Why phylum arthropod is called arthropod?

Arthropoda are the most diverse and arguably the best distributed in species, with the highest count known of described species in any phylum. Over a million species under the phylum have been described and biologists estimate that millions of species have yet to be described. Most of these are in the Insecta class; and about 400,000 of them are beetles (coleoptera).

The actual count of living arthropods at any given moment on Earth is unknown but some estimates place them around ten quintillion (10,000,000,000,000,000,000) which is over a billion for every human. By count, insects are over half of all living animals and by some estimates may constitute up to ninety percent of all living animals on the earth. In biomass, krill (an arthropod crustacean) are believed to have the highest of any animal, weighing in from up to half to three quarters of a billion tonnes, and thus having a huge significance in the food chain.