Expert: Walter Hintz - 6/25/2006

Question
Are you saying that the nervous system is not centralized?  How does it make decisions, like where to go? I guess I am having a hard time imagining a creature that can be very large, not having a brain.
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Followup To

Question -
I have read that jellyfish have no brains.  If this is the case, what directs their chemical functions,etc?

Answer -
The presence of a brain denotes a central nervous system.  The jellyfish has a nervous system and can respond to stimuli without a brain.  There are nerve centers that help co-ordinate activity.

Answer
In a way the entire nervous system is like a brain.  All of the neurons are connected in what is called a nerve net.  A stimulus travels throughout the total animal depending upon the strength of the stimulus.
The jellyfish responds to the following four stimuli.
  light
  balance
  Chemicals
  touch

The jellyfish only responds to stimuli.  It does not make decisions.