Q Concerning germ layers and the presence of coelom how are arthropods characterized?
Arthropods are triploblastic and coelomate beings (they have three germ layers).
Q. Considering the presence of segmentation (metameres) in their body to which other already studied phylum are arthropods proximal?
Considering their metameric feature arthropods are proximal to annelids that as well have segmented bodies. In the embryonic development of some arthropods there are fusions of metameres forming structures like, for instance, the cephalothorax of arachnids.