Hydra Muscle and Neurons Map (IMAGE) Marine Biological Laboratory Caption Hydra are freshwater organisms, only a few millimeters in length. Here, muscle fibers are in sepia and the nerve net is in blue/green. Credit John Szymanski 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.