As animals become more advanced so do their nervous systems. In planiarian we see the first evidence of cephalization, the definition of a central area of the nervous system or a brain. In planarian the nerve impulse is directed from the brain along the parallel central nerve cords to the body. Arthropods, such as the grasshopper, have even greater cephalization. Earthworms have also developed a well defined brain and a single ventral nerve cord that controls the impulses.
Vertebrates have the most defined and developed nervous system.
The vertebreted nervous system is composed of a brain and a dorsal hollow nerve cord, as well as a peripheral nervous system.