“The life of a trematode,” wrote Jennifer Ackerman in the October 1997 issue of NATIONAL GEOGRAPHIC, “is...more complex than anything invented by cartoonist Rube Goldberg and just as improbable.” As an object might whirl and twirl through one of Goldberg’s crazy illustrations, so a single trematode can colonize a series of hosts, from a snail to a snake to a hawk. In a mystery scientists are still plumbing, the parasite can apparently nudge its present host into a situation where it will be eaten by the next host in line. AVERAGE SIZE: 0.12 inch (3 millimeters).