Phylum ArthropodaSpiders, crabs, millipedes, insects, scorpions and lobsters

General informationLargest animal groupHave segmented bodiesBilateral symmetry, 3 cell layers

and a coelomPhylum name means “jointed feet”Has an exoskeleton made out of

protein and chitin (this is heavy and cumbersome, so arthropods can only get so large before they would collapse under there own weight)

Such a big group it is divided into four “subphyla”

Trilobita: extinct nowChelicerta: spiders, scorpions,

etcCrustacea: crabs, lobsters, etcUniramia: millipedes,

centipedes and all insects

Subphylum ChelicertaClass ArachnidaSpiders, scorpions, ticks,

mites…..All have 4 pairs (8) walking

legsMost are carnivorous

Subphylum ChelicerataClass merostomata: horseshoe crabs

“living fossils”Not really crabs at all

All chelicerates have a body divided into a cepalothorax and abdomen

Unique to chelicerates are two pairs of appendages attached near mouth:Chelicerae: spiders use these to

bite/paralyze prey, and then inject enzymes that liquefy the prey so they can suck the “juice” out

Pedipalps: for handling prey, scorpions have large ones modified into claws

All spiders produce silk, some spin webs using “spinnerets”

Ticks and mites are usually parasites

Subphylum Crustacea(not focusing on one class)

Crabs, prawns, lobster, crayfish, copepods, water fleas….

Primarily aquatic Vary widely in size (microscopic to 6

m) Usually have a hard skeleton

reinforced with calcium Usually have 2 pairs of antennae and

a pair of mandibles

Have a head, thorax and abdomen

(these may be fused into cephalothorax)

Appendages vary greatly, crayfish have claws, walking legs, swimmerets and uropods

Subphylum UniramiaClass chilipodaCentipedesI pair of legs per segmentcarnivores

Subphylum UniramiaClass DiplopodaMillipedes2 pairs of legs per segmentscavengers

Subphylum UniramiaClass Insecta

HUGE groupAll have head, thorax and

abdomen3 pairs of legs attached to

thorax