I believe the answer is Medusa Stage. Medusa is form of cnidarian which body is very similar to an umbrella and they are able to free swimming around the environment unlike the other type and hey also tend to have tentacles to catch their preys. One example of this is Jellyfishes.
It is the medusa stage in which the cnidarian life cycle goes through that the organism is mobile, or free-swimming. A well-known example of such an organism, a cnidarian in its medusa stage would be a jellyfish, wholly adapted for free-swimming.
