Cnidarians obtain food, mostly prey animals such as crustaceans and fish, by passively waiting for animals to pass by and touch their tentacles, whereupon they poison them and draw them toward their mouths to be digested. One important exception to this pattern is corals, which instead have a mutualistic relationship with algae which generate food for them via photosynthesis. Cnidarians obtain food very passively, but also have low food requirements.
Cnidarians are extremely simple animals with only simple organs and basic nerve nets. Despite most species being predators, they are purely reactive and do not actively hunt prey. Medusas, such as adult jellyfish, are planktonic organisms that drift with currents, allowing their trailing tentacles to encounter prey by chance. Their swimming motions, using weak muscles in the bells above the tentacles, serve merely to keep them at an optimal depth in the water for their preferred prey.
Polyps, like adult sea anemones, do not drift but instead attach to rocks, moving little if at all. They have a body stalk below their tentacles, but otherwise follow the same strategy as jellyfish. Both types use special stinging cells, called nematocysts, which are barbed and venomous, to attack and kill their prey.