This volume addresses the evolution and diversity of a significant animal group. It is accorded a special status in the GCR series because of the rarity of the fossils. Arthropods have existed for over 500 million years, and their evolutionary history is traced. Although the trilobites are covered in the relevant "Stratigraphy" volumes, the rarer arthropods - and the c.34 sites from which the fossils were recovered - are described here in detail.