• 1) Cl. OLIGOCHAETA (earthworms & relatives, approx. 3,000 spp)
  
  a) remarkably little morphological diversity; mainly detritivores
  b) differ from polychaetes in 8 ways: no parapodia; few setae; highly compartmentalized coelom throughout class; small, simple prostomium (no sensory structures); no eversible proboscis; entirely hermaphroditic; gonads occur in only a few segments; a clitellum secretes the cocoon in which eggs are laid
  c) burrow via retrograde peristaltic waves of body-wall muscles

• 2) Cl. HIRUDINEA (leeches, approx. 500 spp)
  
  a) share 5 traits with oligochaetes: no parapodia; no prostomial appendages; hermaphroditic; gonads occur in only a few segments; a clitellum (seasonal)
  b) differ from oligochaetes in 5 ways: no setae; coelom not compartmentalized; body dorso-ventrally compressed; anterior and posterior suckers formed by fusion of segments; constant number of body segments (33 or 34 throughout class)
  c) some have eversible proboscis or jaws
  d) includes both parasitic and free living groups

TWO MINOR PROTOSTOME PHYLA
• 1) Ph. ECHIURA (innkeeper worms, approx. 100 marine spp)
  
  a) body of two distinct parts: trunk & large flap-like proboscis
  b) have one large coelom & coiled gut with terminal mouth & anus
  c) linked to annelids by: annelid cross, paired setae in setal sacs

• 2) Ph. POGONOPHORA (beard worms, approx. 100 marine spp)
  
  a) live mainly in deep sea; reach large size in vent communities
  b) body of three distinct parts: i) forepart with tentacles, ii) trunk, iii) large segmented opisthoma; produce long chitinous tubes
  c) larvae have a complete gut like an annelid worm; lack a gut as adults but possess symbiotic bacteria in tentacles or trophosome
  d) linked to annelids by features of opisthoma (fully segmented coelom; setae in setal sacs) and spiral cleavage

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