• 1) SubPh.CHELICERATA (sea spiders, horseshoe crabs, spiders, mites)
  
  a) 2 tagmata, bear chelicerae in place of 1st antenna, uniramous legs
  b) Cl. PYCNOGONIDA (~600 marine spp.): 1 pair of ovigers, 4 pair of walking legs, tiny abdomen
  c) Cl. MEROSTOMATA (~50 marine spp.): 'living fossils' because 400 million-year old fossils very similar to living forms, only 4 living species; 5 pairs of walking legs (first 4 with claws), abdominal book gills, tail spine
  d) Cl. ARACHNIDA (~70,000 mostly terrestrial, a few marine & freshwater spp.): most diverse class; 1 pair of pedipalps, 4 pair of walking legs, abdomen lacks walking legs, compound eyes lost,

• 2) SubPh. TRILOBITOMORPHA (extinct trilobites)
  
  a) 3 tagmata; head with 1 pair antennae & 4 pair of thoracic-like legs
  b) variable number of thoracic segments with similar biramous limbs
  c) variable number of abdominal segments fused as pygidium

• 3) SubPh. UNIRAMIA (myriapods & insects)
  
  a) 2nd antennae lost, uniramous limbs, spiracle/trachea system
  b) SupCl. MYRIAPODA: 2 tagmata (head, trunk); the trunk has many similar legs, compound eyes are lost

  - Cl. CHILOPODA: 15 - 177 pairs of legs; 1st pair bears poison claws; mostly predatory
  - Cl. DIPLOPODA: 100 - 700 pairs of legs; adjacent trunk segments fuse yielding 2 pairs of legs per body unit (diplosegment); mostly herbivores

  • c) SupCl. HEXAPODA (=Insecta): 3 tagmata; 2nd maxillae fuse as labium; thorax with 3 pairs of walking legs; abdomen lacks limbs

    - Cl. APTERYGOTA: insects lacking wings
    - Cl. PTERYGOTA: winged insects

    
    - SubCl. EXOPTERYGOTA: hemimetabolous life history
    - SubCl. ENDOPTERYGOTA: holometabolous life history
    
    - most successful arthropod group, but very curiously never successfully invaded the sea

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