The part of the nervous system with a
concentration of nerve cells which
coordinates various body functions. In
vertebrates, the central nervous system
consists of a brain and a dorsal nerve cord
(the spinal cord) within the spinal column.
In worms, insects, and crustaceans, it
consists of a paired ventral nerve cord with
concentrations of nerve cells, known as
ganglia in each segment, and a small brain in
the head. Some simple invertebrates, such as
sponges and jellyfishes, have no central
nervous system but a simple network of nerve
cells called a nerve net.