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Originally Posted by Black Peter
So what do you think about the study mentioned in the OP?
I think the word sleep is being strained.
Life forms change state depending on environmental stimuli. For instance, plants can change depending on sunlight direction and availability. I am sure with enough imagination you could refer to this as sleeping, but really this would be a misuse of the word.
Jellyfish do have a nervous system, however, they do not have a central control area that could be labelled a brain. I can easily imagine that this nervous system could have different states that vary depending on timing and outside stimuli, in fact, this is what the paper claims.
So in effective control of the Jellyfish is distributed over the whole nervous system rather than collected in a central area. I guess you could say that the Jellyfish's whole nervous system acts like a brain as it has a meta state-based structure. Think of a computer made out of bits of string, matchboxes and sticky tape!