How many classes arthropods?

Phylum Arthropoda hosts at least eighteen classes, (not counting those yet to be fully classified):

Trilobita - trilobites (extinct)

Arachnida - Spiders, scorpions, etc.

Merostomata - horseshoe crabs, eurypterids (extinct), etc.

Pycnogonida - sea spiders

Chilopoda - centipedes

Diplopoda - millipedes

Pauropoda - sister group to millipedes

Symphyla - resemble centipedes

Branchiopoda - brine shrimp etc.

Remipedia - blind crustaceans

Cephalocarida - horseshoe shrimp

Maxillopoda - barnacles, copepods, fish lice, etc.

Ostracoda - seed shrimp

Malacostraca - lobsters, crabs, shrimp, etc.

Insecta - insects

Entognatha - wingless

Camptophyllia (extinct)

Marrellomorpha (with one extinct species)