The more facets, the more detail. Homology is a structural similarity regardless of present day function. Recently, molecular biologists indicate that there may be genetic homologies among the wide variety of arthropod limbs. 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.