[A] Agnatha

• The term for the two living groups of Agnathans

  Cyclostomes
• Ancient vertebrate fishes encased in bony armor

  Ostracoderms
• Myxinoidea (Hagfishes)

  • Deep-sea mud burrowers, eel-like scavengers
  • Feed on dead or dying invertebrates
  • Structurally hermaphroditic, but only one gonad is functioning
  • No vertebrae on the notochord of adults 
  • Vertebrate-like elements in embryonic myxinoids
  • Salt concentration inside the body is similar to surroundings
• Petromyzontiformes (Lampreys)

  • Can be parasitic or free-living 
  • Suctorial oval mouth
  • Grasping onto substrate 
  • Clinging onto prey (in parasitic species) 
  • Rough tongue for rasping flesh 
  • All species spawn in freshwater 
  • Ammocoete larva 
  • Suspension feeder 
  • Structures similar to amphioxi and tunicates
• The only living vertebrate whose ancestors never lived in freshwater, but remained in salt water

  Hagfishes or Myxinoidea
• The ammocoete larva is a suspension feeder, all spawn in fresh water
• Ostracoderms

  • Appeared in Late Cambrian 
  • First vertebrates to possess lateral-line system and bone 
  • bone  as exoskeleton armor (bony head shield) 
  • Endoskeleton made of cartilage when present
  • Poor swimmers
  • sediment-feeders