Phylum Sipuncula

Phylum Echiura

Phylum Onychophora

Phylum Arthropoda

• Subphylum Trilobitomorpha (trilobites)

• Subphylum Chelicerata
  • Class Merostomata
  • Subclass Xiphosura
  • Subclass Eutypterida

Recommended Reading

• Order Scorpiones (scorpions)
• Order Palpigradi
• Order Uropygi
• Order Schizomida
• Order Amblypygi
• Order Araneae (spiders)
  • Suborder Orthognatha (Avicularioidea)
    • Family Ctenizidae
    • Family Theraphosidae (Aviculariidae)
    • Family Antrodiaetidae
    • Family Mecicobothriidae
    • Family Dipluidae
    • Family Atypidae

  • Suborder Labidognatha
    • Section Cribellatae
      • Family Hypochilidae
      • Family Hypochilidae
      • Family Filistatidae
      • Family Zoropsidae
      • Family Amaurobiidae
      • Family Dinopidae
      • Family Uloboridae
      • Family Uloboridae
      • Family Dictynidae

    • Section Ecribellatae
      • Family Dysderidae
      • Family Caponiidae
      • Family Segestriidae
      • Family Oonopidae
      • Family Seytodidae
      • Family Diguetidae
      • Family Plectreuridae
      • Family Loxoscelidae
      • Family Leptonetidae
      • Family Prodidomidae
      • Family Gnaphosidae
      • Family Zodariidae
      • Family Homalonychidae
      • Family Hersiliidae
      • Family Pholcidae
      • Family Theridiidae
      • Family Linyphiidae
      • Family Erigonidae
      • Family Mimetidae
      • Family Araneidae
      • Family Theridiosomatidae
      • Family Tetragnathidae
      • Family Thomisidae
      • Family Selenopidae
      • Family Heteropididae
      • Family Ctenidae
      • Family Clubionidae
      • Family Anyphaenidae
      • Family Agelenidae
      • Family Hahniidae
      • Family Pisauridae
      • Family Lycosidae
      • Family Oxyopidae
      • Family Lyssomanidae
      • Family Salticidae

  • Order Ricinulei
  • Order Opiliones
  • Order Cyphophthalmi
  • Order Acarina
  • Order Opiliones
  • Order Acarina
  • Order Pseudoscorpiones
  • Order Solifugae

• Class Pycnogonida

Spiders and Their Kin (A Golden Guide from St. Martin's Press) is the best book on the subject for North America (and beyond). It's a great introduction to arachnids. Highly recommended for every library. 

• Class Cephalocarida
• Class Branchiopoda
  • Order Notostraca
  • Order Conchostraca
  • Order Cladocera
  • Order Anostraca

• Class Remipedia
• Class Maxillopoda
  • Subclass Mystacocarida
  • Subclass Branchiura
  • Subclass Copepoda
    • Order Calanoida
    • Order Harpacticoida
    • Order Cyclopoida
    • Order Poecilostomatoida
    • Order Siphonostomatoida
    • Order Monstrilloida
    • Order Misophryoida
    • Order Mormonilloida

  • Subclass Cirripedia
    • Order Ascothoracica
    • Order Acrothoracica
    • Order Thoracica
    • Order Rhizocephala

• Subclass Myodocopa
  • Order Myodocopida
  • Order Halocyprida

• Subclass Podocopa
  • Order Platycopida
  • Order Podocopida

• Subclass Palaeocopa

• Subclass Phyllocarida
• Subclass Hoplocarida
  • Order Stomatopoda

• Subclass Eumalacostraca
  • Superorder Syncarida
    • Order Anaspidacea
    • Order Bathynellacea

  • Superorder Pancarida
    • Order Thermosbaenacea

  • Superorder Peracarida
    • Order Mysidacea
    • Order Cumacea
    • Order Amphipoda
    • Order Isopoda
    • Order Tanaidacea

  • Superorder Eucarida
    • Order Euphausiacea
    • Order Decapoda