Jellyfish are among the oldest forms of life on Earth, dating back to the Cambrian Period. They're a complex animal, with specialized components. They have a 'neural net', rather than a nervous system, a much looser arrangement of nerve cells. Jellyfish are nearly all predators, feeding on fish fry, plankton, and any smaller animals they can catch, except for a some species which have algae as part of the organism, which absorb sunlight for energy. Some jellyfish, like the tiny Australian box jellyfish, Irukandji which are capable of killing humans, are considered the most venomous animals on Earth.