The cnidarians include jellyfishes, sea anemones, corals, and hydrozoans. Of the roughly 11,000 living cnidarian species, all but a few live in the oceans. The smallest cnidarians can hardly be seen without a microscope. The largest known jellyfish is 2.5 meters in diameter, and some colonial species can reach lengths in excess of 30 meters.
The life cycle of many cnidarians has two distinct stages, one sessile and the other motile, although one or the other of these stages is absent in some groups. Obelia (a hydrozoan) is an example of a cnidarian with a life cycle alternating between polyp (sessile, with asexual reproduction through budding) and medusa (motile, with sexual reproduction) generations.
Obelia belongs to the cnidarian group Hydrozoa. Life cycles are diverse among the hydrozoans, but most of the cycles include a generation in which an animal buds repeatedly to form a colony of polyps. Perhaps the best known hydrozoan is the Portuguese man-of-war. Although this animal resembles a single, large medusa form, in reality it consists of a free-floating colony of polyps. The polyps hang from a gas-filled float and, like the polyps of Obelia, they are specialized. Some are reproductive, some capture prey (these polyps have tentacles that may hang as far as 50 meters into the water), and yet others specialize in digesting prey.
Textbook Reference: Concept 23.2 Some Animal Groups Fall outside the Bilateria