Jellyfish Have No Brain, Blood or Heart

A jellyfish gets by with remarkably little. NOAA’s National Ocean Service describes jellyfish as pretty simple critters, lacking brains, blood, or even hearts. They are mostly water. Only about five percent of the body of a jellyfish is solid matter, and the rest, about 95 percent, is water, which is why a jellyfish out of the sea collapses into a blob. What they do have is an elementary nervous system, or nerve net, which allows them to smell, detect light and respond to other stimuli. Digestion is equally basic. A simple cavity acts as both stomach and intestine, with one opening serving as both the mouth and the anus. Jellyfish belong to the phylum Cnidaria, the same group as sea anemones and corals, and their body parts radiate from a central axis.