Jellyfish are, like the mythical Medusa, both beautiful and potentially dangerous. Found from pole to tropic, these mesmeric creatures form an important part of the sea's plankton and vary in size from the gigantic to the minute. Perceived as alien creatures and seen as best avoided, jellyfish nevertheless have the power to fascinate: with the sheer beauty of their translucent bells and long, trailing tentacles; with a mouth that doubles as an anus; and without a head or brain. Drawing upon myth and historical sources as well as modern scientific advances, this book examines our ambiguous relationship with these ancient and yet ill-understood animals, describing their surprisingly complex anatomy, weaponry and habits, and their vital contribution to the ocean's ecosystem.