Pulsing jellies (VIDEO) Howard Hughes Medical Institute This video is under embargo. Please login to access this video. Caption Of all the animals that scientists know can sleep, Cassiopea jellyfish are the only ones that don't have a centralized nervous system, researchers report. Credit Caltech Usage Restrictions Use with credit. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.