How are Different Groups of Arthropods Related?
Arthropods are the largest of all animal phyla, with over a million described species, estimated at between 6 and 7 million total. Arthropods, whose name means “jointed feet,” are characterized by a hard exoskeleton, segmented bodies, and an open circulatory system. The group includes insects, crustaceans, myriapods (millipedes and centipedes), chelicerates (arachnids and horseshoe crabs), and several extinct groups including trilobites. Arthropod phylogeny is an unsettled scientific topic, and opinions continue to change as new information comes in. Arthropods are almost universally considered monophyletic, meaning they descended from a common ancestor rather than arising multiple times. This is in contrast to the predominant view during the 1970s. A 2001 study of arthropods places the group next to the tardigrades, a phyla of microscopic aquatic animals. Both are in turn related to the velvet worms, a group of sophisticated terrestrial worms with a fossil record stretching back to th
