Sea anemone, cylindrical marine polyp with a ring of tentacles, belonging to the division of the animal kingdom known as Cnideria, or Coelenterata. Anemones are related to the jellyfish. The body of the anemone consists of a hollow sac with a mouth at one end. The base of the sac is fastened to a rock and the mouth is surrounded by a ring of tentacles armed with stinging cells, or nematocysts. Sea anemones feed on fish and other small animals, which they catch with their tentacles and force into their mouths. While most anemones are fixed to rocks, some burrow in the sand, some can float free, and many can creep over the rocks. Certain sea anemones live on the shells of hermit crabs. Sea anemones reproduce by laying eggs that develop into minute, floating larvae. They can also split in two.