Pulsing jellies (VIDEO) Howard Hughes Medical Institute This video is under embargo. Please login to access this video. To view this video please enable JavaScript, and consider upgrading to a web browser that supports HTML5 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.