Lecture 9 - Annelida

• What are the 3 classes under Phylum Annelida?

  • Class Polychaeta
  • Class Oligochaeta
  • Class Hirudinea
• Members of phyla Echiura and Sipuncula are benthic marine animals with unsegmented bodies.
• Molecular sequence data place echiurans within phylum Annelida.
• Echiurans - Sister taxon to Annelida
• Sipunculans - Sister taxon to a clade composed of Annelida and Echiura.
• Annelids are protostome coelomates in superphylum Lophotrochozoa.

  • Spiral, determinate cleavage.
• The evolutionary innovation shown by annelids is segmentation (metamerism).

  • Segmentation evolved separately in annelids, arthropods, and chordates.
• Many annelids have chitinous bristles called setae.

  • Help in locomotion
  • Anchor worm in place
  • Deter predators
• • Polychaetes use setae for filtering food, movement, and attachment
  • Oligochaete help prevent slipping while burrowing
  • Hirudinea use suckers for attachment to prey
• Prostomium – anterior part followed by segmented body.
• Pygidium – terminal portion.
• Peritonia (layers of mesodermal epithelium) of adjacent segments meet to form septa.
• Fluid-filled coelom acts as a hydrostatic skeleton.
• Polychaeta is a paraphyletic class. (Aclitellata)
• Oligochaeta and Hirudinida form a monophyletic group called Clitellata.

  • Characterized by reproductive structure called a clitellum.
• Polychaeta is the largest, most diverse class.

  • May be brightly colored, variable shape.
• Polychaetes have some features other annelids do not:

  • A well developed head.
  • Paired appendages, parapodia, that function as gills and aid in locomotion.
  • No clitellum.
  • Many setae
• Feeding structures of Polychates

  • Errant - Have jaws for feeding
  • Sedentary - Posses tentacles
• • Sedentary forms often have elaborate devices for feeding and respiration. Filter or deposit feeders.
• • Errant forms include pelagic and benthic types and are often predators or scavengers.
• Representative Polychaetes

  • Clam Worms: Nereis
  • Scale worms
  • Fireworms
  • Tubeworms
  • Fanworms or Featherduster worms
  • Parchment Worms
• Clam Worms: Nereis

  • Errant polychaetes
  • Live in mucus-lined burrows near low tide level.
  • Come out of hiding places at night to search for food.
  • Prostomium bears a pair of palps sensitive to touch and taste, a pair of short sensory tentacles, and two small dorsal eyes sensitive to light.
• Scale worms

  • Flattened bodies are covered with broad scales.
  • Some are large, all are carnivores and some are commensals in burrows of other organisms.
• Fireworms

  • Have hollow, brittle setae that contain poisonous secretions.
  • Feed on cnidarians.
• Tubeworms

  • Tube-dwellers
  • May line their burrows with mucus
  • Use cilia or mucus to obtain food
• Fanworms or Featherduster worms

  • Unfurl tentacular crowns to feed.
  • Food moved from radioles to mouth by ciliary action.
• Parchment Worms

  • Lives in a U-shaped tube.
  • Modified segments pump water through tube.
• Clade Siboglinidae

  • Formerly members of phylum Pogonophora (beardworms).
  • Discovered in 1900.
  • 150 species described.
  • Most are small, less than 1 mm in diameter.
  • Giant beardworms that live in deepwater hydrothermal vents are 3 m long and 5 cm in diameter.
• Clade Clitellata

  • Class Oligochaeta and Class Hirudinida
  • Form reproductive structure called a clitellum.
  • Members are derived annelids that lack parapodia.
  • Hermaphroditic (monoecious) animals that exhibit direct development.
• Class Oligochaeta includes earthworms and many freshwater worms.

  • They possess setae, but not as much as polychaetes.
• Class Oligochaeta - Reproduction

  • Earthworms are hermaphroditic – male and female organs in the same animal.
  • When mating, two worms are held together by mucus secreted by the clitellum.
• Class Oligochaeta - Feeding

  • Food is stored in a thin-walled crop.
  • Muscular gizzard grinds food into small pieces.
  • Digestion and absorption occur in intestine.
• Class Oligochaeta - Nervous System and Sense Organs

  • Pair of cerebral ganglia connect around the pharynx to the ganglia of the ventral nerve cord.
  • Neurosecretory cells in brain and ganglia secrete neurohormones.
• Class Oligochaeta - General Behavior

  • Avoid bright light (negative phototaxis).
  • Chemical stimuli are important in locating food.
  • Limited learning ability - primarily trial- and-error learning.
• Class Hirudinea includes the leeches.

  • Primarily freshwater, a few marine & terrestrial.
  • More common in tropical climates.
• Leeches are hermaphroditic and have a clitellum (only appears during breeding season), like oligochaetes.
• Chemical stimuli are important in locating food.
• 10 to 17 pairs of nephridia.
• Coelomocytes and other special cells may assist in excretion. 
• 21 pairs of segmental ganglia in between along a double nerve cord.