Evolution of the Nervous System Nerve Primitive brain n Primitive brain Nerve cords Mouth Subpharyngeal Ventral nerve cord Esophagus with ganglia (a) Nerve net of jellyfish (b) The flatworm nervous system has a (c)The earthworm nervous system has a primitive brain simple brain and ganglia along a nerve cord Forebrain Forebrain Forebrain (d)The fish forebrain is small (e) The goose forebrain is larger. (f) The human forebrain(cerebrum) compared to remainder of brain dominates the brainEvolution of the Nervous System