Jellyfish. Shapeless gelatinous masses lying on the shore like big spongy plastic bags, they inspire in us the same superstitious fear as the ocean depths themselves. Jellyfish are truly alien: formless and translucent, they have no head, no heart, no brain. Deep in the oceans, before any animal trod the earth, they were here. They have flourished for over 600 million years. We still know very little about these translucent blobs that cause terrible burns and serious economic problems for the fishing industry. Even though numerous naturalists, zoologists, biologists and ecologists have been studying them for two hundred years, these creatures remain a mystery. Magnificent assassins, they can kill in seconds. And they can invade in mass; when they strike, we have practically no defense. These terrifying animals have also got mysterious abilities which scientists are using to improve human health - to unlock the mysteries of cancer, aging and genetic disease.