The answer is jointed legs.
Arthropods has another smaller classification, which includes insects, arachnids, crustaceans and myriapods. Some of their common characteristics includes segmented body, jointed legs etc.
Others like hard shells or gills are only present in certain types of Arthropods, which in this case is crustaceans.
Meanwhile Arthropods has exoskeleton, instead of endoskeleton. It's like a hard shell outside of their body, to protect them.
