Jellyfish are ancient marine invertebrates belonging to the phylum Cnidaria, characterized by their gelatinous bodies and radial symmetry. They spend a portion of their life cycle in a free-swimming form, which is what people commonly recognize as a jellyfish. The dome-shaped upper structure, which makes up the bulk of the animal, is correctly termed the bell. It is also sometimes referred to by the broader life-stage name, the medusa, a reference to the mythical Greek figure with snakes for hair.
The Definitive Answer: The Bell
The bell is the umbrella-like or dome-shaped structure that defines the jellyfish’s shape. This hollow structure protects the internal organs and provides the main form of locomotion. The bell acts as a hydrostatic skeleton, using water pressure for structural support, which is why it collapses when removed from the water.
Movement is achieved through rhythmic muscular contractions of the bell’s margin, which push water out from beneath the dome. This action creates a form of jet propulsion, allowing the jellyfish to move vertically and navigate the water column. While many species are passive drifters carried by ocean currents, the bell’s pulsing mechanism provides limited control over direction and speed.
The bell’s shape varies significantly by species, ranging from a flat disc, like the moon jelly, to a pronounced box shape, as seen in Cubozoans (box jellyfish). The term “medusa” is used for this adult, free-swimming phase because of the trailing appendages that hang down, resembling the head of Medusa. Along the outer edge of the bell are small sensory structures called rhopalia, which house balance organs and light-sensing ocelli that aid in orientation and navigation.
Internal Structure of the Bell
The bell’s structure is simple, composed of three main tissue layers and a large, non-living filling. The outermost layer is the epidermis, a thin sheet of cells that serves a protective function against the external environment. This layer also contains sensory cells that allow the jellyfish to perceive stimuli like light and chemicals in the water.
The gastrodermis is the innermost layer, lining the gastrovascular cavity. It is responsible for digestion, containing specialized cells that secrete enzymes to break down captured food. Between the epidermis and gastrodermis is the mesoglea, a thick, gelatinous material that constitutes the majority of the bell’s mass.
The mesoglea gives the jellyfish its common name and characteristic texture, as it is composed of over 95% water. This jelly-like substance contains fibrous proteins, such as collagen, which provide the bell with structural integrity and flexibility. The mesoglea is responsible for the organism’s buoyancy, allowing it to float in the water with minimal effort.
Anatomy Beyond the Top
Hanging down from the underside of the bell, known as the subumbrella, are the appendages responsible for feeding and defense. The center of the subumbrella features a stalk-like structure called the manubrium, which extends downward and terminates with the mouth. The mouth serves a dual purpose, acting as the only opening for both food intake and waste expulsion.
Surrounding the manubrium are the oral arms, which are frilly, extended structures used to maneuver captured prey toward the mouth. These arms are armed with stinging cells, or cnidocytes, which help subdue small organisms. The long, trailing filaments that are the most recognizable feature are the tentacles, which extend from the bell’s margin.
The tentacles are densely packed with cnidocytes, which fire tiny, venomous harpoons called nematocysts upon contact with prey or a threat. They are used for capturing food and protection, with their length varying significantly across species. Once prey is stunned, the tentacles contract to bring the meal closer to the oral arms for transport to the mouth.