<Dl> <Dd> Hydrozoa--hydrozoans </Dd> </Dl> <Dd> Hydrozoa--hydrozoans </Dd> <P> Jellyfish or jellies are softbodied, free - swimming aquatic animals with a gelatinous umbrella - shaped bell and trailing tentacles . The bell can pulsate to acquire propulsion and locomotion . The tentacles may be utilized to capture prey or defend against predators by emitting toxins in a painful sting . Jellyfish species are classified in the subphylum Medusozoa which makes up a major part of the phylum Cnidaria, although not all Medusozoa species are considered to be jellyfish . </P> <P> Jellyfish are found in every ocean, from the surface to the deep sea . Scyphozoans (the "true jellyfish") are exclusively marine, and some hydrozoans with a similar appearance live in freshwater . Large, often colorful, jellyfish are common in coastal zones worldwide . Jellyfish have roamed the seas for at least 500 million years, and possibly 700 million years or more, making them the oldest multi-organ animal . </P>

How do the stinging cells of a jellyfish aid their survival