<P> Jellyfish, comb jellies, and related animals have diffuse nerve nets rather than a central nervous system . In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth . The nerve nets consist of sensory neurons that pick up chemical, tactile, and visual signals, motor neurons that can activate contractions of the body wall, and intermediate neurons that detect patterns of activity in the sensory neurons and send signals to groups of motor neurons as a result . In some cases groups of intermediate neurons are clustered into discrete ganglia . </P> <P> The development of the nervous system in radiata is relatively unstructured . Unlike bilaterians, radiata only have two primordial cell layers, endoderm and ectoderm . Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type . </P> <P> The vast majority of existing animals are bilaterians, meaning animals with left and right sides that are approximate mirror images of each other . All bilateria are thought to have descended from a common wormlike ancestor that appeared in the Ediacaran period, 550--600 million years ago . The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord with an enlargement (a "ganglion") for each body segment, with an especially large ganglion at the front, called the "brain". </P> <P> Even mammals, including humans, show the segmented bilaterian body plan at the level of the nervous system . The spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature . On the limbs, the layout of the innervation pattern is complex, but on the trunk it gives rise to a series of narrow bands . The top three segments belong to the brain, giving rise to the forebrain, midbrain, and hindbrain . </P>

The most recent part of the nervous system to evolve