POLYP

in zoology, one of two principal body forms occurring in members of the animal phylum Cnidaria. The polyp may be solitary, as in the sea anemone, or colonial, as in coral, and is sessile (attached to a surface). The upper, or free, end of the body, which is hollow and cylindrical, typically has a mouth surrounded by extensible tentacles that bear stinging structures called nematocysts. The tentacles capture prey, which is then drawn into the mouth. The lower end of the polyp typically is adapted for attachment to a surface. The body wall consists of an ectodermal, or outer, layer and an endodermal, or inner, layer. One class of cnidarians, anthozoans, is polypoid; most species of the other three classes (hydrozoans, scyphozoans, and cubozoans) alternate in their life cycles between polypoid and medusoid (free-swimming) body forms. Compare medusa. The term polyp also is sometimes applied to an individual in a colony of invertebrate aquatic animals belonging to the phylum Bryozoa. in medicine, any growth projecting from the wall of a cavity lined with a mucous membrane. A polyp may have a broad base, in which case it is called sessile; or it may be a pedunculated polyp, i.e., one with a long, narrow neck. The surface of a polyp may be smooth, irregular, or multilobular. The most common locations of polyps in the human body are the nose, the urinary bladder, and the gastrointestinal tract, especially the rectum and colon. Symptoms of polyps depend upon their location and size. There may be no symptoms, or there may be symptoms resulting from pressure or from mechanical obstruction of all or part of a channel, such as that of the nose or a bowel. Polyps occasionally may bleed. Usually polyps are simple, benign growths, but a small percentage may be either precursors to cancers or may actually contain cancers. For that reason, it is advisable, when possible, to have all polyps removed and examined microscopically.