Phylum Arthropoda is the most luxuriant branch on the tree of life. This program covers phylum characteristic and three major arthropod classes: Crustaceans (copepods, waterfleas, branchiopods, decapods, and barnacles), Chelicerates (scorpions, pseudoscorpions, spiders, ticks, and mites), Uniramians (centipedes, millipedes, and insects).