The free-swimming stage in the life cycle of certain members of the Cnidaria. Medusae are umbrella-shaped, with tentacles round the edge and the mouth in the centre underneath. They swim by pulsations of the body and reproduce sexually. In the Hydrozoa (e.g. Hydra) they alternate in the life cycle with polyps, from which they are produced by budding. In the Scyphozoa, which includes all the common jellyfish, and in the box jellies (Cubozoa), the medusa is the dominant form and the polyp is reduced or absent.