Polyps are one of two forms of the cnidarians. Polyps may be sexual or asexual. The other body form is the medusa. When medusae mate, the result may be polyps, when polyps replicate, the result may be medusae. The details vary between species. Certain animal species are polyps all the time. Jellyfish polyps turn into 10–15 jellyfish and die.
In many cases the asexual buds formed do not separate from the parent but remain, thus forming colonies or 'stocks'. These may reach a great size, and contain a vast number of individuals. Slight differences in the method of budding produce great variations in the form of the colonies. The reef-building corals are polyp-colonies, strengthened by the formation of a firm skeleton.