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Whereas the mammalian brain has a million-million central nerve cells, Aplysia has only 20,000.

Aplysia nerve cells are clustered together in five major bilateral pairs of ganglia, each ganglion containing about 2,000 nerve cells.

An individual ganglion, such as the abdominal ganglion, mediates not one but a family of behaviors. Thus, the simplest behaviors that can be modified by learning may involve less than 100 cells.