They don't have a BRAIN but they have nervous system. A nervous system creates waste and that waste needs to be reclaimed. What's a brain? An exotic collection of neurons... that create waste.

http://neurodojo.blogspot.com/2009/05/jellyfish-nervous-syst...

>Instead, jellyfish have a ring nervous system, located along the margin of the bell. There is definitely a concentration of neurons in that location (although it contains relatively few neurons compared to other animals). Plus, those neurons do serve as an active relay and processing station for sensory and motor activity. Those are two of the main things that central nervous systems do, so there seems to be no good reason to deny that jellyfish have a central nervous system. (Picture from Mackie and Meech, 2000.)

https://en.wikipedia.org/wiki/List_of_animals_by_number_of_n...

Jellyfish: 5,600 neurons.

Box jellyfish: 8,700-17,500 neurons.

Human: 86,000,000,000 neurons.

Notice jellyfish do not have "zero" neurons.

Further, from the wiki: "Not all animals have neurons; Trichoplax and sponges lack nerve cells altogether."

https://www.quora.com/Do-sponges-sleep

"Do sponges sleep?"

"As noted by others, sponges lack nervous systems, [...] sponges don't have nerve cells at all. So NO they don't 'sleep" in any manner similar to other animals."