They do not have a rigid skeleton nor any specialized organ systems, and there are only two tissue layers, with relatively simple internal organization. The specimen of the moon jellyfish, Aurelia pictured below, has been stained to bring out various internal structures that are typical of scyphozoans. The mouth is in the center, surrounded by four palps, or oral arms . The gonads , or sex organs, are visible as four crescent-shaped bodies surrounding the central mouth. The broad, saucer- like bell of the jellyfish is crossed by numerous canals radiating from the center.
Tentacles fringe the edge of the bell.
The jellyfish swims by contracting and relaxing sets of muscles at the margin of the bell. Contraction of the muscles tightens the bottom of the bell, like pulling the drawstrings on a bag. This forces water out through the bottom of the bell, and pushes the jellyfish forward. Relaxing the muscles opens the bell again, preparing for another contraction. In jellyfish with shallow saucer-shaped bodies this may lead to jerky movements, but in the large jellys with highly domed bells, powerful contractions provide stately motion.