Cnidarians [ny-DAIR-ee-uhns] are a phylum of animals that include corals, jellyfish, sea anemones [uh-NEM-uh-nees], and hydras. Cnidarians display only two basic forms, which occur at different stages of their life cycles. The first form is a polyp, which is a tube-shaped body with a mouth surrounded by tentacles. Name the second form, which has a body shaped like an umbrella with tentacles hanging down.