Phylum Annelida

The segmented worms. Our beloved fish food has many interesting relatives.


  1. Unifying Characteristics
    1. Marine, freshwater, terrestrial; most free-living
    2. Vermiform, bilaterally symmetrical, segmented
    3. Hydrostatic skeleton
    4. Chaetae = hard bristle-like structures projecting from the body wall
    5. Triploblastic; body wall muscles circular and longitudinal
    6. Pre-oral ganglia of central nervous system linked by pair of connectives to ventral ganglionated nerve cord
    7. Nephridia and coelomoducts for excretory and reproductive purposes
    8. Protostome, Triploblastic, Coelomate
    9. Closed, tubular circulation
    10. Reproduction may involve copulation; spiral determinate cleavage


    II.   Classification

    1. Class Polychaeta (many bristles)
      1. Parapodia = lateral appendages; biramous appendages = 2 lobed, dorsal lobe = notopodium; ventral lobe = neuropodium
      2. Bristles large and numerous
      3. Distinct cephalization; head region with eyes and tentecles
      4. Dioecious
      5. Planktonic, trochophore larvae
      6. Mostly free-living larvae
      7. Subclasses
        1. Subclasses Errantia
          1. very active
          2. well developes parapods
          3. numerous segments
          4. mostly benthic predators
          5. examples: Aphrodite (Blood Worm), Neries, Glycera
        1. Subclass Sententaria
          1. sedentary, do not move
          2. tube dwellers in soft sediments
          3. parapodia reduced
          4. Examples: Chaetopterus (Parchment Worm), Feather Duster Worm
            B.  Class Oligochaeta
      1. No parapodia
      2. Setae present but reduces; mostly on ventral surface
      3. No distinct head or tentacles
      4. Hermaphroditic
      5. Free-living larvae
      6. Internal fertilization
      7. Examples: Tubifex (low oxygen, good pollution indicators), Branchiobdella (parasites on crayfish gills), Lumbricus terrestris (common earthworm)

            C.  Class Hirudinea

      1. 34 internal segments, many outer annuli
      2. No parapodia or setae
      3. Anterior and posterior suckers
      4. Reduced coelum
      5. Hermaphroditic
      6. Internal fertilization
      7. Feed on blood, anti-coagulant activity (Hirundin).