This crazy photo from the Yomiuri Shimbun shows a diver swimming amongst a swarm of giant jellyfish. These giant sea blobs, known as Echizen kurage (Nomura's jellyfish), inflict heavy damage on Japanese fisheries in the Sea of Japan each year.
Baby jellies start out binding to a surface, like coral or rock and grow into a polyp tree where once it is mature enough, the small jellyfish detaches from the tree and grows and reproduces to start the cycle over again. Wicked stuff.
Medusa (plural medusae) is another word for jellyfish. Medusa is also the word for jellyfish in: Greek, Finnish, Portuguese, Romanian, Hebrew, Serbian, Croatian, Spanish, French, Italian, Hungarian, Polish, Czech, Slovak, Russian and Bulgarian