Medusa (biology)

The medusa is a free-swimming, bell-shaped life stage of many cnidarians. It typically bears tentacles around its margin and often produces eggs or sperm.

Polyp (zoology): A cylindrical, usually attached cnidarian body form with a mouth and tentacles directed upward. Many cnidarians alternate this attached form with a free-swimming medusa.
Cnidaria: A phylum of aquatic animals that includes jellyfish, corals, sea anemones, and hydrozoans. Medusae are body forms found within this animal phylum.
Aurelia aurita: A widespread scyphozoan jellyfish commonly called the moon jelly. Its familiar medusa develops from a strobilating polyp and later produces gametes.
Cubozoa: The cnidarian class of box jellyfish, with medusae typically distinguished by cube-shaped bells and complex eyes. Box jellyfish are medusae with a body plan distinct from familiar scyphozoan forms.
Cnidocyte: A specialized cnidarian cell that fires a capsule, often delivering a toxin or entangling prey. Medusa tentacles use these cells to capture prey and deter threats.
Mesoglea: The gelatinous, mostly acellular layer between the inner and outer cell layers of cnidarians. Its thickness and composition help give the medusa bell its shape and buoyancy.
Strobilation: A form of asexual reproduction in which a polyp segments into disc-like units that become young medusae. Scyphozoan polyps use this process to release juvenile medusae.
Hydromedusa: A medusa stage produced by many hydrozoans, often smaller and structurally simpler than a typical scyphozoan jellyfish. The term highlights how medusa anatomy varies across cnidarian lineages.
Nematocyst: A pressurized capsule inside a cnidocyte that rapidly ejects a thread. This capsule supplies the piercing or entangling action of many medusa stings.
Gastrovascular cavity: A digestive cavity with a single opening that functions in digestion and internal distribution of nutrients. The medusa's central cavity receives food through its mouth and digests it.

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