The nervous system is the part of an animal's body that coordinates its voluntary and involuntary actions and transmits signals between different parts of its body. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In most animal species it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS contains the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body.