Invertebrates are those animals without a backbone (spinal column). Invertebrates include animals such as insects, worms, jellyfish, spiders - these are only a few of the many types of spineless creatures.

For many biologists the nervous system is a particularly fascinating organ system. The nervous system is involved in or is even responsible for many features that are regarded as being characteristic of animals in general.

It is believed that simple nerve nets seen in animals like cnidaria evolved first, followed by nerve cords in bilateral animals – ventral nerve cords in invertebrates and dorsal nerve cords surrounded by a notochord in chordates. Bilateralization led to the evolution of brains, a process called cephalization.