How is this possible ? #300084 - Ask Extension

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How is this possible ? #300084

Asked February 07, 2016, 9:44 PM EST

I was recently looking up different sea creatures. Some of them don't have brains, I was wondering how these creatures are considered animals or even to be living without a Nervous System. I understand for plants, but this I cannot grasp sadly.

Hennepin County Outside United States

Expert Response

While marine invertebrates do not have a central nervous system like you or me, they still have their own version of a nervous system.  They have nerve and nerve-like cells that respond to things in their environment.  For example, when a clam or oyster is exposed to air, these cells trigger the muscles to pull the shell closed to keep the animal from drying out.  In humans, the nerve cells send a signal to the brain, which then tells other cells how to respond.  In invertebrates without a brain, this still occurs.  However mammals can sense something and choose a response.  If you are cold, you might decide to go inside, get a coat, or do something else.  For most of these creatures, they cannot make a choice.  Their cells respond in a set manner. If they detect a predator, they flee, close up their shell, or retract.  Here you can read about the jellyfish "nerve net" https://en.wikipedia.org/wiki/Jellyfish#Nervous_system, or starfish https://en.wikipedia.org/wiki/Starfish#Sensory_and_nervous_systems.
Julie Lively Replied February 08, 2016, 3:02 PM EST

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