Arthropods

Phylum Arthropoda

Arthropods are the most diverse phylum of animals, which includes spiders, scorpions, horseshoe crabs, centipedes, millipedes, insects, crustaceans, etc.

Chelicerates (Subphylum Chelicerata)

Sea Spiders (Class Pycnogonida)
Horseshoe Crabs (Class Merostomata)
Arachnids (Class Arachnida)

Harvestmen (Order Opiliones)
Scorpions (Order Scorpiones)
Pseudoscorpions (Order Pseudoscorpionida)
Sun Spiders (Order Solpugida)
Hooded Tick-spiders (Order Ricinulei)
Mites and Ticks (Order Acari)
Micros Whip Scorpions (Order Palpigradi)
Spiders (Order Araneae)
Tailless Whip Scorpions (Order Amblypigi)
Whip Spiders (Order Uropygi)
Short-tailed Whip Spiders (Order Schizomida)

Myriapods (Subphylum Myriapoda)

Millipedes (Class Diplopoda)
Centipedes (Class Chilopoda)
Pseudocentipedes (Class Symphyla)
Pauropods (Class Pauropoda)

Hexapods (Subphylum Hexapoda)

Insects (Class Insecta)

Crustaceans (Subphylum Crustacea)

Branchiopods (Class Branchiopoda)
Barnacles and Copepods (Class Maxillopoda)
Ostracods (Class Ostracoda)
Lobsters, Crabs, Shrimps, etc (Class Malacostraca)

Mantis Shrimps (Order Stomatopoda)
Sandhoppers, etc (Order Amphipoda)
Woodlice, Sea Slaters, etc (Order Isopoda)
Lobsters, Crabs, Shrimps, etc (Order Decapoda)

References

http://tolweb.org/Arthropoda/2469
http://animaldiversity.ummz.umich.edu/site/accounts/classification/Arthropoda.html#Arthropoda