Arthropods are the group of animals that have jointed appendages. They are distinguished from other animals by their external skeletons and segmented bodies. Arthropods include beetles, spiders, crabs, scorpions and centipedes.
All arthropods are made up of segments, and each segment contains a pair of appendages, even if the segments have become fused together to form what looks like a single mass. The appendages themselves are segmented. Some are uniramous, meaning that the segments do not branch, while other are biramous, meaning that the appendage contains branches.
The segments of an arthropod function as the bones do in a human or other similar creature. They are made up of chitin, a tough fibrous substance that is flexible yet strong. The muscles that power each appendage attach to the inner wall of the segments, a situation exactly opposite to that seen in humans, where the muscles lie on top of rather than inside of the structures to which they attach.
The appendages of arthropods include but are not confined to legs. The antennae or feelers present on many insects and crustaceans are actually modified appendages, and so are the spinnerets of spiders. Maxillae, which resemble jaws, and mandibles, which resemble large fangs, are appendages modified for chewing and biting. Some arthropods have appendages that function as reproductive organs.