Arthropods share a common ancestor with polychaete worms, and may even be a direct descendant of polychaetes. But unlike other eucoelomate invertebrates, the arthropod coelom is greatly reduced in the adult animal. All arthropods have jointed appendages . This evolutionary innovation is probably the key to the stunning success of this diverse group. There are over three times as many species of arthropods as there are of all other animals on Earth, and there may be millions more that we haven't even discovered.
Arthropods do everything with legs or modified legs.
That's one reason arthropods look so alien when we see them up close. They chew sideways, and it's all done with legs. Their bodies are protected by an tough cuticle made of proteins and chitin , a polysaccharide with added nitrogen groups. A cuticle is a tough outer layer of non living organic material. The cuticle of arthropods acts as an exoskeleton.