Pulsing jellies (video) Howard Hughes Medical Institute Share Print E-Mail Loading 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. Share Print E-Mail 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.