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. Gas exchange is from tracheal tubes that go down into body through exoskeleton, book lungs, both. Book lungs are plates of vascularized tissue, like pages of a book.