• 1) Subphylum Chelicerata (sea spiders, horseshoe crabs, spiders, mites)

  a) 2 tagmata, bear chelicerae in place of 1st antenna, uniramous legs

  b) Cl. Pycnogonida: 1 pr ovigers, 4 pr walking legs, tiny abdomen

  c) Cl. Merostomata: 'living fossils', only 4 living species; 5 pr walking legs (first 4 with claws), abdominal book gills, tail spine

  d) Cl. Arachnida: most diverse class; 1 pr pedipalps, 4 pr walking legs, abdomen lacks walking legs, compound eyes lost

• 2) Subphylum Trilobitomorpha (extinct trilobites)

  a) 3 tagmata; head with 1 pr antennae & 4 pr legs like thoracic legs

  b) variable number of thoracic segments with similar biramous limbs

  c) variable number of abdominal segments fused as pygidium

• 3) Subphylum Uniramia (myriapods & insects)

  a) 2nd antennae lost, uniramous limbs, spiracle/trachea system

  b) SupCl. Myriapoda: 2 tagmata, trunk with many similar legs, compound eyes lost

  - Cl.Chilopoda: 15 - 177 pr of legs; 1st pr bear poison claws; mostly predatory
  - Cl. Diplopoda: 100 - 700 pr of legs; adjacent trunk segments fuse yielding 2 pr legs per body unit (diplopodia); mostly herbivorous

  c) SupCl. Hexapoda: 3 tagmata, 2nd maxillae fuse as labium, thorax with 3 pr walking legs, abdomen lacks limbs

  - Cl. Apterygota: insects lacking wings
  - Cl. Pterygota: winged insects
  - SubCl. Exopterygota: hemimetabolous life history
  - SubCl. Endopterygota: holometabolous life history

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