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The term *medusa*, in its primary biological context, refers to the free-swimming, bell-shaped, or umbrella-shaped life stage characteristic of organisms belonging to the phylum Cnidaria, specifically within the classes Scyphozoa (true jellyfish), Cubozoa (box jellyfish), and Hydrozoa (a diverse class containing many colonial forms where the medusa stage is prominent). This sessile polyp stage, contrasting with the motile medusa, is defined by its radial symmetry, typically exhibiting four-fold or eight-fold symmetry in the scyphozoans, though other counts are possible, manifesting as distinct oral arms and marginal tentacles. The structural morphology of the medusa is fundamentally simple yet highly effective for its pelagic existence. The body, or umbrella, is composed primarily of a gelatinous, non-living extracellular matrix known as mesoglea, sandwiched between two layers of epidermis and gastrodermis. This mesoglea provides buoyancy and structural support, often constituting over 95% of the organism’s total mass, contributing to the characteristic translucent and ethereal appearance associated with jellyfish. The oral surface, located on the underside of the bell, features a central mouth, which serves as the sole opening for both ingestion of prey and egestion of waste, a characteristic feature of the blind sac body plan found throughout Cnidaria. Extending from the mouth are usually four or eight oral arms, which are muscular and often frilled, aiding in manipulating captured food items towards the gastric cavity located centrally within the bell structure. Propulsion in the medusa is achieved through rhythmic muscular contractions of the margin of the bell, forcing water out from beneath the umbrella and creating a form of jet propulsion. While this movement allows for vertical migration and horizontal drift with currents, the swimming capabilities are generally weak and largely dependent on environmental flows, classifying most medusae as plankton rather than strong nekton. A defining feature of the medusa stage is the presence of nematocysts, or stinging cells, housed within specialized structures called cnidocytes, which are typically clustered on the tentacles. These harpoon-like organelles deliver toxins through a rapid, explosive discharge mechanism, serving as the primary defense and predation tool for capturing small crustaceans, fish larvae, and other zooplankton. The nervous system of the medusa is decentralized, lacking a centralized brain. Instead, it possesses a nerve net distributed throughout the body, particularly concentrated around the bell margin, forming a nerve ring. This allows for coordinated, rhythmic contractions necessary for swimming and basic sensory processing of environmental stimuli. Sensory structures, known as statocysts, are crucial components often located along the bell margin, typically associated with notches called rhopalia. Statocysts are gravity-sensing organs that allow the medusa to orient itself vertically in the water column, essential for maintaining proper feeding posture and avoiding benthic environments inappropriate for the pelagic stage. In addition to statocysts, many medusae, particularly in the Cubozoa and some Hydrozoa, possess more complex photoreceptive structures, sometimes including rudimentary eyes or even complex lensed eyes capable of forming images, demonstrating a surprising level of visual acuity for an organism lacking a centralized nervous system. The life cycle differentiation between scyphozoan and hydrozoan medusae is significant. In Scyphozoa, the medusa is typically the dominant, sexually mature stage, following a polyp stage that is often solitary and short-lived, or entirely absent in some derived forms. Sexual reproduction occurs exclusively in the medusa stage, releasing gametes into the water column. Hydrozoan medusae, conversely, are often much smaller and may bud asexually from the colonial hydroid (polyp) stage. In many Hydrozoa, the medusa stage is either reduced to a few specialized reproductive buds or, in some cases, entirely suppressed, with the sessile polyp stage becoming the primary form dominating the life cycle. Cubozoan medusae, or box jellyfish, possess a distinctly cuboidal or box-shaped bell, often exhibiting superior swimming capabilities compared to the radial symmetry of scyphozoans. Their rhopalia are highly sophisticated, housing complex, fully functional eyes, which allows them to actively navigate toward light or away from obstacles, contrasting sharply with the passive drift of many true jellyfish. Gastrovascular function involves enzymatic digestion initiated in the gastric cavity, followed by absorption through gastrodermal cells lining the canals that radiate outward from the central stomach to the bell margin. This extensive canal system increases the surface area for nutrient absorption throughout the extensive body tissues. The reproductive strategy involves the release of gametes (sperm and eggs) from gonads located in the gastric region, usually hanging beneath the radial canals or forming distinct structures. Fertilization is typically external, leading to the formation of a planula larva, which is a small, ciliated, bilaterally symmetric organism. The planula larva is the transitional stage, lacking the radial symmetry of the adult medusa or polyp. It eventually settles onto a substrate, metamorphosing into a planuloid polyp, thus completing the alternation of generations characteristic of many Cnidaria, or beginning the development of the adult medusa in forms lacking a distinct polyp stage. Finally, the ecological role of the medusa stage is crucial; as effective predators in the planktonic community, they exert significant top-down control on zooplankton populations, and in some regions, large aggregations (blooms) can profoundly impact fisheries and marine ecosystem structure, cementing the medusa as a highly significant, albeit often ephemeral, component of the marine biota.