All arthropods are believed to have descended from a common ancestor, the annelids, more commonly called segmented worms. They are grouped into three main subphyla: Uniramia, Chelicerata and Crustacea.

The subphylum Uniramia contains the classes Diplopoda, Chilopoda and Insecta. Diplopoda are millipedes and Chilopoda are centipedes. Insects include beetles, flies, butterflies, and many other six legged creatures. Most Uniramians live on land and can fly. They have unbranched legs, unlike crustaceans. Most uniramians breathe using a tracheal system, comprised of spiracles, tracheae and tracheoles. All possess one pair of antennae.

The subphylum Chelicerata contains the classes Arachnida, Pycnogonida and Merostomata. The Arachnids include spiders, mites and scorpions. Class Merostomata are horseshoe crabs and class Pycnogonida are sea spiders. The most obvious defining characteristic of the entire subphylum is their fangs and/or pinchers. All Chelicerates have eight legs and lack antennae.

The subphylum Crustacea contains the class Malacostraca. This class is where marine animals such as shrimp and crabs are found. There are a few terrestrial representatives, such as pill bugs. Crustaceans have branched appendages and their exoskeletons are hardened with calcium carbonate. A very unique characteristic of crustaceans is that they have two pairs of antennae, where insects only have one. They have legs attached to their abdomens in addition to the ones on their thorax and have three pairs of chewing appendages. A final important characteristic of Crustaceans is that they breathe with gills. This limits terrestrial Crustaceans such as pill bugs to very damp environments.

The table below outlines the differences found in the phylum Arthropoda: