The Arthropods
- Chelicerates - includes spiders, mites, scorpions and related organisms. They are characterised by the presence of chelicerae, appendages just above / in front of the mouth. Chelicerae appear in scorpions as tiny claws that they use in feeding, but those of spiders have developed as fangs that inject venom.
- Myriapods - include millipedes, centipedes, and their relatives. Many have body segments, each bearing one or two pairs of legs.
- Crustaceans - lobsters, crabs, barnacles, crayfish, shrimp and many others. They are primarily aquatic (a notable exception being woodlice) and are characterised by having biramous appendages.
- Hexapods - comprise insects and three small orders of insect-like animals with six thoracic legs.