Cnidarians are organisms found exclusively in aquatic habitats. Of the over 10,000 aquatic Cnidarian species discovered, most inhabit marine environments. Cnidarians exhibit two major body forms, polyp or medusae. Polyps consist of a body stalk with a tubular shape with a single opening and multiple tentacles that surround this opening which serves as a mouth and anus. Polyps point upward for filter-feeding. The tentacles are covered with cnidocytes (stinging cells). Cnidocytes are the most important characteristic that distinguishes organisms as belonging to the Cnidarian phylum.Polyps are sessile and remain attached to a substrate. The medusa form is most notably observed in the umbrella like form of jellies. Similar to polyps, they consist of a body with a single opening surrounded by tentacles, but the gelatinous layer is much thicker and the mouth is usually oriented towards the substrate when swimming. Many Cnidarians have the ability to switch between these two morphs over the course of their life.
The photo below depicts a living hard coral polyp with its cnidocyte-covered tentacles protruding from the calcareous cup.
The image below show the medusa form of Cnidarians, as seen in jellies. Check out the Monterey Bay Aquarium’s fantastic Live Jelly Cam!
 Brusca, R.C., & Brusca, G.J.(2003). Invertebrates. Sunderland, MA: Sinauer Associates, Inc.