Hard segmented exoskeleton with head, thorax, abdomen. Each segment had pair of biramous appendages of an inner walking leg and outer branch with gills Probably evolved into marine chelicerates like horseshoe crabs. Other marine chelicerates moved to fresh water, became arachnids, then arachnids moved onto land. Long tail used in locomotion. Chelicerae are 1st pair of appendages.
Spiders are fanglike to penetrate prey, inject poison.
Pedipalps are 2nd pair of appendages. Spiders use pedipalps to hold and chew food. Scorpion pedipalps are pincers. Most are carnivorous. Spiders have 8 eyes in two rows that detect movement and locate stuff.