Medusa stage of a jellyfish—Tom McHugh—Photo Researchers

In zoology, one of the two principal cnidarian body forms; the typical form of the jellyfish. Its name derives from its tentacles, resembling the snakes borne by Medusa in place of hair. The medusoid body is bell- or umbrella-shaped. Hanging downward from the center is a stalklike structure, the manubrium, bearing the mouth at its tip. The mouth opens into the main body cavity, which connects with radial canals extending to the outer rim of the bell. A free-swimming form, the medusa moves by rhythmic muscular contractions of the bell, providing a slow propulsive action against the water. The other principal cnidarian body type is the polyp.

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