Each retinal cell transmits a tiny bit of picture to brain, but with so many, it seems a continuous whole. In contrast, the arthropod compound eye is made of many individual eyes. The eye does not move, nor can it be focused. Each catches a small part of the scene and leaves a coarse-grained picture. 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.