“Jellyfish” is a decidedly unscientific term—the creatures are not fish and are more rubbery than jamlike—but scientists use it all the same (though one I spoke with prefers his own coinage, “gelata”). The word “jellyfish” lumps together two groups of creatures that look similar but are unrelated. The largest group includes the bell-shaped beings that most people envision when they think of jellyfish: the so-called “true jellies” and their kin. The other group consists of comb jellies—ovoid, ghostly creatures that swim by beating their hairlike cilia and attack their prey with gluey appendages instead of stinging tentacles. (Many other gelatinous animals are often referred to as jellyfish, including the Portuguese man-of-war, a colony of stinging animals known as a siphonophore.) All told, there are some 1,500 jellyfish species: blue blubbers, bushy bottoms, fire jellies, jimbles. Cannonballs, sea walnuts. Pink meanies, a.k.a. stinging cauliflowers. Hair jellies, a.k.a. snotties. Purple people eaters.