Arthropoda, a phylum within Kingdom Animalia, is the largest and most diverse of all the phyla in the Animal Kingdom. Four out of five animal species on earth today are arthropods. Ninety percent of arthropods are insects; the other ten percent of this diverse and prolific group includes crabs, spiders, scorpion, and a bucnh of lesser-known animals. All arthropods are bilaterally symmetrical, and their bodies are covered with a tough exoskeleton. This exoskeleton consists of layers of chitin and proteins; its hard covering protects the animal, and provides points of attachment for muscle cells. There are five main groups of arthropods: horseshoe crabs, arachnids, crustaceans, centipedes and millipedes, and insects. Overall, Arthropoda is the most successful phylum in terms of diversity, geographical distribution, and population.