Other genes affect whether crustacean limbs are for feeding or for swimming. Familiar ones, e.g., lobsters, crayfish, and crabs, have a body plan of a cephalothorax under the protection of a fused shell or carapace . Gills are both off of the body and off of limbs. Their mouthparts of chelicerae and pedipalps are very limb-like. For instance, spiders have book lungs or tracheae to minimize water loss from moist respiratory surfaces.
Arachnids often have mating rituals.
Development of the young is direct, with no larval stages. Pedipalps modified into pincers. Spiders poison their prey, then pour digestive enzymes into the punctured body to create a liquid meal. Pedipalps are modified in male spiders to help with sperm delivery. 8 simple eyes keep watch.