<P> Cnidaria that carry photosynthetic symbionts may have the opposite problem, an excess of oxygen, which may prove toxic . The animals produce large quantities of antioxidants to neutralize the excess oxygen . </P> <P> All cnidarians can regenerate, allowing them to recover from injury and to reproduce asexually . Medusae have limited ability to regenerate, but polyps can do so from small pieces or even collections of separated cells . This enables corals to recover even after apparently being destroyed by predators . </P> <P> Cnidarian sexual reproduction often involves a complex life cycle with both polyp and medusa stages . For example, in Scyphozoa (jellyfish) and Cubozoa (box jellies) a larva swims until it finds a good site, and then becomes a polyp . This grows normally but then absorbs its tentacles and splits horizontally into a series of disks that become juvenile medusae, a process called strobilation . The juveniles swim off and slowly grow to maturity, while the polyp re-grows and may continue strobilating periodically . The adults have gonads in the gastroderm, and these release ova and sperm into the water in the breeding season . </P> <P> This phenomenon of succession of differently organized generations (one asexually reproducing, sessile polyp, followed by a free - swimming medusa or a sessile polyp that reproduces sexually) is sometimes called "alternation of asexual and sexual phases" or "metagenesis", but should not be confused with the alternation of generations as found in plants . </P>

What are the 5 main features of cnidarians