Segmented Worms
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Phylum Annelida

  • Includes duster worms, earthworms, & leeches
  • Abundant in all habitats
  • Have a true coelom fully lined with mesoderm
  • Body divided into external segments called metameres (metamerism)

  • Metameres correspond to internal segments
  • Have a one-way digestive system with a mouth & anus
  • Well developed brain & sensory organs
  • Fluid-filled coelom provides hydrostatic skeleton
  • Most have external bristles or setae that aid movement
  • Setae may be modified into flashy appendages called parapodia


  • Includes 3 classes based on number of setae & presence or absence of parapodia
  • Classes of segmented worms — Oligochaeta, Polychaeta, & Hirudenia

Class Oligochaeta

  • Have no parapodia & few setae
  • Includes earthworms


  •  Bodies may have over 100 metameres
  • Internal partitions called septa
  • Distinct anterior & posterior ends
  • Cephalization (head with sense organs) shows specialization for burrowing
  • Have both circular & longitudinal muscles for movement
  • Have external, saddle-shaped structure called clitellum that forms a cocoon containing eggs & sperm
  • Prostomium or lip digs through soil as earthworm feeds on organic matter
  • Pharynx is a muscular organ behind the mouth to help suck in food
  • Food temporarily stored in crop, ground in gizzard, and digested & absorbed in intestine
  • Wastes called castings pass out through anus
  • Closed circulatory system with 5 pairs of aortic arches or hearts
  • Dorsal blood vessel carries blood posteriorly to cells & ventral blood vessel returns blood anteriorly

  • Secrete mucus to keep skin moist so oxygen will dissolve & diffuse into body
  • Long tubules called nephridia filter wastes from blood & excrete it through pores
  • Simple brain, no eyes, & dorsal and ventral nerve cords
  • Sensitive to light, touch, moisture, chemicals, temperature, & vibrations
  • Hermaphrodites exchange sperm & cross-fertilize
  • Sperm sacs store the worm’s own sperm & seminal receptacles store exchanged sperm

Class Hirudenia

  • No setae or parapodia
  • Includes leeches
  • Have anterior & posterior suckers for attachment


  • Some suck blood from hosts, while others are scavengers or predators
  • Mouth’s of blood-sucking leeches with chitinous teeth & secrete anticoagulant
  • Found in freshwater
  • Flattened dorso-ventrally
  • Hermaphrodites that cross-fertilize

Class Polychaeta

  • Marine
  • Includes sandworms & clamworms
  • Have paddle-like parapodia to move
  • Take in oxygen through parapodia
  • Some are free-swimming predators with strong jaws to feed on small animals
  • Many live commensally with sponges, mollusks, & echinoderms
  • Well-developed head with antenna & specialized mouthparts