Their appendages make them very motile. They are segmented creatures, but the segments are fused into functional groups such as the head, abdomen, and thorax. They have an exoskeleton tha tis composed of chitin.
Some arthropods have calcium in addition to chitin in their exoskeleton for extra strength.
The exoskeleton protects from predators and dessication. Some arthropods transport oxygen directly to their muscles via trachae. Chelicerates, crustacea, and uniramates are the three subphyla in this phylum. The chelicerates subphyla is comprised mainly of arachnids. They do not have antennae.