Clitellum secretes a cocoon of mucus

• slides along worm picking up eggs and sperm

• cocoon stays in soul while embryos develop

Clitellata: Subclass Hirudinea

• “leeches”

• mostly freshwater

• predators and parasites

  • use jaws to slit skin of host

  • secrete anesthetic and anti-coagulant

  • use crop to store large amounts of blood

• used in medicine