This document discusses the phylum Arthropoda. It is divided into four main classes: Crustacea, Arachnida, Myriapoda, and Insecta. Crustaceans have hard exoskeletons and eat with mandibles, while arachnids have eight legs and book lungs. Myriapods have many leg-bearing segments and live in soil/under debris. Insects are the largest group and have three body segments, three pairs of legs, and one or two pairs of wings. Arthropods are ecologically important as a food source, through pollination, and by controlling pest populations.