The Phylum Arthropoda is a very diverse group of species that evolved more than 500 million years ago. They contain a segmented body covered by a protective exoskeleton containing chitin, which provides rigid structural support and places for muscle attachment. Arthropoda is the largest phylum, with more species of arthropods than species in all other phylums combined, and there are more than one million known arthropod species. This makes the phylum the most diverse of all. Arthropods also have segmented bodies that are fused together to create an integrated unit. Lastly, Arthropods are found in almost every environment from deserts to grasslands, and they are divided into four groups: Myriapoda, Chelicerata, Crustacea, and Hexapoda. These groups categorize them by body features, such as Myriapoda for many pairs of legs, Chelicerata for having a specialized mouth part, Crustacea for having a body that is divided into three parts, and Hexapoda for bodies that are broken up into three sections.