Scyphozoa: More on Morphology
Like the other cnidaria, scyphozoans have no head and no special organs for respiration or excretion. 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 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. The contraction of the peripheral muscles is controlled by a loose network of nerves which circles the bottom of the bell. There is no controlling brain or central nervous system to aid coordination.
At the bell margin are eight sets of eyespots which are sensitive to light, and eight statocysts, which help the jellyfish maintain its orientation for swimming. Also associated with these are chemosensory pits, perhaps used in the detection of food. These sense organs occur in eight pockets around the edge of the bell, and you can just make out these notches in the above picture.
As you may have guessed, scyphozoans have eight-fold radial symmetry. This symmetry is present in both stages of the life cycle the polyp and the medusa. The polyp phase is a stalked organism which remains attached to some hard surface, and looks much like a Hydra. The medusa is the free-swimming phase which is usually thought of as a jellyfish. This stage has a gelatinous umbrella, called a bell, with the mouth located underneath. This is shown in the diagram below.
Below and surrounding the mouth are usually four oral arms; in some giant jellyfish, these oral arms may be enlarged to as much as 40 meters long, trailing behind the bell as it pulses through the water. There are also smaller tentacles fringing the bell of the medusa. The oral arms bear the infamous stinging cells called cnidocysts, which are used both for defense and for paralyzing prey. Many jellyfish cnidocysts have little effect on humans, but some may injure swimmers, causing fever, cramping, or even death. In some cases, the first exposure creates hypersensitivity to the sting, such that later exposure results in shock. Even beached jellyfish can be dangerous, since the cells are activated by pressure contact, and are not controlled by the jellyfish itself.