The four groups or subphyla of arthropods are the Chelicerata, the Crustacea, the Uniramia and the Trilobita. Of these four, only three of the groups are living today; the Trilobita are extinct and found only in fossils.
Arthropods account for between 75 and 80 percent of all living organisms on the Earth. They are found in almost every ocean, freshwater and land ecosystem in the world. Although varied in many ways, all arthropods have exoskeletons that provide support and protection for their bodies. Some examples of arthropods are crabs, lobsters, centipedes, millipedes, spiders, scorpions, ticks, mites, beetles, ants and wasps.