main characteristics - protostomes, largest animal phylum

• level of organization - organ systems
• body symmetry - bilateral
• alimentary structures - 2-hole tube
• triploblastic
• hemocoelomate - has blood cavity (hemocoel)
• chitin exoskeleton
  • shed/renewed as organisms grow
• jointed/paired appendages
• much segmentation
  • tagmosis - fusion of segments into functional units
• open circulatory system
• respiratory system w/ spiracles, tracheae, book lungs/gills
• sexual, dioecious reproduction (sometimes parthenogenic)

subphylum Chelicerata - have chelicerae for stinging, absorption of food

• pedipalps - used to sense surroundings (sort of like antennae)
• body segments - cephalothorax, abdomen
• book gills/lungs, tracheae
• Class Merostomata - Horseshoe crabs
• Class Arachnida - spiders, scorpions, ticks, mites

subphylum Crustacea - mandibles for chewing instead of chelicerae

• biramous - double-branched appendages
• compound eyes (multiple lenses)
• gills, book lungs
• 2 pair of antennae
• Class Malacostraca - shrimp, crabs, crayfish, krill
• Class Branchiopoda (water fleas), Copepoda (Copepods)

subphylum Uniramia - uniramous (single branch appendages) 

• mandibles
• 1 pair of antennae
• Class Chilopoda - centipedes
  • carnivores
  • 1 pair of legs per segment
• Class Diplopoda - millipedes
  • herbivores
  • 2 pairs of legs per segment
• Class Insecta (Hexapoda) - insects
  • 6 legs
  • head, thorax, abdomen
  • modified mouthparts