These wholly marine, usually free-swimming animals are found in all seas and regions of the world. Almost any form of food is suitable for these carnivorous creatures, e.g., from free-floating detritus to small fishes. Most prey/foodstuffs are trapped and/or stung by its nematocyst-armed tentacles and then drawn into its bottom located mouth, then directly to its stomach. Some species trap plankton on mucus covered top areas, which are then transferred to the mouth area. Some species are capable of inflicting serious wounds to people, and even specimens washed up on shores should be by-passed, as their nematocysts stay active long after the animal has died.
Where aquarists are concern, interest in maintaining some of these creatures has grown over the last decade and for more information on this subject recommend reading Jim Stime's article - The Mid-Water Zone in the 'Article' section of this website or contacting him directly at www.aquarium-design.com for more details.