Members of the phylum Arthropoda are characterized by jointed appendages and an exoskeleton of chitin. More than one million species of arthropods are currently known to science, and many biologists believe that millions more will be identified. Arthropods are the most biologically successful group of animals on Earth today. This is because they are the most diverse organisms and live in a greater range of habitats than do the members of any other animal phylum, from land to the oceans. One major reason for their abundance is one of their classes—the Insecta class, the organisms we all fondly (or not too fondly) call insects; they are the most successful organisms on Earth in terms of diversity, geographic distribution, number of species, and number of individuals. In fact, to date, more than 900,000 different kinds of insects are known (and amazingly, it is estimated that ten quintillion (10,000,000,000,000,000,000) individual insects are alive at any given time!