Underneath the bell, in the center, lies the only way in or out of a jellyfish - its mouth. Once inside, we find no brain, no heart and no spine - just a simple stomach and four gonads, or reproductive organs. In every other scallop along the edge of the bell, we find a bundle of sensory nerves. This bundle allows the jelly to detect light, smells and stay upright while swimming. In some jellies the bell is home to tiny crab or fish hitchhikers.
Tentacles Fringing the bell is a row of tentacles.
These vary in length from the slight fringe of the moon jelly to the longer tentacles of some pelagic jellies, which might reach over 100 feet. Four oral arms hang from the middle of a jellyfish's bell, surrounding its mouth. Oral arms are also covered with stinging nematocysts. These cells contain a hollow, barbed thread that fires on impact or in response to a chemical cue. Many are toxic, used to paralyze or kill their prey.