|Sea pens (which include sea pansies) are not just one animal. They are a colony of small minute organisms called polyps or zooids. They consist of a body or common tissue mass, called the rachis, which the polyps attach to. The rachis can be long and thin to flat and round. The polyps develop a division of labor among themselves for feeding, reproduction and water circulation. One polyp usually develops into a stalk, called the peduncle, which is used to anchor into the substrate of sand or mud. They can disappear completely into the substrate and then re-emerge after a disturbance has passed. Many are beautifully colored, and when inflated, they can be twice as big as they are when deflated. Many give off bioluminescence when disturbed. Sea pens are related to corals and are usually termed "soft corals". Sea anemones, hydroids and jellyfishes are in the same phylum.