Aristotle classified organisms by grouping them by similar characteristics. These groups were called genera and he further divided the organisms within the genera.
His worked consisted of two main groups of animals, those with blood and those without. This grouping was similar to the modern day division between vertebrates and invertebrates. There were five classifications of blood animals: mammals, reptiles, birds, whales and fish. The nonblood animals were called cephalopods and included crustaceans, insects, scorpions, shelled animals and plant-like animals. His work did contain some errors, but it remained a strong foundations for biological studies for many years after his death.